Can all mammals swim?

Can all mammals swim?

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When I checked it seemed trivial to answer: yes, all mammals can swim. But research on the internet provided different information. I found:

  • people and primates cannot swim, but can be taught how to swim
  • giraffes can't swim
  • someone claimed elephants can't swim, but this video ad shows the reverse
  • porcupines nor rhinos can't swim
  • at least some bats can swim, but according to this source there's insufficient data.

Of each hit, I found other hits that claimed the reverse, sometimes with proof. Common sense tells me all mammals can swim, but is this true?

With respect to the giraffe claim, this article seems relevant:

D. M. Henderson, D. Naish, Predicting the buoyancy, equilibrium and potential swimming ability of giraffes by computational analysis, J Theoretical Biology 265 (2010) 151-159.

It cites several non-"random person on the internet" claims that giraffes cannot swim:

It is generally thought that giraffes cannot swim, but relevant observations are few. Shortridge (1934) and Goodwin (1954) state that giraffes were poor waders and unable to swim. Crandall (1964) discussed a case where a captive giraffe escaped from a carrying crate, ran to the end of a jetty, and fell into the water. The animal reportedly sank without making any attempt to swim. MacClintock (1973, p. 54) stated 'Giraffes cannot swim. Rivers are barriers they do not cross'. Wood (1982, p. 20) noted that 'Because of its extraordinarily anatomical shape the giraffe is one of the very few mammals that cannot swim - even in an emergency! Deep rivers are an impassable barrier to them, and they will avoid large expanses of water like the plague'.

They then go on to show that a model giraffe could plausibly swim, writing: "For practical and ethical reasons we are unable to use live giraffes… "

They conclude:

In summary, the results and speculations of this study show that it is not impossible that a giraffe could propel itself in water, but in terms of energy efficiency relative to that of the horse, it would appear that the costs of aquatic locomotion might be too high. It is reasonable to expect that giraffes would be hesitant to enter water knowing that they would be at a decided disadvantage compared to being on solid ground.

I have found videos of porcupines, armadillos, anteaters… all swimming. The porcupine seemed to have no trouble at all.

As for apes - quite well it seems.

Rhinos: The water looks deeper than it's head at times, for sure. But whether it is actually swimming or can just/almost reach the bottom is unclear.


It seems to me that if these animals couldn't manage at all in the water, that they wouldn't likely do so, from an evolutionary standpoint.

I'm sure not all manage terribly well, but the videos seem to show that they managed at least.

Pretty cool. :)

Can Moose Swim?

Moose are the largest members of the deer family that lives in North America, Canada & Europe. These huge animals are known for their long & round snouts, humped back, thin legs, and a massive body. When we say massive, we really mean it.

An adult moose can weigh as much as 650Kg and its height ranges from 5 to 6.5 feet. I don’t think one would need an explanation as to why they are called the tallest mammals in the northern hemisphere. There are quite a few interesting things about the moose that are beyond the scope of this article. For now, let us focus only on whether a Moose can swim or not.

So, can moose swim? Yes, Moose can swim well. They are excellent swimmers and divers, capable of diving almost 20ft under the water. They can hold their breath underwater for one whole minute. The large nostrils act as valves to keep the water out during the dive.

In this article we’ll discuss some interesting facts about the swimming of moose, the reason why they swim, how do they swim with their heavy body and a lot more. Let’s get going!

What Animals Cannot Swim?

Although they spend much of their time in the water, hippopotamuses are one of the few animals unable to swim. Their inability to swim is partially due to their density. Instead, they walk or gallop along the bottom of the river.

The Portuguese man-of-war, a siphonophore that resembles a large jellyfish, is also unable to swim despite residing in water. Instead, the animal drifts with the current in hope of finding food and avoiding predators. The Portuguese man-of-war is capable of adjusting its density to either increase or decrease its depth.

Despite the myth that the giraffe cannot swim, scientific observation has proven that the long-necked animal is capable of floating in water and attempting a type of dog-paddle. Similarly, although popular myth considers the great ape incapable of swimming, great apes who have been raised by and live around humans have been observed swimming, sometimes even learning to enjoy it. While they lost their instinct to swim during the process of evolution, they did not lose the physical capability.

Almost all animals, even those who live in areas without much water, are capable of swimming or of learning to swim. This latent ability is due to evolutionary development and first appeared in terrestrial animals around the Early to Middle Cambrian period.

155 Mammals

By the end of this section, you will be able to do the following:

  • Name and describe the distinguishing features of the three main groups of mammals
  • Describe the likely line of evolutionary descent that produced mammals
  • List some derived features that may have arisen in response to mammals’ need for constant, high-level metabolism
  • Identify the major clades of eutherian mammals

Mammals , comprising about 5,200 species, are vertebrates that possess hair and mammary glands. Several other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument, and internal anatomy. Modern mammals belong to three clades: monotremes, marsupials, and eutherians (or placental mammals).

Characteristics of Mammals

The presence of hair , composed of the protein keratin , is one of the most obvious characteristics of mammals. Although it is not very extensive or obvious on some species (such as whales), hair has many important functions for most mammals. Mammals are endothermic, and hair traps a boundary layer of air close to the body, retaining heat generated by metabolic activity. Along with insulation, hair can serve as a sensory mechanism via specialized hairs called vibrissae, better known as whiskers. Vibrissae attach to nerves that transmit information about tactile vibration produced by sound sensation, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s hair stands “on end” to warn enemies, or possibly to make the mammal “look bigger” to predators.

Unlike the skin of birds, the integument (skin) of mammals, includes a number of different types of secretory glands. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin, providing water resistance and lubrication for hair. Sebaceous glands are located over most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water, but also contains metabolic waste products, and sometimes compounds with antibiotic activity. In most mammals, eccrine glands are limited to certain areas of the body, and some mammals do not possess them at all. However, in primates, especially humans, sweat glands are located over most of the body surface and figure prominently in regulating the body temperature through evaporative cooling. Apocrine glands , or scent glands, secrete substances that are used for chemical communication, such as in skunks. Mammary glands produce milk that is used to feed newborns. In both monotremes and eutherians, both males and females possess mammary glands, while in marsupials, mammary glands have been found only in some opossums. Mammary glands likely are modified sebaceous or eccrine glands, but their evolutionary origin is not entirely clear.

The skeletal system of mammals possesses many unique features. The lower jaw of mammals consists of only one bone, the dentary , and the jaw hinge connects the dentary to the squamosal (flat) part of the temporal bone in the skull. The jaws of other vertebrates are composed of several bones, including the quadrate bone at the back of the skull and the articular bone at the back of the jaw, with the jaw connected between the quadrate and articular bones. In the ear of other vertebrates, vibrations are transmitted to the inner ear by a single bone, the stapes. In mammals, the quadrate and articular bones have moved into the middle ear ((Figure)). The malleus is derived from the articular bone, whereas the incus originated from the quadrate bone. This arrangement of jaw and ear bones aids in distinguishing fossil mammals from fossils of other synapsids.

The adductor muscles that close the jaw comprise two major muscles in mammals: the temporalis and the masseter. Working together, these muscles permit up-and-down and side-to-side movements of the jaw, making chewing possible—which is unique to mammals. Most mammals have heterodont teeth, meaning that they have different types and shapes of teeth (incisors, canines, premolars, and molars) rather than just one type and shape of tooth. Most mammals are also diphyodonts , meaning that they have two sets of teeth in their lifetime: deciduous or “baby” teeth, and permanent teeth. Most other vertebrates with teeth are polyphyodonts, that is, their teeth are replaced throughout their entire life.

Mammals, like birds, possess a four-chambered heart however, the hearts of birds and mammals are an example of convergent evolution, since mammals clearly arose independently from different groups of tetrapod ancestors. Mammals also have a specialized group of cardiac cells (fibers) located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.

The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows mammals to produce urine with a high concentration of solutes—higher than that of the blood. Mammals lack a renal portal system, which is a system of veins that moves blood from the hind or lower limbs and region of the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes. A urinary bladder is present in all mammals.

Unlike birds, the skulls of mammals have two occipital condyles, bones at the base of the skull that articulate with the first vertebra, as well as a secondary palate at the rear of the pharynx that helps to separate the pathway of swallowing from that of breathing. Turbinate bones (chonchae in humans) are located along the sides of the nasal cavity, and help warm and moisten air as it is inhaled. The pelvic bones are fused in mammals, and there are typically seven cervical vertebrae (except for some edentates and manatees). Mammals have movable eyelids and fleshy external ears (pinnae), quite unlike the naked external auditory openings of birds. Mammals also have a muscular diaphragm that is lacking in birds.

Mammalian brains also have certain characteristics that differ from the brains of other vertebrates. In some, but not all mammals, the cerebral cortex, the outermost part of the cerebrum, is highly convoluted and folded, allowing for a greater surface area than is possible with a smooth cortex. The optic lobes, located in the midbrain, are divided into two parts in mammals, while other vertebrates possess a single, undivided lobe. Eutherian mammals also possess a specialized structure, the corpus callosum, which links the two cerebral hemispheres together. The corpus callosum functions to integrate motor, sensory, and cognitive functions between the left and right cerebral cortexes.

Evolution of Mammals

Mammals are synapsids, meaning they have a single, ancestrally fused, postorbital opening in the skull. They are the only living synapsids, as earlier forms became extinct by the Jurassic period. The early non-mammalian synapsids can be divided into two groups, the pelycosaurs and the therapsids. Within the therapsids, a group called the cynodonts are thought to have been the ancestors of mammals ((Figure)).

