Mother kangaroos are able to rear multiple joeys (young kangaroos) at different stages of development at the same time. According to Wikipedia, "the mother is able to produce two different kinds of milk simultaneously for the newborn and the older joey still in the pouch." What biological mechanisms are able to ensure that the correct joey gets the correct kind of milk? Do different teats produce different types of milk and somehow the joeys know which teat to suck from; or is each teat able to deliver both types of milk and the teat is able to tell from the mouth conformation (or something else) which type of milk to deliver?
The younger Joey remains attached to the teat from which it receives its milk for the period of nine months until it leaves the pouch. So that would be teat from which milk suitable for the newborn is available (reference 1 and reference 2). The older Joeys also have an incredible sense of smell by which it can detect the teat from which it gets the milk it requires but it stands to reason that they cant get milk from the teat to which the younger joey is attached. (reference)
Kangaroo care also called skin-to-skin contact, is a technique of newborn care where babies are kept chest-to-chest and skin-to-skin with a parent, typically their mother (occasionally their father).
Kangaroo care, named for the similarity to how certain marsupials carry their young, was initially developed in the 1970s to care for preterm infants in countries where incubators were either unavailable or unreliable. More recently, the term skin-to-skin care is also used to describe the technique of placing full-term newborns immediately after birth on the bare chest of their mother or father. There is evidence that it is effective in reducing infant mortality, the risk of hospital-acquired infection, increasing weight gain, increasing rates of breastfeeding, and other advantages for both mother and baby.
How do I do kangaroo care?
Your nurse will typically help you get started with kangaroo care in the hospital. A few basic tips for getting started with kangaroo care include:
- Removing your bra and wearing a shirt that opens in the front. You can also use a hospital gown that opens in the front for kangaroo care. Screens can usually be provided for your privacy.
- Placing the baby — only wearing a diaper and hat — on your bare chest. Your baby will be in an upright position, with his or her chest against your chest.
- Covering the baby’s back. Once you’re settled skin-to-skin, drape a blanket or your shirt or gown over your baby’s back. Keep your baby warm and comfortable while snuggled against your chest.
- Relaxing together. During your session, try and relax as you hold your baby. Remember to breathe normally and focus on your child.
- Planning on multiple sessions. You should plan to do kangaroo care more than once — at least one hour, four or more times each week. However, the number of times you will be able to do kangaroo care in one day will be up to your nurse. Talk to your care team about the best schedule for your baby.
- Letting your baby rest. This is a great time to let your baby rest and relax on you. Allow your baby to snuggle in and fall asleep during the session. Remember, this isn’t time to play with your baby.
Moms aren’t the only ones that can do kangaroo care. A baby can also benefit from skin-to-skin time with dad. Actually, the different feel of the father’s body will provide a different stimulation to the baby.
There will be times when you can’t do kangaroo care with your child. If your baby has arterial monitoring lines, is on an oscillator or is receiving another type of treatment you may not be able to do kangaroo care.
Is there anything I shouldn’t do while practicing kangaroo care?
There are a few things you shouldn’t do when you are practicing kangaroo care with your baby. The most important thing is to be focused on your baby during this time. Spending time skin-to-skin with your child can help your baby in the first few days, weeks and months of life. This activity can also be a great chance to bond with your child.
When you are doing kangaroo care, make sure you:
- Put away your cell phone. Having your phone out during kangaroo care is not only a distraction from your child, but it can be a safety issue.
- Are healthy. If you aren’t feeling well or have a current illness, it’s best to not do kangaroo care until you are feeling better.
- Can spend at least 60 minutes each session skin-to-skin with your baby.
- Have clean and healthy skin (no perfumes, skin rashes, open skin lesions or cold sores).
- Don’t smoke before kangaroo care.
Last reviewed by a Cleveland Clinic medical professional on 06/29/2020.
- March of Dimes. Kangaroo Care. Accessed 7/1/2020.
- Campbell-Yeo M, Disher T, Benoit B, Johnston C. Understanding kangaroo care and its benefits to preterm infants. Pediatric Health, Medicine and Therapeutics. 2015 6: 15-32. Accessed 7/1/2020.
- Jefferies A. Kangaroo care for the preterm infant and family. Paediatrics Child Health. March 2012 17(3): 141-143. Accessed 7/1/2020.
- American Pregnancy Association. Care For The Premature Baby. Accessed 7/1/2020.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
The word kangaroo derives from the Guugu Yimithirr word gangurru, referring to eastern grey kangaroos.   The name was first recorded as "kanguru" on 12 July 1770 in an entry in the diary of Sir Joseph Banks this occurred at the site of modern Cooktown, on the banks of the Endeavour River, where HMS Endeavour under the command of Lieutenant James Cook was beached for almost seven weeks to repair damage sustained on the Great Barrier Reef.  Cook first referred to kangaroos in his diary entry of 4 August. Guugu Yimithirr is the language of the people of the area.
A common myth about the kangaroo's English name is that it was a Guugu Yimithirr phrase for "I don't know" or "I don't understand".  According to this legend, Cook and Banks were exploring the area when they happened upon the animal. They asked a nearby local what the creatures were called. The local responded "kangaroo", said to mean "I don't know/understand", which Cook then took to be the name of the creature.  Anthropologist Walter Roth was trying to correct this legend as far back as in 1898, but few took note until 1972 when linguist John B. Haviland in his research with the Guugu Yimithirr people was able to confirm that gangurru referred to a rare large dark-coloured species of kangaroo.   However, when Phillip Parker King visited the Endeavour River region in 1819 and 1820, he maintained that the local word was not kangaroo but menuah perhaps referring to a different species of macropod.  There are similar, more credible stories of naming confusion, such as with the Yucatán Peninsula. 
Kangaroos are often colloquially referred to as "roos".  Male kangaroos are called bucks, boomers, jacks, or old men females are does, flyers, or jills and the young ones are joeys.  The collective noun for a group of kangaroos is a mob, court, or troupe. 
There are four extant species that are commonly referred to as kangaroos:
- The red kangaroo (Osphranter rufus)  is the largest surviving marsupial anywhere in the world. It occupies the arid and semi-arid centre of the country. The highest population densities of the red kangaroo occur in the rangelands of western New South Wales. Red kangaroos are commonly mistaken as the most abundant species of kangaroo, but eastern greys actually have a larger population.  A large male can be 2 metres (6 ft 7 in) tall and weighs 90 kg (200 lb). 