As with birds, a key characteristic of synapsids is endothermy, rather than the ectothermy seen in many other vertebrates (such as fish, amphibians, and most reptiles). The increased metabolic rate required to internally modify body temperature likely went hand-in-hand with changes to certain skeletal structures that improved food processing and ambulation. The later synapsids, which had more evolved characteristics unique to mammals, possess cheeks for holding food and heterodont teeth, which are specialized for chewing, mechanically breaking down food to speed digestion, and releasing the energy needed to produce heat. Chewing also requires the ability to breathe at the same time, which is facilitated by the presence of a secondary palate (comprising the bony palate and the posterior continuation of the soft palate). The secondary palate separates the area of the mouth where chewing occurs from the area above where respiration occurs, allowing breathing to proceed uninterrupted while the animal is chewing. A secondary palate is not found in pelycosaurs but is present in cynodonts and mammals. The jawbone also shows changes from early synapsids to later ones. The zygomatic arch, or cheekbone, is present in mammals and advanced therapsids such as cynodonts, but is not present in pelycosaurs. The presence of the zygomatic arch suggests the presence of masseter muscles, which close the jaw and function in chewing.

In the appendicular skeleton, the shoulder girdle of therian mammals is modified from that of other vertebrates in that it does not possess a procoracoid bone or an interclavicle, and the scapula is the dominant bone.

Mammals evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period, some 205 million years ago. One group of transitional mammals was the morganucodonts , small nocturnal insectivores. The jaws of morganucodonts were “transitional,” with features of both reptilian and mammalian jaws ((Figure)). Like modern mammals, the morganucodonts had differentiated teeth and were diphyodonts. Mammals first began to diversify in the Mesozoic era, from the Jurassic to the Cretaceous periods. Even some small gliding mammals appear in the fossil record during this time period. However, most of the Jurassic mammals were extinct by the end of the Mesozoic. During the Cretaceous period, another radiation of mammals began and continued through the Cenozoic era, about 65 million years ago.

Living Mammals

There are three major groups of living mammals: monotremes (prototheria), marsupials (metatheria), and placental (eutheria) mammals. The eutherians and the marsupials together comprise a clade of therian mammals, with the monotremes forming a sister clade to both metatherians and eutherians.

There are very few living species of monotremes : the platypus and four species of echidnas, or spiny anteaters. The leathery-beaked platypus belongs to the family Ornithorhynchidae (“bird beak”), whereas echidnas belong to the family Tachyglossidae (“sticky tongue”) ((Figure)). The platypus and one species of echidna are found in Australia, and the other species of echidna are found in New Guinea. Monotremes are unique among mammals because they lay eggs, rather than giving birth to live young. The shells of their eggs are not like the hard shells of birds, but have a leathery shell, similar to the shells of reptile eggs. Monotremes retain their eggs through about two-thirds of the developmental period, and then lay them in nests. A yolk-sac placenta helps support development. The babies hatch in a fetal state and complete their development in the nest, nourished by milk secreted by mammary glands opening directly to the skin. Monotremes, except for young platypuses, do not have teeth. Body temperature in the three monotreme species is maintained at about 30°C, considerably lower than the average body temperature of marsupial and placental mammals, which are typically between 35 and 38°C.

Over 2/3 of the approximately 330 living species of marsupials are found in Australia, with the rest, nearly all various types of opossum, found in the Americas, especially South America. Australian marsupials include the kangaroo, koala, bandicoot, Tasmanian devil ((Figure)), and several other species. Like monotremes, the embryos of marsupials are nourished during a short gestational period (about a month in kangaroos) by a yolk-sac placenta, but with no intervening egg shell. Some marsupial embryos can enter an embryonic diapause, and delay implantation, suspending development until implantation is completed. Marsupial young are also effectively fetal at birth. Most, but not all, species of marsupials possess a pouch in which the very premature young reside, receiving milk and continuing their development. In kangaroos, the young joeys continue to nurse for about a year and a half.

Eutherians (placentals) are the most widespread and numerous of the mammals, occurring throughout the world. Eutherian mammals are sometimes called “placental mammals” because all species possess a complex chorioallantoic placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. There are about 4,000 species of placental mammals in 18 to 20 orders with various adaptations for burrowing, flying, swimming, hunting, running, and climbing. In the evolutionary sense, they have been incredibly successful in form, diversity, and abundance. The eutherian mammals are classified in two major clades, the Atlantogenata and the Boreoeutheria. The Atlantogeneta include the Afrotheria (e.g., elephants, hyraxes, and manatees) and the Xenarthra (anteaters, armadillos, and sloths). The Boreoeutheria contain two large groups, the Euarchontoglires and the Laurasiatheria. Familiar orders in the Euarchontoglires are the Scandentia (tree shrews), Rodentia (rats, mice, squirrels, porcupines), Lagomorpha (rabbits and hares), and the Primates (including humans). Major Laurasiatherian orders include the Perissodactyla (e.g., horses and rhinos), the Cetartiodactyla (e.g., cows, giraffes, pigs, hippos, and whales), the Carnivora (e.g., cats, dogs, and bears), and the Chiroptera (bats and flying foxes). The two largest orders are the rodents (2,000 species) and bats (about 1,000 species), which together constitute approximately 60 percent of all eutherian species.

Section Summary

Mammals are vertebrates that possess hair and mammary glands. The mammalian integument includes various secretory glands, including sebaceous glands, eccrine glands, apocrine glands, and mammary glands.

Mammals are synapsids, meaning that they have a single opening in the skull behind the eye. Mammals probably evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period. A key characteristic of synapsids is endothermy, and most mammals are homeothermic.

There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to young. Marsupials give birth to very immature young, which typically complete their development in a pouch. Eutherian mammals are sometimes called placental mammals, because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. All mammals nourish their young with milk, which is derived from modified sweat or sebaceous glands.

20.1 Mammalian Traits

Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. Examples of mammals include bats, whales, mice, and humans. Clearly, mammals are a very diverse group. Nonetheless, they share many traits that set them apart from other vertebrates.

Characteristics of Mammals

Two characteristics are used to define the mammal class. They are mammary glands and body hair (or fur).

  1. Female mammals have mammary glands. The glands produce milk after the birth of offspring. Milk is a nutritious fluid. It contains disease-fighting molecules as well as all the nutrients a baby mammal needs. Producing milk for an offspring is called lactation.
  2. Mammals have hair or fur. It insulates the body to help conserve body heat. It can also be used for sensing and communicating. For example, cats use their whiskers to sense their surroundings. They also raise their fur to look larger and more threatening (see Figure below).

Most mammals share several other traits. The traits in the following list are typical of, but not necessarily unique to, mammals.

  • The skin of many mammals is covered with sweat glands. The glands produce sweat, the salty fluid that helps cool the body.
  • Mammalian lungs have millions of tiny air sacs called alveoli. They provide a very large surface area for gas exchange.
  • The heart of a mammal consists of four chambers. This makes it more efficient and powerful for delivering oxygenated blood to tissues.
  • The brain of a mammal is relatively large and has a covering called the neocortex. This structure plays an important role in many complex brain functions.
  • The mammalian middle ear has three tiny bones that carry sound vibrations from the outer to inner ear. The bones give mammals exceptionally good hearing. In other vertebrates, the three bones are part of the jaw and not involved in hearing.
  • Mammals have four different types of teeth. The teeth of other vertebrates, in contrast, are all alike.

Structure and Function in Mammals

Many structures and functions in mammals are related to endothermy. Mammals can generate and conserve heat when it’s cold outside. They can also lose heat when they become over-heated. How do mammals control their body temperature in these ways?

How Mammals Stay Warm

Mammals generate heat mainly by keeping their metabolic rate high. The cells of mammals have many more mitochondria than the cells of other animals. The extra mitochondria generate enough energy to keep the rate of metabolism high. Mammals can also generate little bursts of heat by shivering. Shivering occurs when many muscles contract a little bit all at once. Each muscle that contracts produces a small amount of heat.

Conserving heat is also important, especially in small mammals. A small body has a relatively large surface area compared to its overall size. Because heat is lost from the surface of the body, small mammals lose a greater proportion of their body heat than large mammals. Mammals conserve body heat with their hair or fur. It traps a layer of warm air next to the skin. Most mammals can make their hair stand up from the skin, so it becomes an even better insulator (see Figure below). Mammals also have a layer of fat under the skin to help insulate the body. This fatty layer is not found in other vertebrates.

How Mammals Stay Cool

One way mammals lose excess heat is by increasing blood flow to the skin. This warms the skin so heat can be given off to the environment. That’s why you may get flushed, or red in the face, when you exercise on a hot day. You are likely to sweat as well. Sweating also reduces body heat. Sweat wets the skin, and when it evaporates, it cools the body. Evaporation uses energy, and the energy comes from body heat. Animals with fur, such as dogs, use panting instead of sweating to lose body heat (see Figure below). Evaporation of water from the tongue and other moist surfaces of the mouth and throat uses heat and helps cool the body.

Eating and Digesting Food

Maintaining a high metabolic rate takes a lot of energy. The energy must come from food. Therefore, mammals need a nutritious and plentiful diet. The diets of mammals are diverse. Except for leaf litter and wood, almost any kind of organic matter may be eaten by mammals. Some mammals are strictly herbivores or strictly carnivores. However, most mammals will eat other foods if necessary. Some mammals are omnivores. They routinely eat a variety of both plant and animal foods. Most mammals also feed on a variety of other species. The few exceptions include koalas, which feed only on eucalyptus plants, and giant pandas, which feed only on bamboo. Types of mammalian diets and examples of mammals that eat them are given in Table below. How would you classify your own diet?

rabbit, mouse, sea cow, horse, goat, elephant, zebra, giraffe, deer, elk, hippopotamus, kangaroo

aardvark, anteater, whale, hyena, jackal, dolphin, wolf, weasel, seal, walrus, cat, otter, mole

bear, badger, mongoose, fox, raccoon, human, rat, chimpanzee, pig, monkey

Different diets require different types of digestive systems. Mammals that eat a carnivorous diet generally have a relatively simple digestive system. Their food consists mainly of proteins and fats that are easily and quickly digested. Herbivorous mammals, on the other hand, tend to have a more complicated digestive system. Complex plant carbohydrates such as cellulose are more difficult to digest. Some herbivores have more than one stomach. The stomachs store and slowly digest plant foods.

Mammalian teeth are also important for digestion. The four types of teeth are specialized for different feeding functions, as shown in Figure below. Together, the four types of teeth can cut, tear, and grind food. This makes food easier and quicker to digest.

Lungs and Heart of Mammals

Keeping the rate of metabolism high takes a constant and plentiful supply of oxygen. That’s because cellular respiration, which produces energy, requires oxygen. The lungs and heart of mammals are adapted to meet their oxygen needs.

The lungs of mammals are unique in having alveoli. These are tiny, sac-like structures. Each alveolus is surrounded by a network of very small blood vessels (see Figure below). Because there are millions of alveoli in each lung, they greatly increase the surface area for gas exchange between the lungs and bloodstream. Human lungs, for example, contain about 300 million alveoli. They give the lungs a total surface area for gas exchange of up to 90 square meters (968 square feet). That’s about as much surface area as one side of a volleyball court!

Mammals breathe with the help of a diaphragm. This is the large muscle that extends across the bottom of the chest below the lungs. When the diaphragm contracts, it increases the volume of the chest. This decreases pressure on the lungs and allows air to flow in. When the diaphragm relaxes, it decreases the volume of the chest. This increases pressure on the lungs and forces air out.

The four-chambered mammalian heart can pump blood in two different directions. The right side of the heart pumps blood to the lungs to pick up oxygen. The left side of the heart pumps blood containing oxygen to the rest of the body. Because of the dual pumping action of the heart, all of the blood going to body cells is rich in oxygen.

The Mammalian Brain

Of all vertebrates, mammals have the biggest and most complex brain for their body size (see Figure below). The front part of the brain, called the cerebrum, is especially large in mammals. This part of the brain controls functions such as memory and learning.

The brains of all mammals have a unique layer of nerve cells covering the cerebrum. This layer is called the neocortex (the pink region of the brains in Figure above). The neocortex plays an important role in many complex brain functions. In some mammals, such as rats, the neocortex is relatively smooth. In other mammals, especially humans, the neocortex has many folds. The folds increase the surface area of the neocortex. The larger this area is, the greater the mental abilities of an animal.

Intelligence of Mammals

Mammals are very intelligent. Of all vertebrates, they are the animals that are most capable of learning. Mammalian offspring are fed and taken care of by their parents for a relatively long time. This gives them plenty of time to learn from their parents. By learning, they can benefit from the experiences of their elders.

Social Living in Mammals

Many mammals live in social groups. Here are some examples:

  1. Herbivores such as zebras and elephants live in herds. Adults in the herd surround and protect the young, who are most vulnerable to predators.
  2. Lions live in social groups called prides. Adult females in the pride hunt cooperatively, which is more efficient than hunting alone. Then they share the food with the rest of the pride. For their part, adult males defend the pride’s territory from other predators.

Locomotion in Mammals

Mammals are noted for the many ways they can move about. Generally, their limbs are very mobile. Often, they can be rotated. Many mammals are also known for their speed. The fastest land animal is a predatory mammal. Can you guess what it is? Racing at speeds of up to 112 kilometers (70 miles) per hour, the cheetah wins hands down. In addition, the limbs of mammals let them hold their body up above the ground. That’s because the limbs are attached beneath the body, rather than at the sides as in reptiles (see Figure below).

Limb Positions in Reptiles and Mammals. The sprawling limbs of a reptile keep it low to the ground. A mammal has a more upright stance.

Mammals may have limbs that are specialized for a particular way of moving. They may be specialized for running, jumping, climbing, flying, or swimming. Mammals with these different modes of locomotion are pictured in Figure below.

The deer in the Figure above is specialized for running. Why? It has long legs and hard hooves. Can you see why the other animals in the figure are specialized for their particular habitats? Notice how arboreal, or tree-living animals, have a variety of different specializations for moving in trees. For example, they may have:

  • A prehensile, or grasping, tail. This is used for climbing and hanging from branches.
  • Very long arms for swinging from branch to branch. This way of moving is called brachiation.
  • Sticky pads on their fingers. The pads help them cling to tree trunks and branches.

Lesson Summary

  • Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. The mammal class is defined by the presence of mammary glands and hair (or fur). Other traits of mammals include sweat glands in their skin, alveoli in their lungs, a four-chambered heart, and a brain covering called the neocortex.
  • Mammals have several ways of generating and conserving heat, such as a high metabolic rate and hair to trap heat. They also have several ways to stay cool, including sweating or panting. Mammals may be herbivores, carnivores, or omnivores. They have four types of teeth, so they can eat a wide range of foods. Traits of the heart and lungs keep the cells of mammals well supplied with oxygen and nutrients.
  • Mammals have a relatively large brain and a high level of intelligence. They also have many ways of moving about and may move very quickly.

Lesson Review Questions


1. List five traits that are shared by all mammals, including the two traits that are used to define the mammal class.

2. Describe how mammals stay warm.

3. What is the function of sweating?

4. Identify mammals that are herbivores, carnivores, and omnivores.

5. What are alveoli? What is their function?

Apply Concepts

6. A certain mammal has very long forelimbs. What does that suggest about where the animal lives and how it moves?

Think Critically

7. Explain how mammalian teeth differ from the teeth of other vertebrates. How are mammalian teeth related to endothermy?

8. Compare and contrast the mammalian brain with the brains of other vertebrates. How is the brain of mammals related to their ability to learn?

Points to Consider

Most mammals are born as live young, as opposed to hatching from eggs. Giving birth to live young has certain advantages over egg laying.


Horizontal swimming

During the past 20 years a large number of studies have examined the biomechanics and energetics of swimming in mammals. Often the animals were placed in water flumes and required to swim continuously against a current generated by a pump (see Williams, 1999 for a review). The addition of a metabolic chamber on the water surface provided the test animals with a place to breathe and permitted the collection of expired gases for respirometry. Alternative methods have examined small mammals such as beavers swimming submerged between metabolic test chambers ( Allers and Culik, 1997). Assessing the swimming energetics of large, fast marine mammals such as dolphins or whales presents a unique challenge and has required novel experimental approaches. These have included training dolphins to match their speed with that of a moving boat in open water ( Williams et al., 1993a) or to swim to metabolic stations ( Ridgway et al., 1969 Yazdi et al., 1999). Swimming costs have also been estimated from field respiratory rates of killer whales ( Kriete, 1995) and gray whales ( Sumich, 1983).

Based on these allometric regressions, it appears that the total cost of transport for many swimming mammals is significantly higher than predicted for fish of comparable body mass ( Brett, 1964). For example, the cost of transport for swimming in the North American mink is 19 times that of salmonid fish ( Williams, 1983) human swimmers have transport costs that are 15 to 23 times the predictions ( Holmer, 1972). Despite specialization for aquatic locomotion, marine mammals also demonstrate elevated transport costs in comparison to fish. The energetic costs for swimming in marine mammals range from 2 to 4 times the predicted values for comparably sized fish ( Williams, 1999).

Several factors appear to contribute to the comparatively high energetic cost of horizontal swimming in mammals. First, under the conditions of these tests, stroking is more or less continuous, where stroking is defined as the movement of a propulsive surface to produce thrust that results in forward motion of the swimmer. Whether in a flume or chasing a boat, continuous stroking was often necessary for the animal to maintain prolonged periods of constant speed in a horizontal plane regardless of position on the water surface or submerged. Second, as mentioned above, drag forces are considerably higher if the swimmer remains at or near the water surface than if it submerges during swimming. The addition of wave drag during surface swimming has been shown to increase the energetic cost of swimming in some mammals by two-fold ( Williams, 1989). A third factor contributing to elevated swimming energetic costs, particularly in semi-aquatic mammals, is the efficiency of the propulsor ( Fish, 1993). Drag-based propulsion characteristic of many semi-aquatic mammals is less efficient in terms of thrust generation and energetic cost than lift-based propulsion typical of marine mammals and fish. Lastly, the ability to retain endogenous heat, that is the cost of endothermy, explains in part the difference in total energetic cost of swimming between marine mammals and fish ( Williams, 1999).