- The eastern grey kangaroo (Macropus giganteus)  is less well-known than the red (outside Australia), but the most often seen, as its range covers the fertile eastern part of the country. The range of the eastern grey kangaroo extends from the top of the Cape York Peninsula in northern Queensland down to Victoria, as well as areas of southeastern Australia and Tasmania. Population densities of eastern grey kangaroos usually peak near 100 per km 2 in suitable habitats of open woodlands. Populations are more limited in areas of land clearance, such as farmland, where forest and woodland habitats are limited in size or abundance. 
- The western grey kangaroo (Macropus fuliginosus)  is slightly smaller again at about 54 kg (119 lb) for a large male. It is found in the southern part of Western Australia, South Australia near the coast, and the Murray–Darling basin. The highest population densities occur in the western Riverina district of New South Wales and in the western areas of the Nullarbor Plain in Western Australia. Populations may have declined, particularly in agricultural areas. The species has a high tolerance to the plant toxin sodium fluoroacetate, which indicates a possible origin from the southwest region of Australia. 
- The antilopine kangaroo (Osphranter antilopinus)  is, essentially, the far northern equivalent of the eastern grey and western grey kangaroos. It is sometimes referred to as the antilopine wallaroo, but in behaviour and habitat it is more similar to the red, eastern grey and western grey kangaroos. Like them, it is a creature of the grassy plains and woodlands, and gregarious. Its name comes from its fur, which is similar in colour and texture to that of antelopes. Characteristically, the noses of males swell behind the nostrils. This enlarges nasal passages and allows them to release more heat in hot and humid climates. 
In addition, there are about 50 smaller macropods closely related to the kangaroos in the family Macropodidae. Kangaroos and other macropods share a common ancestor with the Phalangeridae from the Middle Miocene.  This ancestor was likely arboreal and lived in the canopies of the extensive forests that covered most of Australia at that time, when the climate was much wetter, and fed on leaves and stems.  From the Late Miocene through the Pliocene and into the Pleistocene the climate got drier, which led to a decline of forests and expansion of grasslands. At this time, there was a radiation of macropodids characterised by enlarged body size and adaptation to the low quality grass diet with the development of foregut fermentation.  The most numerous early macropods, the Balbaridae and the Bulungmayinae, became extinct in the Late Miocene around 5–10 mya.  There is dispute over the relationships of the two groups to modern kangaroos and rat-kangaroos. Some argue that the balbarines were the ancestors of rat-kangaroos and the bulungmayines were the ancestors of kangaroos.  while others hold the contrary view. 
The middle to late bulungmayines, Ganguroo and Wanburoo lacked digit 1 of the hind foot and digits 2 and 3 were reduced and partly under the large digit 4, much like the modern kangaroo foot. This would indicate that they were bipedal. In addition, their ankle bones had an articulation that would have prohibited much lateral movements, an adaptation for bipedal hopping.  Species related to the modern grey kangaroos and wallaroos begin to appear in the Pliocene. The red kangaroo appears to be the most recently evolved kangaroo, with its fossil record not going back beyond the Pleistocene era, 1–2 mya. 
The first kangaroo to be exhibited in the Western world was an example shot by John Gore, an officer on Captain Cook's ship, HMS Endeavour, in 1770.   The animal was shot and its skin and skull transported back to England whereupon it was stuffed (by taxidermists who had never seen the animal before) and displayed to the general public as a curiosity. The first glimpse of a kangaroo for many 18th-century Britons was a painting by George Stubbs. 
Comparison with wallabies
Kangaroos and wallabies belong to the same taxonomic family (Macropodidae) and often the same genera, but kangaroos are specifically categorised into the four largest species of the family. The term wallaby is an informal designation generally used for any macropod that is smaller than a kangaroo or a wallaroo that has not been designated otherwise. 
Kangaroos are the only large animals to use hopping as a means of locomotion. The comfortable hopping speed for a red kangaroo is about 20–25 km/h (12–16 mph), but speeds of up to 70 km/h (43 mph) can be attained over short distances, while it can sustain a speed of 40 km/h (25 mph) for nearly 2 km (1.2 mi).  During a hop, the powerful gastrocnemius muscles lift the body off the ground while the smaller plantaris muscle, which attaches near the large fourth toe, is used for push-off. Seventy percent of potential energy is stored in the elastic tendons.  At slow speeds, it employs pentapedal locomotion, using its tail to form a tripod with its two forelimbs while bringing its hind feet forward. Both pentapedal walking and fast hopping are energetically costly. Hopping at moderate speeds is the most energy efficient, and a kangaroo moving above 15 km/h (9.3 mph) maintains energy consistency more than similarly sized animals running at the same speed. 
Kangaroos have single-chambered stomachs quite unlike those of cattle and sheep, which have four compartments.   They sometimes regurgitate the vegetation they have eaten, chew it as cud, and then swallow it again for final digestion. However, this is a different, more strenuous, activity than it is in ruminants, and does not take place as frequently. 
Different species of kangaroos have different diets, although all are strict herbivores. The eastern grey kangaroo is predominantly a grazer, and eats a wide variety of grasses, whereas some other species such as the red kangaroo include significant amounts of shrubs in their diets. Smaller species of kangaroos also consume hypogeal fungi. Many species are nocturnal,  and crepuscular,   usually spending the hot days resting in shade, and the cool evenings, nights and mornings moving about and feeding.
Because of its grazing habits, the kangaroo has developed specialized teeth that are rare among mammals. Its incisors are able to crop grass close to the ground and its molars chop and grind the grass. Since the two sides of the lower jaw are not joined or fused together, the lower incisors are farther apart, giving the kangaroo a wider bite. The silica in grass is abrasive, so kangaroo molars are ground down and they actually move forward in the mouth before they eventually fall out, and are replaced by new teeth that grow in the back.  This process is known as polyphyodonty and, amongst other mammals, only occurs in elephants and manatees.
Absence of digestive methane release
Despite having herbivorous diets similar to ruminants such as cattle, which release large quantities of digestive methane through exhaling and eructation (burping), kangaroos release virtually none. The hydrogen byproduct of fermentation is instead converted into acetate, which is then used to provide further energy. Scientists are interested in the possibility of transferring the bacteria responsible for this process from kangaroos to cattle, since the greenhouse gas effect of methane is 23 times greater than carbon dioxide per molecule. 