In view of the high energetic cost of swimming in mammals, it is not surprising that marine adapted species have developed a number of behavioral strategies that enable them to avoid the work of continuous stroking. Porpoising is a behavioral strategy used by small cetaceans and pinnipeds moving at high speed near the water surface. Theoretically, this behavior allows the swimmer to avoid the high costs associated with swimming continuously near the water surface by interrupting locomotion and leaping into the air ( Au and Weihs, 1980 Blake, 1983). Wave riding is another strategy that enables the swimmer to avoid continuous stroking. In a study involving bottlenose dolphins trained to swim freely or wave-ride next to a moving boat, we demonstrated a reduction in heart rate, respiration rate and calculated energetic costs for animals riding the bow wave of a boat at 3.8 m·sec −1 . This behavior enabled bottlenose dolphins to nearly double their forward travelling speed with only a 13% increase in energetic cost ( Williams et al., 1992).

Although energetically advantageous when swimming near the water surface, both wave-riding and porpoising have been described for only a limited number of marine mammal species moving at high speeds. These locomotor strategies are not possible during slow transit, in large marine mammals such as elephant seals and whales, or in polar regions where ice covers the water surface. Instead, transit swimming is often accomplished by a sawtooth series of sequential dives that allows the animals to remaining submerged except for brief surface intervals to breathe ( Crocker et al., 1994 Slip et al., 1994 Davis et al., 2001).

Vertical diving

With data readily available for the cost of swimming in mammals, it seems reasonable to presume that the cost of diving can be calculated. Data from time depth recorders and velocity meters deployed on free ranging marine mammals provide information about the duration, distance and speed of the diver. When combined with the relationships for oxygen consumption and speed from swimming experiments, a theoretical diving cost can be determined. Because marine mammals rely on stored oxygen to maintain aerobic processes during a dive, maximum dive durations supported by these reserves (termed the aerobic dive limit, ADL Kooyman, 1989) can be calculated by dividing the oxygen store by swimming metabolic rates. This calculation provides an upper limit for the energetic cost of an aerobic dive. Dives exceeding the ADL require a switch to anaerobic metabolism with the consequent detrimental effects associated with increased plasma lactate (see Butler and Jones, 1997 for a review).

Such calculations for diving bottlenose dolphins resulted in a paradox. Descent and ascent durations, and swimming speed were measured with time-depth/velocity recorders carried by dolphins trained to dive in a straight line path to submerged targets ( Williams et al., 1999). On a relatively short dive to 57 m we calculated that dolphins used 34% (11.1 mlO2·kg −1 ) of the total oxygen store in the blood, muscles and lungs. On deep dives to 206 m metabolic calculations indicated that the oxygen reserves were exhausted after only three quarters of the dive had been completed. Yet, there was little increase in post-dive plasma lactate to indicate a change to anaerobic metabolism ( Williams et al., 1993b, 1999).

The discrepancy was resolved by recording the locomotor behavior of the dolphins during the complete dive. Video cameras placed on the diving dolphins revealed the use of several different swimming gaits rather than continuous stroking ( Fig. 2). During descent the dolphins switched from active stroking to prolonged gliding. Ascents began with active stroking followed by stroke and glide swimming, and ended with a short glide to the surface ( Skrovan et al., 1999). Similar experiments with elephant seals ( Davis et al., 2001, Fig. 2), Weddell seals and even blue whales ( Williams et al., 2000) reveal identical changes in locomotor patterns during diving. Dive descents for these marine mammals typically begin with a period of active stroking followed by gliding to depth. Ascent is characterized by large amplitude, continuous strokes followed by stroke and glide swimming. Depending on the species, the animal may change to a short final glide to the surface.

The absolute duration of gliding during the descent depends on the maximum depth of the dive. For dolphins, phocid seals and the blue whale, the percentage of time gliding increased significantly with depth of the dive. Nearly 80% of the descent was spent gliding for dives exceeding 200 m ( Williams et al., 2000). Interestingly, the depth at which gliding began was similar for the marine mammals examined ( Fig. 3). Glide initiation depth increased from 20 m to 70 m as maximum depth of the dive increased to 200 m. A plateau in the glide initiation depth was reached at approximately 80 m for dives exceeding 200 m. The similarity in pattern for these glide depths suggests the influence of physical factors on the diver.

The mammalian lung at depth

To understand how marine mammals accomplish these prolonged gliding periods we need to examine the structural and functional characteristics of the mammalian lung at depth. Because the lung capacities of many marine mammals are large in comparison to those of terrestrial mammals on a lean weight basis, Kooyman (1973) proposed that the lungs play a role in buoyancy control at sea. Relatively small changes in lung volume depending on whether the animal inhales or exhales could tip the balance between positive or negative buoyancy, and whether an animal floats or sinks when resting on the water surface.

When diving, rapid changes in hydrostatic pressure will also alter lung volume with consequent changes in buoyancy. The magnitude of these changes appears to be associated with morphological modifications coincident with adaptations for a marine lifestyle ( Fig. 4). A unique feature of the lungs of marine adapted mammals is cartilaginous reinforcement of the small airways ( Scholander, 1940 Denison and Kooyman, 1973). Such reinforcement provides a rigid system to the level of the alveoli that permits the progressive collapse of the airways in response to increases in pressure. As a result, compliant alveoli will compress rapidly at depth emptying gas into the reinforced airways. The structural and functional effects of airway reinforcement have been tested both in the laboratory and at sea. While the alveoli of terrestrial mammals such as dogs trap air during simulated dives, those of sea lions show a progressive collapse from the alveoli to the reinforced airways with increases in pressure ( Denison et al., 1971). Pressure chamber tests on Weddell seals and northern elephant seals ( Kooyman et al., 1970), and on the excised lungs of bottlenose dolphins ( Ridgway et al., 1969) show similar patterns of progressive collapse of the airways with exposure to increased pressure. Differences in the oxygen and carbon dioxide content of expired air of dolphins trained to dive or station at depth ( Ridgway et al., 1969) have demonstrated that alveolar collapse is complete once the animals reach 70 m in depth ( Ridgway and Howard, 1979). Alveolar volume is considerably reduced at depths of less than 30 m in Weddell seals and elephant seals ( Kooyman et al., 1970). Likewise, the lungs of large whales including fin whales and sei whales ( Scholander, 1940) and pilot whales ( Olsen et al., 1969) show evidence of progressive alveolar collapse in response to increased hydrostatic pressure.

The structural and functional changes that occur at depth in the marine mammal lung appear to serve many roles. First, the movement of alveolar contents away from gas exchange surfaces and into the conducting airways of the lungs enables marine mammals to avoid the deleterious effects of nitrogen narcosis and decompression sickness ( Scholander, 1940). Second, strengthening of the peripheral airways permits exceptionally rapid tidal ventilation and respiratory gas exchange when marine mammals surface to breathe ( Kooyman and Sinnett, 1979). Furthermore, these same changes in lung volume enable marine mammals to take advantage of the increase in hydrostatic pressure to facilitate prolonged periods of passive gliding during descent ( Fig. 5). Skrovan et al., (1999) described the interrelationships between lung volume, dive depth and buoyancy for the bottlenose dolphin. As air spaces compress with depth the volume of the dolphin decreases without an accompanying reduction in mass, and the animal becomes less buoyant. The theoretical buoyant forces associated with this collapse range from 24.3 N when the dolphin is near the water surface and the lungs are fully inflated, to a negative buoyancy of −25.7 N when the lungs are deflated at 67.5 m in depth. Measured deceleration rates of gliding dolphins correlated directly with the calculated changes in buoyant forces coincident with lung compression ( Skrovan et al., 1999). Thus, the progressive increase in hydrostatic pressure and subsequent lung collapse with depth led to a progressive increase in the ability of dolphins to glide during descent until maximum lung compression occurred at approximately 70 m. A similar interrelationship between hydrostatic pressure, lung volume and buoyancy likely occurs in seals. Although phocid seals exhale prior to submergence, even the small changes in volume that occur as the alveoli compress will alter buoyancy during the course of a dive ( Webb et al., 1998).

These physical and anatomical changes with depth influence the locomotor behavior of diving marine mammals ( Fig. 3). During shallow (<100 m) dives, seals and dolphins initiate short glides early during descent. Deep divers such as the elephant seal ( Davis et al., 2001) and Weddell seal begin prolonged gliding at 60–86 m for dives exceeding 200 m in depth. Even the largest diver in the ocean, the blue whale, appears to follow this pattern and begins gliding at approximately 18 m when performing dives to 36–88 m in depth ( Williams et al., 2000). The depth at which gliding begins undoubtedly depends on many factors including body composition and buoyancy characteristics of the species, initial lung volume, and the type of dive (i.e., transit, exploratory, foraging). Short, shallow dives in which oxygen reserves are not limiting permit greater flexibility in locomotor behavior compared to dives approaching physiological limits ( Williams et al., 1993b). On prolonged dives when the balance between speed and energetic efficiency is critical marine mammals incorporate extended glide periods that often begin when lung compression is near complete ( Fig. 3).

Energetic benefits of intermittent locomotion at depth

Because the contraction of skeletal muscle expends energy, behaviors such as gliding that reduce overall locomotor effort should be manifest as a decrease in energetic cost. This view is supported in simple calculations for the cost of diving by phocid seals and dolphins ( Table 1). In this example the rate of oxygen consumption for harbor seals is 4.6 mlO2·kg −1 ·min −1 during rest and 12.9 mlO2·kg −1 ·min −1 during swimming at approximately 2.0 m.sec −1 ( Davis et al., 1985). Rates of oxygen consumption determined for bottlenose dolphins are 4.6 mlO2·kg −1 ·min −1 during rest and 8.1 mlO2·kg −1 ·min −1 during swimming at 2.1 m·sec −1 ( Williams et al., 1992). Assuming that oxygen consumption during passive gliding approximates resting levels, then a theoretical dive to 200 m by an adult harbor seal will require 43 mlO2·kg −1 if the animal continuously strokes during descent and ascent. A dive incorporating prolonged periods of gliding as in Figure 2 will need only 29.5 mlO2·kg −1 . For dolphins, a stroking dive to 200 m in depth will use 37.8 mlO2·kg −1 compared to 33.8 mlO2·kg −1 for a gliding dive of similar depth.

The benefit of these energetic savings becomes apparent when the size of the oxygen reserve available during submergence is considered. The total oxygen store for an adult, 145 kg bottlenose dolphin is 33.0 mlO2·kg −1 ( Williams et al., 1993b), and 65.0 mlO2·kg −1 for a 24 kg harbor seal ( Kooyman, 1989). In terms of the total oxygen reserve available, the gliding seal performing a 200 m dive realizes a 23% savings and the dolphin a 12% savings. Initially, these savings may appear trivial. However, a 12% savings in the oxygen store for the diving dolphin translates into an additional 1.0 min of gliding or 0.5 min of swimming at 2.0 m·sec −1 assuming the metabolic rates described above. For the phocid seal, a 23% saving in the oxygen store represents 3.0 additional minutes of gliding or 1.1 min of swimming at 2.0 m·sec −1 .

Many additional factors not accounted for in these simple calculations will also affect the actual cost of diving in marine mammals. Metabolic depression during submergence, dive depth, gliding duration, angles of descent and ascent, velocity, the use of stroke and glide locomotion, and the interactive effects of drag and buoyant forces during a dive will influence total energetic requirements. For example, metabolic depression during diving ( Hochachka, 1992) would have an additional conserving effect on oxygen reserves. Several studies have also demonstrated that stroke and glide locomotion can reduce the energetic cost of swimming by 15–50% in fish ( Weihs, 1974 Fish et al., 1991). Stroke and glide propulsion is the preferred mode of locomotion for many species of marine mammal during the ascent portion of a dive ( Williams et al., 2000). Consequently, the energetic savings described in these calculations probably represent a conservative estimate depending on the type of dive and gaits selected by the animal. Certainly, the use of interrupted forms of swimming to complete a dive appears to provide an energetic advantage when compared to continuous swimming for the same dive ( Table 1).

Recent measurements of the post-dive oxygen consumption of Weddell seals provide direct evidence of the energetic benefits of gliding ( Williams et al., 2000). In these studies instrumented adult seals were placed in an isolated ice hole located on the Antarctic sea ice. The hole was covered with a metabolic hood for the collection of respiratory gases and subsequent determination of post-dive oxygen consumption. The seals were free to dive in surrounding waters that exceeded 500 m in depth. Strategic placement of the hole required that the animals return to the metabolic hood to breathe following each dive ( Kooyman et al., 1980). By combining measurements of the underwater locomotor behavior of the seals ( Davis et al., 1999) with post-dive metabolic rate we found that interrupted swimming during a dive resulted in a 9.2–59.6% energetic savings for the Weddell seals ( Williams et al., 2000). Figure 6 demonstrates the difference in energetic costs for dives with and without interrupted swimming periods. Two groups of dives by Weddell seals covering equal distances (1,750–1,850 m) but varying in swimming pattern and depth were compared. Deep dives (231 ± 27 SEM m, n = 12) that facilitated gliding due to changes in pressure with depth resulted in a significant (at P = 0.043) 35% reduction in recovery oxygen consumption compared to shallow dives (55 ± 7 SEM m, n = 4) covering the same distance with nearly continuous stroking. From these results, the incorporation of interrupted forms of locomotion during diving appears to provide an energetic advantage for the diving Weddell seal. Further studies will be needed to determine if this is a general phenomenon for other marine mammal species.

In summary, the evolutionary history of marine mammals has resulted in physiological and morphological characteristics that contribute to elevated costs during swimming. Interrupted forms of locomotion, including wave-riding and porpoising when near the water surface or gliding when descending on a dive, enables marine mammals to mitigate some of these costs. By increasing overall energetic efficiency, these locomotor behaviors allow marine mammals to increase travelling speed for little additional energetic input when swimming, and to prolong the duration of a dive by conserving limited oxygen stores when submerged.

Table 1. Energetic costs for theoretical 200 m dives by a phocid seal and dolphin.*

Aquatic Locomotion

Animals that live in aquatic environments exhibit many different forms of locomotion. Some animals crawl or burrow into the bottom of a body of water. Others swim through the water using a variety of different appendages. Still others float freely, following the currents wherever they go. Aquatic organisms range in size from microscopic to the blue whale, the largest animal that has ever lived.


Aquatic invertebrates swim through the water, crawl along the bottom, or burrow into the bottom. In swimming, muscular activity propels the animal by pushing against the water. On the bottom, muscular activity moves the animal around by interacting with the bottom. Some bottom dwellers simply crawl around on the bottom in a manner exactly like terrestrial locomotion. Others take advantage of the weightless environment to move in ways unique to the water environment.

Aquatic invertebrates have developed two distinct modes of swimming. One mode uses hydraulic propulsion. Jellyfish are a good example of this type of locomotion. They have umbrella-shaped bodies, with the "handle" of the umbrella containing the digestive system. The outer margin of the top of the umbrella, or medusa, is a band of muscles that can contract rapidly. As the muscles contract (just like closing an umbrella) water is expelled forcefully and the jellyfish is propelled along. Scallops use a similar locomotion. They are the best swimmers among bivalves, but at its best, the motion is jerky and poorly controlled. It is used mostly to escape predators. Rapid clapping movements of the two shells create a water jet that propels the scallop.

Cephalopods, such as the squids and octopi, are also mollusks that use water-jet propulsion. Adult cephalopods have lost most of their heavy shell. Many squid are excellent swimmers and can swim forward or backward by undulating flaps along each side of their bodies. All cephalopods are much better swimmers than any other species of mollusk. The mantle of cephalopods encloses a cavity that contains the gills and other internal organs. It also includes, on its bottom surface, a narrow opening called a siphon. When the circular muscles surrounding the cavity simultaneously contract, water is forced through the siphon. This propels the cephalopod in a direction opposite to the direction of the siphon. Thus the siphon also provides directional control.


Some fishlike animals use a purely undulatory motion to move themselves. Almost all fish use undulatory movement to some extent and supplement that motion with muscular effort by fins.

An eel swims by undulating its entire body in a series of waves passing from head to tail. This type of movement is called anguilliform (eel-like) locomotion. During steady swimming, several waves simultaneously pass down the body from head to tail. The waves move faster as they approach the animal's tail.

While eels have a body with a fairly blunt anterior and constant diameter for the rest of the length of the body, most fish have a body that tapers at both anterior and posterior ends. For these fish, undulatory motion is not the most efficient. So most fish exhibit carangiform locomotion, in which only the rear half of the body moves back and forth. The fastest swimming fish use this method of locomotion, so it is apparently the most efficient one. In contrast, ostraciiform locomotion uses only the tail fin to sweep back and forth. This is slower and apparently less efficient.

Whales and other cetaceans use undulatory body waves, but the waves move the whale's body up and down instead of from side to side. The elongated tail region of whales produces a form of carangiform locomotion apparently as effective as that of the swiftest fish. Fish, whales, and other aquatic vertebrates have some arrangement of fins distributed around their bodies. They all have a caudal (tail) fin, vertical in fish and horizontal in cetaceans. Aquatic vertebrates also have a large dorsal fin and a pair of large fins (or flippers) on the sides of their bodies close to the front. The caudal fin is the primary means of locomotion. The lateral fins do most of the steering. The dorsal fin or fins provide stability.

Tetrapodal vertebrates.

Tetrapodal vertebrates (four-legged vertebrates) that use undulatory locomotion include crocodilians, marine lizards, aquatic salamanders, and larval frogs. However, adult frogs and other tetrapods primarily use appendicular locomotion. Many aquatic tetrapods move primarily by using the hind legs. However, sea turtles, penguins, and fur seals have evolved short hind legs with webbed feet used primarily as rudders. These animals use their powerful forelegs, which have evolved into flippers.

Diving birds, such as cormorants and loons, are propelled by their webbed hind feet. Loons are the best adapted for diving. Their body, head, and neck are elongated and slender the hind legs have moved far back to the posterior end of the body the lower legs are short and the feet are completely webbed.

Frogs and some freshwater turtles have elongated rear legs with enlarged, webbed feet. Other aquatic turtles (such as snapping turtles) are relatively poor swimmers. These turtles walk on the bottom of the lake or stream with limb movements very similar to those used on land except that they can move faster in water than they can on land.

Many mammals have swimming movements identical with their terrestrial limb movements. Most aquatic mammals — such as sea otters, hair seals, and nutria — use their hind legs and frequently their tails for swimming. The feet have some degree of webbing. Fur seals and polar bears swim mainly with forelimbs.

see also Flight Skeletons.

Elliot Richmond


Alcock, John. Animal Behavior: An Evolutionary Approach. Sunderland, MA: Sinauer Associates, 1997.

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishers, 1989.

Gould, James L., and Carol Grant Gould. The Animal Mind. New York: W. H. Freeman & Company, 1994.

Gray, James. Animal Locomotion. London: Weidenfield and Nicolson, 1968.

Hertel, Heinrich. Structure, Form, and Movement. New York: Reinhold, 1966.