Social and sexual behavior
Groups of kangaroos are called mobs, courts or troupes, which usually have 10 or more kangaroos in them. Living in mobs can provide protection for some of the weaker members of the group.  The size and stability of mobs vary between geographic regions,  with eastern Australia having larger and more stable aggregations than in arid areas farther west.  Larger aggregations display high amounts of interactions and complex social structures, comparable to that of ungulates.  One common behavior is nose touching and sniffing, which mostly occurs when an individual joins a group.  The kangaroo performing the sniffing gains much information from smell cues. This behavior enforces social cohesion without consequent aggression. During mutual sniffing, if one kangaroo is smaller, it will hold its body closer to the ground and its head will quiver, which serves as a possible form of submission.  Greetings between males and females are common, with larger males being the most involved in meeting females. Most other non-antagonistic behavior occurs between mothers and their young. Mother and young reinforce their bond through grooming. A mother will groom her young while it is suckling or after it is finished suckling.  A joey will nuzzle its mother's pouch if it wants access to it.
Sexual activity of kangaroos consists of consort pairs.  Oestrous females roam widely and attract the attention of males with conspicuous signals.  A male will monitor a female and follow her every movement. He sniffs her urine to see if she is in oestrus, a process exhibiting the flehmen response. The male will then proceed to approach her slowly to avoid alarming her.  If the female does not run away, the male will continue by licking, pawing, and scratching her, and copulation will follow.  After copulation is over, the male will move on to another female. Consort pairing may take several days and the copulation is also long. Thus, a consort pair is likely to attract the attention of a rival male.  As larger males are tending bonds with females near oestrus, smaller males will tend to females that are farther from oestrus.  Dominant males can avoid having to sort through females to determine their reproductive status by searching for tending bonds held by the largest male they can displace without a fight. 
Fighting has been described in all species of kangaroos. Fights between kangaroos can be brief or long and ritualised.  In highly competitive situations, such as males fighting for access to oestrous females or at limited drinking spots, the fights are brief.  Both sexes will fight for drinking spots, but long, ritualised fighting or "boxing" is largely done by males. Smaller males fight more often near females in oestrus, while the large males in consorts do not seem to get involved. Ritualised fights can arise suddenly when males are grazing together. However, most fights are preceded by two males scratching and grooming each other.  One or both of them will adopt a high standing posture, with one male issuing a challenge by grasping the other male's neck with its forepaw. Sometimes, the challenge will be declined. Large males often reject challenges by smaller males. During fighting, the combatants adopt a high standing posture and paw at each other's heads, shoulders and chests. They will also lock forearms and wrestle and push each other as well as balance on their tails to kick each other in the abdomen. 
Brief fights are similar, except there is no forearm locking. The losing combatant seems to use kicking more often, perhaps to parry the thrusts of the eventual winner. A winner is decided when a kangaroo breaks off the fight and retreats. Winners are able to push their opponents backwards or down to the ground. They also seem to grasp their opponents when they break contact and push them away.  The initiators of the fights are usually the winners. These fights may serve to establish dominance hierarchies among males, as winners of fights have been seen to displace their opponent from resting sites later in the day.  Dominant males may also pull grass to intimidate subordinate ones. 
Kangaroos have a few natural predators. The thylacine, considered by palaeontologists to have once been a major natural predator of the kangaroo, is now extinct. Other extinct predators included the marsupial lion, Megalania and Wonambi. However, with the arrival of humans in Australia at least 50,000 years ago and the introduction of the dingo about 5,000 years ago, kangaroos have had to adapt. Wedge-tailed eagles and other raptors usually eat kangaroo carrion but wedge-tailed eagles are known to hunt young or small kangaroos. Goannas and other carnivorous reptiles also pose a danger to smaller kangaroo species when other food sources are lacking.
Along with dingoes, introduced species such as foxes, feral cats, and both domestic and feral dogs, pose a threat to kangaroo populations. Kangaroos and wallabies are adept swimmers, and often flee into waterways if presented with the option. If pursued into the water, a large kangaroo may use its forepaws to hold the predator underwater so as to drown it.  Another defensive tactic described by witnesses is catching the attacking dog with the forepaws and disembowelling it with the hind legs.
Kangaroos have developed a number of adaptations to a dry, infertile country and highly variable climate. As with all marsupials, the young are born at a very early stage of development—after a gestation of 31–36 days. At this stage, only the forelimbs are somewhat developed, to allow the newborn to climb to the pouch and attach to a teat. In comparison, a human embryo at a similar stage of development would be about seven weeks old, and premature babies born at less than 23 weeks are usually not mature enough to survive. When the joey is born, it is about the size of a lima bean. The joey will usually stay in the pouch for about nine months (180–320 days for the Western Grey) before starting to leave the pouch for small periods of time. It is usually fed by its mother until reaching 18 months.
The female kangaroo is usually pregnant in permanence, except on the day she gives birth however, she has the ability to freeze the development of an embryo until the previous joey is able to leave the pouch. This is known as embryonic diapause, and will occur in times of drought and in areas with poor food sources. The composition of the milk produced by the mother varies according to the needs of the joey. In addition, the mother is able to produce two different kinds of milk simultaneously for the newborn and the older joey still in the pouch.
Unusually, during a dry period, males will not produce sperm, and females will conceive only if enough rain has fallen to produce a large quantity of green vegetation. 
Kangaroos and wallabies have large, elastic tendons in their hind legs. They store elastic strain energy in the tendons of their large hind legs, providing most of the energy required for each hop by the spring action of the tendons rather than by any muscular effort.  This is true in all animal species which have muscles connected to their skeletons through elastic elements such as tendons, but the effect is more pronounced in kangaroos.
There is also a link between the hopping action and breathing: as the feet leave the ground, air is expelled from the lungs bringing the feet forward ready for landing refills the lungs, providing further energy efficiency. Studies of kangaroos and wallabies have demonstrated, beyond the minimum energy expenditure required to hop at all, increased speed requires very little extra effort (much less than the same speed increase in, say, a horse, dog or human), and the extra energy is required to carry extra weight. For kangaroos, the key benefit of hopping is not speed to escape predators—the top speed of a kangaroo is no higher than that of a similarly sized quadruped, and the Australian native predators are in any case less fearsome than those of other countries—but economy: in an infertile country with highly variable weather patterns, the ability of a kangaroo to travel long distances at moderately high speed in search of food sources is crucial to survival.