Muybridge, Eadweard. Animals in Motion. New York: Dover Publications, 1957.

Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer Associates, 1987.

Tricker, R. A. R., and B. J. K. Tricker. The Science of Movement. New York: American Elsevier Publishing Company, 1967.

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Mammal Diversity

Welcome to Mammal Diversity, the Burke Museum’s exploration of the diversity of Earth's mammals. Our tree diagram, down below, shows you the pathways of relatedness and historical evolution of today’s 29 different mammal orders. This phylogenetic tree also shows you that all of our modern mammals were derived from a common ancestor that lived over 200 million years ago. Click on each order for photos and a who’s–who of its members.

Mammals belong to the group of animals that have a backbone, or column of vertebrae. These vertebrate animals include various fishes, amphibians, reptiles, birds, and mammals. The vertebrates all descended from a common ancestor that lived over half a billion years ago. The Class Mammalia evolved later in the history of life on Earth, in the early Mesozoic, about 210 million years ago. Today we find mammals across all of Earth’s continental land masses, on the ground, in the ground, above the ground, and in the air, as well as throughout the oceans. Scientists recognize more than 5,400 species of mammals world–wide.

What distinguishes mammals from other vertebrates? Mammals have hair on their bodies for insulation and protection. Most of them have quite a bit of hair, but some, like whales, armadillos, and humans don’t have so much. Mammal mothers provide their newborn with milk from their mammary glands. Three special tiny bones (the hammer, anvil, and stirrup) that conduct sound through the middle ear of mammals to the hearing nerve in the inner ear also represent unique mammalian characteristics. Mammals also have a single lower jaw bone (the dentary) on each side of their jaw. In fossil mammals, we can’t find hair or milk, but we can identify these special bones that are only found in the mammalian skull.

The branching diagram above is a phylogenetic tree of modern mammals, showing how all modern mammals are related to one another. It is based on available scientific evidence for the evolutionary relationships among major living groups (orders) of mammals. The most recent common ancestor of all mammals lived about 210 million years ago, indicated by the black dot at the base of the tree. The earliest mammals descended from reptile ancestors. They were small, land–inhabiting creatures, not completely like the mammals alive today. Rocks from the Triassic (earliest) period of the Mesozoic era contain these fossil records. These earliest mammals laid eggs, just as reptiles do. Among today’s mammals only the platypus and the echidnas (Order Monotremata) lay eggs to produce their young but, being mammals, they nourish their hatchlings with milk. Monotremes are the oldest of modern orders. Throughout the first 100 million years of their 210–million year history, mammals were basically small, generalized land–dwelling creatures. Compared to reptiles, mammals were few in number and probably many of them were active during the night. Among the earliest groups of modern mammals to evolve were the ancestors of today’s marsupials. These appeared in the fossil record during the later Mesozoic, in the Cretaceous, which is the time of the dinosaurs. Marsupials give birth to tiny, very immature young that are nourished with milk for a long developmental period during the early stages of this development the young are continuously attached to their mother’s teats in a pouch (marsupium). Three orders of marsupial mammals (Didelphimorphia, Paucituberculata, Microbiotheria) live today in South and Central America (and marginally in North America). Four orders of the best known and most diverse marsupials are found in Australasia (Australia, New Guinea, and nearby islands). These are: Diprotodontia, Peramelemorphia, Dasyuromorphia, Notoryctemorphia. During the early evolutionary history of marsupials the present continents of Australia, Antarctica, and South America were joined, as part of the massive southern continent “Gondwana”.

The remaining 21 modern orders of mammals (often called the Eutherian mammals) branched off from one another relatively rapidly around 70 to 50 million years ago. This rapid diversification is recognized, in evolutionary terms, as an adaptive radiation. This means that mammals expanded the range of body forms and ways of making a living, or niches, by which species were able to survive. The resulting great variety of body forms, modes of locomotion, diets, skull forms, and dental anatomy that we see today mostly arose during this rapid radiation in the early Cenozoic, or early Paleogene. By the time of the Eocene and Oligocene epochs, mammals were represented by modes of locomotion that included not only walking, but running, crawling, hopping, climbing, gliding, and even flying and swimming. They came to occupy many ecological niches and every geographic corner of Earth. The whales and dolphins (Cetacea) represent one of the most dramatic of these evolutionary transformations. As descendants of the small, terrestrial, four–legged Mesozoic mammalian ancestors, whales developed new adaptations in the early Paleogene and took on a new body form, including loss of the hind limbs. With front limbs that evolved into flippers and with their fish–like body form, the Cetaceans became swimmers and divers. This evolutionary process allowed them to return successfully from the life of their mammal ancestors on land to the seas, inhabited by their much earlier ancient fish ancestors. Whales include the largest mammal species. Furthermore the largest animal of any kind, including dinosaurs, ever known to have lived on Earth, is the Blue Whale.

The adaptive radiation of modern mammals in the Cenozoic era resulted in the great variety of body forms and modes of locomotion that suit mammals for life in all of Earth’s major environmental media–land, water, and air. Depending on where it lives, an animal has different requirements for moving itself around (locomotion). The same general body plan and common set of bones in the skeletons of the early mammal ancestors have evolved into the diverse array of modern body forms. For example, the same bones of the arm or foreleg are modified into elongated running legs in hoofed mammals, into wings in bats, flippers in whales, and even “shovels” in moles. These and other evolutionary changes in mammals have all occurred since the time when the earliest common ancestors of today’s mammals derived the historic first characteristics that distinguished them from reptiles.

The diets, feeding behavior, and ecology of mammals have influenced the evolution of the shape of the skull and kinds of teeth in the jaws. Mammals use their teeth to seize and, in the case of some predators, to kill their food. Also, unlike many other kinds of animals, mammals use their jaws and teeth to break up and chew pieces of their food that are then further digested in the stomach. (Have you ever seen a bird, a reptile, or an amphibian chewing food?) Most mammals have a variety of different kinds of teeth in their jaws–incisors, canines, premolars, and molars. The numbers, shapes, and sizes of these different kinds of teeth, as well as the shape of the skull have become matched to deal with the feeding behavior and kind of food eaten by each species. Special extreme cases are also interesting. The tropical American anteaters have no teeth at all. An anteater just uses its tongue to slurp up ants and termites. Some of the biggest whales also have no teeth, but instead their mouths contain the brush–like filtering material called “baleen.” This material is used to pick up small shrimp–like “krill” that the so–called baleen whales filter from seawater. Mammals that are specialized for plant–eating (herbivory), whether they are small rodents or large elk, antelopes, or zebras, have flat, hard–ridged rear teeth (premolars and molars) that they use to grind grasses and other green browse plants. Specialized carnivores, such as members of the cat and dog families, have sharper, pointier rear teeth for piercing, tearing, and even shearing pieces of flesh. The longest and sharpest teeth in these carnivores are their canines. Other mammals such as humans, bears, and raccoons are omnivores and have more generalized rear teeth that are neither extremely flat nor extremely pointed they are, instead, somewhat flat, but with rounded bumps or “cusps.” The incisors, in the front of the mouth, are the first teeth to grab and in some cases to cut the food. The long and pointed canines are piercing teeth, used mainly by predatory meat–eating mammals in fact they are absent in most herbivores.

Most mammal females bear their young alive, but members of the order Monotremata (platypus and echidnas) lay eggs from which the young must hatch before they can be nourished by mother’s milk. Note that some other vertebrates, including some sharks, bony fishes, lizards, and snakes give birth to live young, rather than hatching eggs.

You have to travel around the world to see all the different kinds of mammals–kangaroos in Australia, giraffes in Africa, and mountain goats in North America. Why is this? The answer comes from knowing about Earth history. For example, the northern continents were joined together and the southern continents were joined together at the time of the early evolution of mammals, but then they became separated. This means that the new, smaller continents often maintained species that descended from common ancestors who originated on the big supercontinents. That’s how mammals now on different continents were able to share ancestry. At various times since then, connections have been reestablished between north and south. For example, North and South America were separated for a long time until they were reconnected at the “Panama Land Bridge” about 3 million years ago. These kinds of historic continental connections and reconnections explain the unevenness in the geographic diversity and evolutionary relatedness of mammals (and of course other organisms) across the globe. Although ancient Marsupials (a group of seven orders) have been recorded as fossils on most continents, their successes were greatest beginning at the time when South America, Antarctica, and Australia were connected today we still find the greatest successes in Australia and South America–a heritage of the great late Cretaceous supercontinent of Gondwana. Over the entire Earth, scientists now recognize 29 different orders that make up the Class Mammalia. In Washington we have only nine of these orders, or just less than one–third of the world’s mammal biodiversity in terms of orders. In terms of species, the State of Washington has 141 species of mammals, which is just less than three percent of the 5400 mammal species found on Earth.

“Mammal Diversity” web design, graphics, images, and production by George Wang, May 2009.

Mammals: Locomotion, Sense Organ and Origin | Vertebrates

Mammals occupy the highest position in the ladder of evolution. The structural diversi­ties amongst the different groups of mammals are profound. They vary in size from that a field mouse barely 2.5 cm in length to that of a whale attaining a length of more than 30 metres.

A shrew, Sorex minutus (order Insectivora) is the smallest living mammal weighing about 3 grammes in comparison to whale, the Balaenoptera which is the largest mammalian form of about 122 tonnes in weight. Elephants are largest among the land mammals. The giraffe is the longest for its elongated neck.

Mammals have colonised all environ­ments in course of their evolution. Adaptive radiation at its zenith is encountered amongst the mammals.

2. Locomotion in Mammals:

The mammals are basically quadruped animals. The legs are like the ‘towers of the bridge’ and the backbone is the ‘arched can­tilever system’ supported by the ‘towers’. This whole system carries the animal and helps to secure food, shelter and other biological needs.

Mammals exhibit extensive adaptive radiation for locomotion. The skeletal frame­work becomes greatly modified in relation to diverse modes of locomotion.

Plantigrade (Ambulatory)—the central type of locomotion:

The ancestral mammals were plantigrade, i.e., the feet (soles) and toes touched the ground during locomotion. This type of loco­motion is observed in human beings. The other mammals which serve as examples of this type of locomotion are: opossums, bears, raccoon, shrews, mice, etc.

This is the central type of locomotion from which other types of locomotion have radiated in mammals. The mammals under this category walk on the entire foot and are typically five-toed. The metatarsals and metacarpals are not fused and are longer than the phalanges.

The wrist and ankle bones permit movement in various planes. In larger mammals (exemplified by bears) the locomotion is ambulatory while in smaller forms (e.g., shrews, opossums, etc.) the locomotion tends toward the cursorial types. Human beings practice an ambulatory bipedal plantigrade type of locomotion.

Cursorial (Running) type of locomotion:

Surface-oriented larger mammals depen­ding on speed for catching prey or survival show cursorial locomotion. Larger mammals including carnivores, horses, zebras, deer, pronghorn, antelopes, cattle, bison, giraffe show this type of locomotion. Cursorial type of locomotion reaches its peak in ungulates living on the plains. Cursorial mammals have an elongated body and neck.

The elongated neck is used to shift the centre of gravity forward when the animal attains momentum during locomotion. Odocoileus virginianus (white-tailed deer) stretches its neck far for­ward when it moves at its greatest speed.

The limbs become lengthened with the tendency towards fusion or loss of metacarpal and metatarsal bones into the cannon bones. The joint surfaces become tongue-and-groove types restricting the movement of the limbs in a single plane parallel to the long axis of the body.

Depending on the degree of contact with the ground, cursorial locomotion is divi­ded into two types:

Only the toes touch­ing the ground, i.e., walking on toes. Most of the carnivores, cats, dogs, etc., are digitigrade animals. It is estimated that Cheetah (Acinonyx jubatus) attains a maximum speed of 60-65 mph (100 km/hr). It gallops in a ‘measuring-worm fashion’.

Only the tips of toes touch the ground, i.e., walking on tips of toes.

Zebra (Equus), deer (Odocoileus), pronghorn (Antilocapra americana), moose (Alces americana), African antelopes etc.

Jumping type of locomotion:

The jumping type of locomotion is closely similar to that of cursorial type. Some mammals always move by jumping and use both saltatorial and ricochetal jumping depending on speed. Dipodomys (Kangaroo rats) and Zapus (Jumping mice) use both methods of jumping.

When four feet are used in jumping. Rabbits are the best exam­ples. Rabbits, hares and jumping mice possess longer hind limbs. The hind limbs are more muscular than the forelimbs. The forelimbs are usually used for digging or manipulation. The neck becomes short.

When only the hind limbs are used in jumping. Kangaroos are the best examples.

Amphibious, aquatic and marine locomotion:

Adaptations for living and swimming in water are all secondary. The mammals under this category have evolved from previous terrestrial mammals.

Depending on the degree of modifications they are divided into the following types:

The mammals of this type include the beaver (Castor), musk rat (Ondatra), nutria (Myocaster), otter (Lutra), mink (Mustela) and many others. An increase in thickness and quality of hair is usually encountered in these mammals.

The tail becomes modified for aquatic locomotion. It is dorsoventrally flattened in beaver and lateral­ly flattened in musk-rat and nutria. The surface area of the feet has been increased by web­bing or addition of stiff hairs.

The mammals under this category spend most of the time in water and usually come to land for reproduction. The typical examples are seals and hippopotamus. The forelimbs and the hind limbs become high­ly modified into paddle or fin for swimming.

These mammals never come to land. The typical examples are whales. They underwent adaptations to live in water and never leave it. The body is ovoid with short and rigid neck. The skin is hairless except for vibrissae.

Presence of subcutaneous fat (blubber) is a physiological adaptation. The tail becomes modified into a horizontal fluke which serves as the propelling organ. Besides, there are many other morphological as well as physiological adaptations for marine life.

There are many mammals who spend their entire life in the underground. They become specially adapted for this mode of life. The pocket gophers and moles are the typical representatives. Most of them are small in size.

The digging apparatus becomes highly evolved in this group of mammals. There are some semi-fossorial type [e.g., badger (Taxidea)] which spends much of its time above the ground. In fossorial mammals the profile of the head is triangular and flat (e.g., Spalax). Besides modifications of skull the post-cranial portion of the skeleton becomes also modified.

The forelimbs together with the pectoral girdle are modified for digging effi­ciency in different ways (Fig. 10.120). The forelimb may be provided with sesamoid and hetero-tropic bones. In Scalopus the palmar regions are furnished with stiff hairs.

In much semi-fossorial type (Taxidea) the claws become greatly elongated. These claws grow at a quicker rate to make up for the wear and tear for digging. In pocket gopher (Thomomys bot-tae) three centre claws grow 0.23 mm/day or over 0.84 cm/year.

Graviportal (Recti-grade or Sub-unguligrade) type:

This type of locomotion is best illustrated by an elephant which means movement on pil­lars. The limbs are pillar-like and the articula­ting ends are flattened. Each limb has five dig­its arranged in a circle around its edge and an elastic tissue pad is present under the foot.

Many mammals, specially living in forest areas, have become modified to live on trees. This mode of living is named as arboreal. Arboreal mammals are able to climb the trees and use their branches as the highways.

Modifications for holding onto tree branches are observed in these mammals. Tree squirrels and sloths have well-developed claws. Some arboreal mammals possess prehensile tail. The tarsier develops adhesive discs on the front toes.

Sloths spend most of their time hanging upside down the trees and lead a sedentary life. The skeletal system becomes greatly modified. The neck is short with the unusual number of cervical vertebrae (i.e., seven in number) in Choleopus tridactylus (two-toed sloth), but in Bradypus tridactylus (three-toed sloth) there are nine cervical vertebrae.

Strong shoulder girdle, well-formed clavicle, increase in the number of ribs are some of the impor­tant arboreal adaptations.

The squirrels use the trees for climbing and jumping rather than hanging. In typical forms the body is elongated, the hind limbs have well-developed musculature, well- developed and sharp claws and well-formed sense organs.

This is a specialised type of arboreal loco­motion which means swinging from branch to branch by using the forelimbs only. The fore­limbs become greatly lengthened. In gibbons the forelimbs may touch the ground. The stereoscopic vision is very good which helps a gibbon to have a 12-metre-jump from one branch to another with precision.

Volant, gliding and glissant:

Volant and glissant are interchangeable names for gliding type of locomotion. The flying squirrels (Rodentia), flying phalangers (Mursupialia) and the flying lemur (Dermoptera) have developed extra sustaining surfaces formed by the flap of skin (Patagium).

The patagium may extend from the fore to hind limbs on both sides. In colugo (Cynocephalus) the patagium connects the head, forelimbs, hind limbs and tail. Gliding has evolved at different times in different groups of Mammalia.

True flight (flying) exists in bats only. The ‘winged hands’ are the lifting surfaces required for true flight. The hand and arm have modi­fied into a wing. Greatly elongated radius and digits ll-IV of the hand support the patagium. The hind limbs can rotate 180° when it remains suspended upside down from a branch of tree.

Methods of Terrestrial locomotion:

The method, most frequently practiced by the great majority of land mammals, is four-footed or quadrupedal locomotion.

The mechanics of locomotion can be translated in the following way:

In walk, the animal raises the two diagonally opposite feet, for example the anterior left and posterior right, it advances them while the other diagonal pair support and propel the body. The animal then replaces on the ground the feet it has forwarded and raises the other two. This diagonal gait is exhibited by the members of the cat family and dog family.

In pacing the legs of the same side are moved simultaneously. Thus when the two feet on the right side are on the ground the other two on the left side are raised, and just when the latter are put down the others are raised. Giraffe, brown bear and camel exhibit such pacing.

It is nothing but succession of leaps. The animal throws itself by means of the hind legs, extends its body in the air and lands on the forefeet.

In certain mammals a very pecu­liar progression by leaps is observed in which the tail plays an important role. Australian kangaroos progress in this fashion. When it walks slowly, it supports its body partly on its forelegs and partly on its tail and then raises the hind-legs together, then in a second move­ment, it is supported on its hind-feet, while its forefeet and tail are raised.

When the animal accelerates its gait, it employs its hind-legs and during the leap, the tail acts as counter­balance and is held horizontal.

Man alone is bipedal and walks with the help of the two hind limbs. Chimpanzees and gorillas are partially bipedal. They cannot stand upright and while walking they generally touch the ground with their digits of the forelimbs.

Methods of Arboreal locomotion:

Many mammals spend their entire lives on trees, descending to the ground rarely or acci­dentally. This is true for certain monkeys and apes as well as for many rodents and marsu­pials. The techniques of arboreal locomotion are most probably derived directly from the techniques of walking.