New research has revealed that a kangaroo's tail acts as a third leg rather than just a balancing strut. Kangaroos have a unique three-stage walk where they plant their front legs and tail first, then push off their tail, followed lastly by the back legs. The propulsive force of the tail is equal to that of both the front and hind legs combined and performs as much work as what a human leg walking can at the same speed. 
A DNA sequencing project of the genome of a member of the kangaroo family, the tammar wallaby, was started in 2004. It was a collaboration between Australia (mainly funded by the State of Victoria) and the National Institutes of Health in the US.  The tammar's genome was fully sequenced in 2011.  The genome of a marsupial such as the kangaroo is of great interest to scientists studying comparative genomics, because marsupials are at an ideal degree of evolutionary divergence from humans: mice are too close and have not developed many different functions, while birds are genetically too remote. The dairy industry could also benefit from this project. 
Eye disease is rare but not new among kangaroos. The first official report of kangaroo blindness took place in 1994, in central New South Wales. The following year, reports of blind kangaroos appeared in Victoria and South Australia. By 1996, the disease had spread "across the desert to Western Australia".  Australian authorities were concerned the disease could spread to other livestock and possibly humans. Researchers at the Australian Animal Health Laboratories in Geelong detected a virus called the Wallal virus in two species of midges, believed to have been the carriers.   Veterinarians also discovered fewer than 3% of kangaroos exposed to the virus developed blindness. 
Reproduction and life cycle
Kangaroo reproduction is similar to that of opossums. The egg (still contained in the shell membrane, a few micrometres thick, and with only a small quantity of yolk within it) descends from the ovary into the uterus. There it is fertilised and quickly develops into a neonate. Even in the largest kangaroo species (the red kangaroo), the neonate emerges after only 33 days. Usually, only one young is born at a time. It is blind, hairless, and only a few centimetres long its hindlegs are mere stumps it instead uses its more developed forelegs to climb its way through the thick fur on its mother's abdomen into the pouch, which takes about three to five minutes. Once in the pouch, it fastens onto one of the four teats and starts to feed. Almost immediately, the mother's sexual cycle starts again. Another egg descends into the uterus and she becomes sexually receptive. Then, if she mates and a second egg is fertilised, its development is temporarily halted. This is known as embryonic diapause, and will occur in times of drought and in areas with poor food sources. Meanwhile, the neonate in the pouch grows rapidly. After about 190 days, the baby (joey) is sufficiently large and developed to make its full emergence out of the pouch, after sticking its head out for a few weeks until it eventually feels safe enough to fully emerge. From then on, it spends increasing time in the outside world and eventually, after about 235 days, it leaves the pouch for the last time.  The lifespan of kangaroos averages at six years in the wild  to in excess of 20 years in captivity, varying by the species.  Most individuals, however, do not reach maturity in the wild.  
The kangaroo has always been a very important animal for Aboriginal Australians, for its meat, hide, bone, and tendon. Kangaroo hides were also sometimes used for recreation in particular there are accounts of some tribes (Kurnai) using stuffed kangaroo scrotum as a ball for the traditional football game of marngrook. In addition, there were important Dreaming stories and ceremonies involving the kangaroo. Aherrenge is a current kangaroo dreaming site in the Northern Territory. 
Unlike many of the smaller macropods, kangaroos have fared well since European settlement. European settlers cut down forests to create vast grasslands for sheep and cattle grazing, added stock watering points in arid areas, and have substantially reduced the number of dingoes.
Kangaroos are shy and retiring by nature, and in normal circumstances present no threat to humans. In 2003, Lulu, an eastern grey which had been hand-reared, saved a farmer's life by alerting family members to his location when he was injured by a falling tree branch. She received the RSPCA Australia National Animal Valour Award on 19 May 2004.   
There are very few records of kangaroos attacking humans without provocation however, several such unprovoked attacks in 2004 spurred fears of a rabies-like disease possibly affecting the marsupials. The only reliably documented case of a fatality from a kangaroo attack occurred in New South Wales in 1936. A hunter was killed when he tried to rescue his two dogs from a heated fray. Other suggested causes for erratic and dangerous kangaroo behaviour include extreme thirst and hunger. In July 2011, a male red kangaroo attacked a 94-year-old woman in her own backyard as well as her son and two police officers responding to the situation. The kangaroo was capsicum sprayed (pepper sprayed) and later put down after the attack.  
Kangaroos-even those that are non domesticated-  can communicate with humans, according to a research study.  
Conflict with vehicles
Nine out of ten animal collisions in Australia involve kangaroos. A collision with a vehicle is capable of killing a kangaroo. Kangaroos dazzled by headlights or startled by engine noise often leap in front of cars. Since kangaroos in mid-bound can reach speeds of around 50 km/h (31 mph) and are relatively heavy, the force of impact can be severe. Small vehicles may be destroyed, while larger vehicles may suffer engine damage. The risk of harm or death to vehicle occupants is greatly increased if the windscreen is the point of impact. As a result, "kangaroo crossing" signs are commonplace in Australia.
Vehicles that frequent isolated roads, where roadside assistance may be scarce, are often fitted with "roo bars" to minimise damage caused by collision. Bonnet-mounted devices, designed to scare wildlife off the road with ultrasound and other methods, have been devised and marketed.
If a female is the victim of a collision, animal welfare groups ask that her pouch be checked for any surviving joey, in which case it may be removed to a wildlife sanctuary or veterinary surgeon for rehabilitation. Likewise, when an adult kangaroo is injured in a collision, a vet, the RSPCA Australia or the National Parks and Wildlife Service can be consulted for instructions on proper care. In New South Wales, rehabilitation of kangaroos is carried out by volunteers from WIRES. Council road signs often list phone numbers for callers to report injured animals.
The kangaroo is a recognisable symbol of Australia. The kangaroo and emu feature on the Australian coat of arms. Kangaroos have also been featured on coins, most notably the five kangaroos on the Australian one dollar coin. The Australian Made logo consists of a golden kangaroo in a green triangle to show that a product is grown or made in Australia.
Registered trademarks of early Australian companies using the kangaroo included Yung, Schollenberger & Co. Walla Walla Brand leather and skins (1890) Arnold V. Henn (1892) whose emblem showed a family of kangaroos playing with a skipping rope Robert Lascelles & Co. linked the speed of the animal with its velocipedes (1896) while some overseas manufacturers, like that of "The Kangaroo" safety matches (made in Japan) of the early 1900s, also adopted the symbol. Even today, Australia's national airline, Qantas, uses a bounding kangaroo for its logo. 