In mounting a tree trunk alter­nate grasping with the hand and foot of one or the other diagonal is practiced by some mon­keys, and three toed ant-eater, Tamandua. Some animals climb by alternate movement of the limbs of one side and then of the other. Others climb by successive holds first by the forelimbs and then by the hind ones.

Possession of a prehensile tail facilitates arbo­real locomotion. In leaping from one branch to another, spider monkeys use hands, arms and tail. Some flying squirrels possess scales beneath the tail and the scales act as anti-skids.

The special mode of progres­sion of gibbons and spider monkeys has been termed Brachiation. It corresponds in a way to the bipedal walk but here the front limbs are used. The mechanism of the two-handed loco­motion is simple.

First one of the hands grasps a branch and draws the body forward. Then the body oscillates on that pivot made by the hand and the other hand extends to hold another branch. If the animal wants to go from one branch to another little far off branch a short glide occurs between the holds.

Aquatic Locomotion:

Aquatic mammals swim in water with the aid of modified forelimbs or hind limbs and with the undulation of tail. Many terrestrial animals can swim.

Aortic Arches of Vertebrates:

Aortic arches are paired blood vessels that emerge from the ventricle of the heart which are basically similar in number and disposition in different vertebrates during the embryonic stages.

Embryonic arterial arches:

During the embryonic stages six pairs of arterial arches develop in most gnathostomes and are named according to the name of the visceral clefts.

The aortic arches are designated by Roman numerals (I—VI, Fig. 10.145A). The first aortic arch (I) is called mandibular aortic arch that proceeds upwards on either side of the pharynx and turn backwards as lateral longitudinal tubes, called radices aortae or lateral aortae which both join mesially to form the common dorsal aorta.

The second aortic arch becomes hyoid arch. The third (III), fourth (IV), fifth (V) and sixth (VI) are called branchial arches. Table 58 relates the modification of embryonic arterial arches in adult in different vertebrates.

3. Sense Organs in Mammals:

They have the same general plan of struc­ture as encountered in birds and reptiles.

Organ of Jacobson is well- developed in lower groups of mammals. The olfactory mucosa has become elaborate in higher mammals because of the convolutions of the ethmoturbinal bones. The nasal cham­ber has lost its sensory functions in the toothed whales where the olfactory nerves are almost vestigial.

The structure of the eye resembles that of other vertebrates. The pecten of birds and reptiles is absent. The sclera is composed of condensed fibrous tissue. Most of the mam­mals are provided with three eyelids in each eye. The upper and lower eyelids are opaque and are provided with hair.

The third eyelid is transparent and hairless. In higher forms of the third eyelid is vestigial. Its vestige can be seen as a pink-fold in the inner canthus of each eye. The eyeball and the eyelids are kept mois­tened by the secretion of a lachrymal, a harderian and a series of meibomian glands for each eye. Following are the accounts of eye and its associated structures in mammals.

The eyes are ill-developed and almost functionless in burrowing insectivores, moles and marsupial, Notoryctes. The eyes of whales are small. In platanista it is vestigial. The eyes of the whales are variously modified. In them the cornea is flat and the lens is round. The sclera is thick and the eyelids are provided with specialised lid muscles for protection against pressure.

Cartilage is absent in the sclera. A tapetum lucidum is present in many nocturnal forms. Tapetum lucidum is a layer of light reflecting crystals located on the choroid coat adjacent to the retina. Hoofed mammals possess a tape-turn fibrosum. In it the portion of the choroid coat’ is made up of a tendinous type of con­nective tissue which glistens in a manner simi­lar to a fresh tendon.

Carnivores, seals and lower primates possess another type of tape­tum, called tapetum cellulosum. It is com­posed of several layers of cells filled with small crystals of unknown organic material. The pupil is round in most forms. A vertical slit is characteristic of the cat family. The vertical slit in them becomes round at night.

The slit is transversely arranged in many ungulates and whales. The portion of the iris bordering the pupil is modified to form an irregular, pig­mented and fringe-like umbraculum in gazelle and camel. This is a device to protect the eye from excessive glare.

The retina of the eye bears rods and cones. But in lower orders like Edentata, Chiroptera and certain shrews the cones are absent. It has been shown that rods detect the differences in the intensity of light and hence the nocturnal animals usually have rods only. Rods contain a purple pigment, called rhodopsin, which is destroyed by light but is instantly manufactured by vitamin A.

The cones of the eye are sensitive to bright light and to various colours. Recently three more pigments have been detected in the human eyes. They are red-sensitive etythrolabe, green-sensitive chlorolabe and blue- sensitive cyanolabe. The capacity for colour vision, however, is restricted amongst the primates only.

4. Hearing and Balance in Mammals:

Amongst the verte­brates the ear is best developed in mammals. The accessory parts and the membranous labyrinth have become very complex in mam­mals. All mammals excepting monotremata, cetacea and sirenia possess large external pinna.

This is supported by cartilage and of various sizes and shapes in different mam­mals. It helps in collecting sound waves and it is turned in different directions by the mammals excepting man. Opening of the external auditory passage lies at the base of the pinna.

The external passage leads up to the tympanic membrane. The walls of the passage may be membranous or cartilaginous or osseous in part. The passage is formed of a series of incomplete rings in Tachyglossus. The middle ear or tympanic cavity is enclosed by periotic and tympanic bones.

The cavity communicates with the pharynx by the Eustachian tube. The inner wall of the tympanic cavity bears fenestra ovalis or fenestra rotunda.

A chain of auditory ossicles—the malleus, incus and stapes— run between the tympanic membrane and rotunda. These are the smallest bones in the body and show variation in form.

The stapes is with a foramen and the perforation is made by a minute artery as in rabbit. Stapes is rod- shaped in manis. The membranous labyrinth of the internal ear is with a specially deve­loped cochlea. The cochlea is spirally coiled and is absent in monotremata.

History of the animal kingdom has been from the beginning a continuous process of evolution and highest in this evolutionary series stand mammals unique amongst the ani­mals. It is an accepted idea that mammals originated from some groups of vertebrates that lie lower in the scale of the ladder of evo­lution.

These lower vertebrates are constituted by the Fishes, Amphibians, Reptiles and Birds. Of these the Fishes and the Birds can be eliminated since the former is too low and the latter is too specialised in their organisations.

Ancestry through Amphibia:

Amphibian ancestry of mammals through Hypotheria, a stage intermediate between amphibia and mammals, was advocated by T. H. Huxley (1880).

The points on which Huxley based his arguments were:

(a) Presence of two occipital condyles in both amphibia and mammals

(b) Presence of left aortic arch in mammals. As the left aortic arch is weak in reptiles they can­not hold the line of ancestry of mammals.

It is true that there are two occipital condyles in amphibia and mammals. But the source of the condyles is different. In amphib­ia the occipital condyles are derivatives of exoccipitals whereas in mammals the sources are the basioccipitals. So Huxley’s theory of the origin of mammals through Hypotheria is not justified.

Ancestry through Reptiles:

Paleontological evidences establish the fact that mammals arose from reptiles. Fossils of Synapsida that have been discovered in the carboniferous strata indicate many characters leading to mammalian line. The advanced forms of Synapsida showed a tendency towards reduc­tion of skull bones and teeth differentiation.

From the Synapsida arose the Therapsids during late Permian and upper Triassic.

The Therapsids approached mammalian organisa­tion by having:

(a) Strong and enlarged lateral temporal fossa,

(c) Enlarged dentary and teeth differentiation,

(d) Double occipital condyles and

(e) Reduced quadrate and quadrato jugal.

The Therapsids diverged into Dicynodont and Theriodontia and during mid-Triassic from Theriodointia arose Cynodontia and Ictidosauria. It is believed that one of them or both gave rise to primitive mammals.

Different views regarding the origin of mam­mals:

Barghusen (1968), Crompton and Jenkins (1968), Crompton (1969) supported the monophyletic origin of mammals. According to them mammals have evolved from cynodonts, the last group of therapsids appeared in the late Permian.

By the end of Triassic non-mammalian cynodonts included some mammal-like groups and had given rise to mammals themselves. Recently Pough et al., (1996) presented the idea that the three groups of living mammals—the monotremes, marsupials and eutherians are all derived from the ‘holothere’ lineage.

It is quite possible that the mammals may have had a polyphyletic origin — that several groups of mammals-like reptiles contributed to the ancestry of the early mammals and in the Jurassic times the threshold had been crossed from reptiles to mammals. This view is almost supported by Olson (1959), Carter (1967), Simpson (1971), Bellairs and Attridge (1975), Griffiths (1978) and Young (1981).

Romer and Parsons (1986):

The mam­mals are descended from reptiles, but the fossil record shows that the reptilian line leading to them, the subclass synapsida, diverged almost at the base of the family tree of that class. Their relationship to the existing reptilian orders is thus exceedingly remote (Fig. 10.160).

What animals can't swim?

I was thinking about this and the only animal I can think of that instinctively sucks at swimming would be humans. Maybe other primates? Google only gave me some half-ass answers so I wonder what the smart people here thought?

Humans and primates actually have a surprisingly strong affinity to water. Monkeys enjoy swimming and there is even a (poorly supported and not scientifically accepted) theory that modern humans spent part of their evolution as aquatic animals.

As for species that can't swim? Well I can't say I can find any papers on the subject of which animals swim and which animals drown, for obvious reasons. Though I do know for a fact drowning is a major risk for many bird species (eg Quails kept as pets require marbles in there water bowls to prevent drowning).

Watch the video: Warum können Babys tauchen? - Welt der Wunder (October 2022).