The kangaroo and wallaby feature predominantly in Australian sports teams names and mascots. Examples include the Australian national rugby league team (the Kangaroos) and the Australian national rugby union team (the Wallabies). In a nation-wide competition held in 1978 for the XII Commonwealth Games by the Games Australia Foundation Limited in 1982, Hugh Edwards' design was chosen a simplified form of six thick stripes arranged in pairs extending from along the edges of a triangular centre represent both the kangaroo in full flight, and a stylised "A" for Australia. 
Kangaroos are well represented in films, television, books, toys and souvenirs around the world. Skippy the Bush Kangaroo was a popular 1960s Australian children's television series about a fictional pet kangaroo. Kangaroos are featured in the Rolf Harris song "Tie Me Kangaroo Down, Sport" and several Christmas carols.
The kangaroo has been a source of food for indigenous Australians for tens of thousands of years. Kangaroo meat is high in protein and low in fat (about 2%). Kangaroo meat has a high concentration of conjugated linoleic acid (CLA) compared with other foods, and is a rich source of vitamins and minerals.  Low fat diets rich in CLA have been studied for their potential in reducing obesity and atherosclerosis.  
Kangaroo meat is sourced from wild animals and is seen by many as the best source of population control programs  as opposed to culling them as pests where carcasses are left in paddocks. Kangaroos are harvested by highly skilled, licensed shooters in accordance with a strict code of practice and are protected by state and federal legislation.  
What Makes a Kangaroo a Kangaroo
Two UConn biologists are part of an international team that sequenced a kangaroo genome for the first time.
A tammar wallaby at about two weeks of age. (Photo provided by Andrew Pask)
Ten years in the making, the sequencing of the tammar wallaby’s genome was published last week, and two UConn biologists in the College of Liberal Arts and Sciences are among the senior authors.
An adult tammar wallaby. (Andrew Pask/UConn Photo)
The tammar wallaby – a small kangaroo weighing about 30 pounds (the size of a large beagle) – is the first Australian marsupial to be sequenced. The genome sequence will provide scientists with new insights into the evolution of mammals, and into human reproduction and development. Because the kangaroo baby, known as a joey, develops outside the mother in a pouch, biologists can access information that could not be studied in utero on a human fetus.
One of the surprising findings in the sequencing was how many tammar wallaby genes are conserved, or look similar and seem to have similar roles, as human genes, even though humans and kangaroos diverged in their evolutionary path 150 million years ago, says Rachel O’Neill, professor of genetics and genomics in the Department of Molecular and Cell Biology, one of the principal investigators.
A tammar wallaby at about two weeks of age. (Photo provided by Andrew Pask)
That makes the tammar an “awesome model,” she says, for studying the evolution of genomes and chromosome structure. Tammars also make good models for understanding more about mammalian reproduction, says Andrew Pask, associate professor of genetics and genomics in the molecular and cell biology department, also a principal investigator.
From this project, scientists will be able to learn more about milk production (the wallaby produces two different types of milk from four nipples) how mammals develop and grow and how nutrition in early development can affect adult health outcomes, says Pask.
They will also learn more about what makes kangaroos hop, as they investigate further the HOX genes responsible for the tammar’s powerful hind legs.
The tammar wallaby was the first Australian marsupial discovered by Europeans. An excerpt from the Genome Biology paper records an early description:
“Their manner of procreation is exceeding strange and highly worth observing below the belly the female carries a pouch into which you may put your hand inside the pouch are her nipples, and we have found that the young ones grow up in this pouch with the nipples in their mouths. We have seen some young ones lying there, which were only the size of a bean, though at the same time perfectly proportioned so that it seems certain that they grow there out of the nipples of the mammae from which they draw their food, until they are grown up.” Francisco Pelseart, captain of the Dutch Indies ship Batavia, while shipwrecked off the coast of Western Australia, 1629.
The wallaby’s unusual method of reproduction, with the baby emerging at about one month – when it is a kidney-bean-sized fetus – and crawling up the mother’s tummy into the exterior pouch, also provides potential clues for the development of antibiotic-resistant drugs. The pouch is dirty and full of bacteria (“pouch jam” is the name given to the black material typically found in a kangaroo’s pouch), but the international team of researchers found novel proteins in the wallaby’s milk and secretions in the pouch jam that protect the baby, which has not yet developed an immune system.
“It’s a great model when you’re thinking about anti-microbials,” says O’Neill.
The wallaby baby also has a remarkable sense of smell – the study found it has as many as 1,500 olfactory receptor genes. These allow the newborn embryo to locate the mother’s pouch and the correct teat: only one of the four provides the milk it needs at the newborn stage.
The paper, with dozens of authors and researchers around the world associated with it – from Baylor College of Medicine in Texas, to Japan, to the UK – was published Aug. 19 in Genome Biology. The manager of the project and lead author is Professor Marilyn Renfree of the University of Melbourne, Australia. Four UConn graduate students are also authors on the paper – James Lindsay, Thomas Heider, William O’Hara, and Dawn Carone. Carone contributed to the work but is now a postdoctoral associate at the University of Massachusetts Medical Center.
Andrew Pask, right, with Asao Fujiyama, from the Japanese team, and Marilyn Renfree of the University of Melbourne, the lead author. (Photo provided by Andrew Pask)
The large consortium of researchers spread around the world used various technologies in sequencing the genome. For years, researchers in Australia have been mapping the tammar wallaby genome – both O’Neill and Pask, who overlapped as Ph.D. students at LaTrobe University in Australia – had worked on the map. But taking the bits and pieces of the map and placing them on “scaffolds,” or structures that allow scientists to assemble the full genome, began a decade ago.
UConn’s contribution toward the end of the project was critical, using a next-generation 454 sequencer purchased through a National Science Foundation grant to O’Neill and Linda Strausbaugh, professor of genetics and genomics, and director of the Center for Applied Genetics and Technology, where it is housed. Where older sequencers available at the start of the project could process 96 sequences overnight, the new generation can process billions of sequences in a few days.
Dealing with the resulting terabytes (1 terabyte is 1,000 gigabytes) of data then becomes a challenge, but a UConn supercomputer, the SGI Altix, nicknamed “The Dude,” which was obtained through a large equipment grant from the Provost’s Office a couple of years ago, made that possible.
Professor Rachel O’Neill. (Daniel Buttrey/UConn Photo)
“That supercomputer is gold,” says O’Neill. Even so, the UConn team worked “a lot of 100-hour work weeks” over the summer to validate the genome assembly, do all the annotations, and wrap up the project, while getting anxious calls from the researchers at other institutions.
O’Neill was surprised to find that the tammar wallaby’s genome was very compact – smaller than the human genome, even though researchers had expected it to be larger. That makes it ideal for studying, since the chromosomes are large and easily identifiable. The amount of non-coding RNA in the genome was also an exciting find, she says. Non-coding RNA is often involved in regulating protein pathways, but it doesn’t actually make a protein. It is the subject of a massive, emerging new field of study, she says, and the tammar wallaby findings will allow scientists to identify new non-coding RNA in humans.
The study also provides new information about a gene that is essential in human reproduction. It is essential for developing normal testes in all mammals, and the wallaby project will help determine what part of the development process the gene affects, Pask says.
About 26 scientific papers will be published as a result of the sequencing. Some are in progress others have already appeared. While the sequencing is considered definitive, it will be tweaked on a fine scale and updated as studies continue.
Says Pask, “I think we’ve created a really unique resource that, hopefully, the scientific community will now utilize.”
Return of the living thylacine
Few extinct animals capture the imagination like the Tassie tiger.
On the islands of the Dampier Archipelago, just off the coast of north-west Western Australia, giant piles of rusty, iron-rich boulders tumble into the brilliant turquoise waters of the Indian Ocean. Six thousand years ago, these islands were hilltops emerging from a wide coastal plain teeming with life. Aboriginal people recorded these animals by carving petroglyphs into the deep-red rocks.
Among the images are more than 20 thylacines, also known as Tasmanian tigers. These wolf-like, carnivorous marsupials carried their young in a pouch like kangaroos, sported tiger-like stripes on their backs and had jaws capable of an impressive 120-degree gape. They were once common across much of Australia and New Guinea.
The thylacine vanished from the Australian mainland about 3,000 years ago, probably as a result of a drying climate and the loss of dense vegetation. It maintained a toehold in forested Tasmania, only to be hunted to extinction by Europeans from the 1800s. The last known tiger died in Hobart Zoo in 1936.
Australia’s roll call of extinct species includes car-sized relatives of the wombat, lion-like predators and giant flightless birds. But the thylacine holds a special place in the public consciousness. Frequent ‘sightings’ and quests to find evidence of a living thylacine manifest hopes it might not really be lost.
In recent times, that hope has translated into possible ‘de-extinction’ through cloning.
Specimens from 450 thylacines are in museums around the world. Most are skin and bones, but 13 pouch young (joeys) were preserved in alcohol or formaldehyde. The Melbourne Museum has one so well-preserved that a team led by Andrew Pask at the University of Melbourne announced, in 2017, the successful sequencing of its entire genome. It is the most intact genome obtained for an extinct species.
The Melbourne joey’s own life might have been cut short, but its DNA may be a blueprint to resurrect the entire species. No one thinks it will happen soon but, as University of New South Wales palaeontologist and incurable ‘de-extinction’ champion Michael Archer puts it: “It’s a brave geneticist these days who’ll say what’s impossible in the next decade or two.”
Archer was perhaps the first person to dare to dream of cloning the thylacine. In 1996, when Dolly the sheep made history as the first mammal to be cloned, he declared doing the same with a thylacine was “a matter of not if but when”.
Dolly’s DNA originated from the mammary cell of an adult ewe. The cell’s nucleus, containing the DNA, was sucked out and transferred into a sheep egg whose own nucleus had been removed. The transferred nucleus ‘rebooted’ the egg’s development, creating a clone of the original ewe.
There is no chance of doing the same with a thylacine. Museum specimens can deliver thylacine DNA but not a viable nucleus or egg. So how do you clone something without these seemingly essential ingredients? Geneticist George Church, at Harvard University, has pioneered a way.
It is somewhat like the cloning strategy imagined in Jurassic Park. The fictional genetic engineers source dinosaur DNA from amber-preserved mosquitoes that dined on dinosaur blood. Gaps in the dinosaur DNA are filled by reptilian, bird or amphibian DNA.
In a similar manner, Church is heading an effort to clone the mammoth by using the DNA of its closest living relative, the Asian elephant, to fill in the missing bits of mammoth DNA.
What takes the scenario from fiction to reality is CRISPR. This latest tool in the genetic engineer’s kit is a set of enzymes used by bacteria to target and destroy foreign DNA. In 2015 genetic engineers co-opted CRISPR to target and alter DNA within living cells. Church’s goal is to ‘edit’ key tracts of elephant code to convert them into mammoth code, rather like turning a modern novel into medieval-era prose.
Church’s team have identified 1642 genes that differ between the species. In February 2017 Church announced the successful conversion of 45 of those genes. “We already know about the ones to do with small ears, subcutaneous fat, hair and blood,” he said, predicting a hybrid elephant-mammoth embryo “could happen in a couple of years”.
Once an edited facsimile of a mammoth nucleus has been created, it could be placed into an Asian elephant egg and then into a womb. Church is also looking into technologies for artificial wombs.
By the time Dolly the sheep was cloned, acquiring a thylacine’s DNA blueprint from a museum specimen was a tantalising possibility. Short sequences of DNA were already being extracted from mammoths and other long-dead specimens. Archer, then at the Australian Museum in Sydney, attempted to extract DNA from a thylacine in the museum’s collection – a six-month-old pup preserved in alcohol in 1886 – but the DNA was too fragmented to be useful.
Given those difficulties, Pask in Melbourne thought sequencing the thylacine genome would be impossible. His team focused instead on sequencing the genomes of living species – the platypus, tammar wallaby and dunnart. The goal was to compare their blueprints to placental mammals like us and trace how genes had evolved since these mammalian relatives had diverged.
Success at reading marsupial genomes emboldened the scientists to take another shot at the thylacine. In 2008 they reported a milestone: isolating a fragment of thylacine DNA so intact its code was still readable. A computer program recognised the DNA as the code for a gene – Col 2A1 – that directs the development of cartilage and bone. The researchers inserted the gene fragment into a mouse embryo, together with a chemical tag that made the gene glow blue wherever it was active. Blue patterns appeared in the embryo’s developing skeleton, meaning the code was good enough to work in a living creature.
The finding was encouraging. Even if scientists could never read a complete thylacine genome, they might glean important information from studying its genes – such as clues about how this cousin of the kangaroo evolved the body shape of a wolf.
Pask’s team spent 10 years taking samples from 40 thylacine specimens worldwide. “Most of the museum samples had really, really badly damaged DNA,” he says. He had almost given up hope when, in 2010, he came across a specimen on his doorstep. In a dusty cabinet in the bowels of the Melbourne Museum, preserved in a jar of ethanol, was a four-week-old joey taken from its dead mother’s pouch in 1909.
Pask’s team sampled its DNA. Unlike all the other specimens, the joey retained strings of DNA 1,000 letters in length – long enough to mean the entire three-billion-letter genome might be puzzled back together. Pask believes the DNA’s good condition might be due to the specimen missing the standard formalin fixation, instead going straight into ethanol.
The sample not only yielded long strings of DNA but plenty of them. Crucially that allowed Pask’s team to read every bit of the DNA sequence 60 times over using different strands. This enabled them to correct inevitable errors in the century-old material.
Imagine finding an old car manual with many pages missing. You would struggle to make use of it. But with 60 tattered incomplete copies you could probably compile a whole manual. Pask is similarly confident the blueprint is accurate enough to instruct the building of a thylacine. So too is Archer, who has lost none of his enthusiasm for bringing back extinct species. “It’s the roadmap for getting a thylacine back,” he says.
Cloning a thylacine will be more challenging than Church’s project to resurrect the mammoth using the Asian elephant. Their ancestors diverged just six million years ago, and they share about 99% of their genes. There is no equivalent species for the thylacine.
Pask suggests Western Australia’s numbat, whose genome he plans to sequence, might provide the best starting DNA blueprint. It is one of the thylacine’s closest living relatives, last sharing a common ancestor 30 million years ago. The diminutive termite-eating creature has stripes, but that’s where the similarity ends. Adult numbats are slightly bigger than a squirrel, whereas adult thylacines weighed about 30 kg. Despite this, Pask says as much as 95% of their DNA may be identical.
That still leaves an awful lot of numbat DNA to edit, making it an expensive proposition. But, as with all other genetic technologies, the costs are likely to fall fast. Pask will wait and watch while other de-extinction projects, particularly that of the mammoth and a similarly advanced effort to resurrect the passenger pigeon of North America, perfect the technologies.
The next series of steps are the most unpredictable: cloning an embryo, implanting it into a surrogate and gestating the pouch young.
Getting cloning to work is a major challenge. The techniques used to create Dolly are notoriously difficult to apply to different species. It was only in 2017 – more than 21 years after Dolly – that it was successfully replicated in a primate, with Chinese scientists producing two genetically identical long-tailed macaques.
Once researchers get a thylacine-recoded numbat egg to start developing into an embryo, gestating it is also far from straightforward. For humans and sheep, both placental mammals, the science of implanting embryos into a womb is well-established. Not so for marsupials, where implantation takes place much later. In placentals we know how to prime a mother with hormones to accept an embryo, but this knowledge is completely lacking in marsupials.
To master assisted reproduction in marsupials, Pask has turned to a different thylacine relative, the tiny mouse-like dunnart. They breed well in captivity and produce a litter of up to 20 young twice a year. Nevertheless, he says, “it will be a decade before we get a really good handle on a lot of this stuff in marsupials”.
Pregnancy is also a very different proposition to placental mammals. A marsupial still looks something like a foetus when it is born, typically two weeks after conception. About the size and shape of a pink jellybean, it must crawl up its mother’s abdomen and into her pouch, where it latches onto a teat to suckle. Its mother’s milk, like a placenta, changes its composition to guide most of the joey’s development.
This two-stage gestation does offer intriguing possibilities. A thylacine embryo might be gestated in the uterus of a smaller marsupial, and then transferred to the pouch of a larger one – perhaps a kangaroo. Cross-fostering is a well-established technique to help bolster the populations of endangered rock wallabies. In 2014 a rock wallaby successfully fostered a baby tree kangaroo in its pouch.
Another option is hand rearing, already widely employed for rescued kangaroos and also for Tasmanian devils captive bred to save the species from the devil facial tumour disease (DFTD) that has decimated wild populations.
Once a thylacine joey has weaned, at about nine months, there would be a new set of hurdles. Would it behave like a thylacine?
Little is known about natural behaviours, such as hunting or mating, as the thylacine was scarcely observed in the wild. “Many behaviours are innate,” Pask says, “but there would be a large subset that they probably learnt from individuals around them. Learned behaviour is more common in species that use complex decision-making to hunt prey, and preserved thylacine brains reveal a well-developed frontal cortex, indicating good memory and capacity to learn.”
We do know thylacines did not fare well in captivity. The Royal Zoological Society of NSW noted in 1939: “The thylacine does not take kindly to captivity, and rarely lives under such conditions for any length of time.” From 1850 to 1931, 224 were kept at zoos in cities including Washington DC, New York, Berlin and Paris. London Zoo had 20 over the years. Some died during journeys, others stopped eating and fell ill. None bred. While our skill at keeping animals has increased enormously, there is no guarantee resurrected thylacines would do better.
Understanding how a species might fare is important, says Beth Shapiro, an evolutionary biologist at the University of California, Santa Cruz, and author of How to Clone a Mammoth: The Science of De-Extinction (2015). “Populations living in captivity, possibly for decades, need not only to survive but must also learn how to live,” she says. “They need to learn how to feed and protect themselves, how to interact with others, how to avoid predation, how to choose a mate, and how to provide parental care.”
You also need a population with genetic variety, Shapiro says. Pask suggests it might be possible to edit such variation into the genome. “If you can get over the hurdle of making all those millions of edits to the genome to make it look like a thylacine in the first place,” he says, then introducing variability into immune system genes “is nothing”.
If all these hurdles can be overcome, the end goal of any de-extinction effort surely must be to reintroduce animals to the wild. One potential issue for some de-extinction candidates – appropriate habitat – is not a problem. Reserves cover about half of Tasmania today. “The habitat is the same, the animals they ate are still there,” says Archer. “There’s no question it could be put back into the bush of Tasmania.” There is also good reason to do so: “The thylacine was Tasmania’s key carnivore. Getting it back is about restabilising ecosystems currently under threat.”
That still may not be enough to convince everyone we should bring back thylacines. Many argue de-extinction projects take the focus away from the vital work to save other species from extinction.
“If you have the millions of dollars it would take to resurrect a species and choose to do that, you are making an ethical decision to bring one species back and let several others go extinct,” Canadian conservation biologist Joseph Bennett has said. “It would be one step forward, and three to eight steps back.”
Yet what is true today may not be true tomorrow. Pask agrees that, right now, resources should go to saving endangered marsupials. “If, however, in 10 to 15 years’ time it becomes relatively inexpensive, then I think it is definitely worth pursuing.” Having hunted the thylacine to extinction, he says, “we owe it to the species to bring it back”.
It may not be entirely thylacine, but one day, a century or so from now, a creature that looks and behaves like one might be found quietly slipping between piles of rusty rocks that bear its likeness, etched millennia ago.
John Pickrell is a Sydney-based science writer and the author of Weird Dinosaurs and Flying Dinosaurs.
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Marsupial Interesting Facts
What type of animal is a marsupial?
Any organism belonging to the infraclass Metatheria or Marsupialia of class Mammalia is termed as marsupials. Marsupials are basically characterized by the presence of pouches. The premature young marsupials live in the mother's pouch till they reach the desired age. The young marsupials crawl from the birth canal to the nipple of the mother located in the pouch and stay there feeding themselves to continue their development. Kangaroo, wombat, the extinct thylacine, koala, Cuscus, New Guinea, Tasmanian devil, and wallabies are some of the Australian marsupials that fall under this category of Biological classification.
What class of animal does a marsupial belong to?
Marsupials belong to the class Mammalia. Being mammals, marsupials give birth to the young of their own kind and possess mammary glands for the nourishment of their babies like other mammals. The young one's stay in the mother's pouch till they achieve the desired developmental level. Marsupials are found in different regions of Australia, North America, and South America.
How many marsupials are there in the world?
The total number of marsupials in the world account for a total of 334 species. Out of the assigned total, about 235 species of extant marsupials are present in Australia. While about 99 remaining species are vastly distributed in the habitats of South America and Central America. Some of the Australian species have become extinct.
Where does a marsupial live?
It is usually assumed that it is only Australia that provides home to the species of marsupials. However besides Australia which is definitely the dwellings of a large majority of these variety of organisms South, Central and North America are also the habitation ground of marsupials.
What is a marsupial's habitat?
The selection of habitat by any organisms is largely affected by various factors like their eating habits, structure, size and shape of their bodies, and breeding. As in case of marsupials the factors affecting inhabitations are no different. In fact these factors have led to adaptation of these organisms to a large variety of environments. Their habitats are distributed over a wide range of geographical distribution. For instance the Red Kangaroo are the occupants of grassland, desert habitats and scrubland, while the Long Tailed Planigale are found commonly in the black soil plains or clay-soiled woodlands. Some marsupials like moles are used to burrowing while others like flying squirrels are adapted to gliding through forest. Some marsupials are even found in aquatic habitats such as water opossum (Didelphis virginiana). Not much change is observed in the geographic distribution of the habitat of marsupials, even after millions of years later.
Who do marsupials live with?
Most masupials travel alone, except for kangaroos, who stay huddled together. They can be found in various regions spanning from South America to Australia.
How long does a marsupial live?
The average lifespan of marsupials ranges from as long as 1-26 years, the variations largely depend upon the different types of species categorized under Marsupials.
How do they reproduce?
Marsupials are viviparous mammalians that is they directly give birth to the young ones. In fact, reproduction in marsupials is one of the prominent and defining characteristics of these species. After fertilization, the progeny is delivered by the females at their embryonic stage (the period after implantation, during which all of the major organs and structures of the embryo are formed.) The offspring at this stage is delivered from the womb of the female to the pouch for further development and remain there almost unto the juvenile stage. In place of placenta the structure formed in the uterus of the female marsupials is yolk sac. Yolk sac, along with the combinations of secretions and hormones, provides the required nutrition and nourishment to the embryo for about three to seven weeks for these mammals. The short gestation period results in a small and immature embryo for these mammals. The developing embryo is provided with nourishment such as milk in the pouch. After reaching the juvenile stage the progeny tends to temporarily leave their pouch, however, occasionally return for warmth. This continues for these mammals until the juvenile marsupial is mature enough to survive on their own.
What is their conservation status?
The conservation status of marsupials ranges from Extinct to Least Concerned. However, in general a number of species listed under marsupials are often observed to be competing and struggling for survival.
For example, the Kangaroo, though not endangered are often threatened due to hunting or other environmental calamities. These true facts about the marsupial kangaroo are worrying for many people.
While in case of mountain pygmy opossum, these marsupials are listed as critically endangered according to the IUCN list. In fact, in 2008 their total population was calculated to be less than 2000 resulting due to factors like loss of habitat, predation by several other organisms, and environmental change.
Whereas in case of thylacine, extensive hunting, diseases and human intrusion into their environment has been quoted as factors responsible for the extinction of these carnivorous marsupials.
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Kangaroos are grazers, meaning they like to eat what they can find. Ideally, you could feed them something like brood mare pellet or goat tex, but if a kangaroo finds a bush close by, he will nibble on that too. Make sure that you keep water nearby, but if you give them access to something like a pond or other body of water, be careful that you aren't inviting bacteria to your kangaroos. If you give them a body of water, make sure there is gravel besides it since kangaroos don't like to get muddy.
Kangaroos mate and breed like many other animals. The difference is in how their young, called "Joeys," develop. After the sperm fertilizes the egg, it goes to the mother's uterus but instead of a placenta being formed, the yolk feeds the embryo until it is time to be born, about 28 days. After this, the mother grooms her pouch and the joey emerges from an opening at the base of her tail, called a cloaca. At this point the fetus is pink and about the size of a lima bean. The only part of his body that is more developed are his arms, which help him, along with instinct, climb up his mother's belly and find his way into her pouch. The joey still has no jaws to feed, so the mother's nipple grows into the young's mouth and very gently forces milk into its mouth. When he is stronger, he will be able to detach and feed on his own until he has a light coat of fur and leaves the pouch. The second pregnancy occurs much differently since the mother mates again almost immediately, but the fertilized egg will stop growing when it has reached about 100 cells, then it stops growing until the other joey leaves the pouch, at which time the second one will replace him.
Owning a kangaroo as a pet is an interesting and exciting venture, but they do have needs, and it is only after you seriously consider those needs and how to effectively fill them should you get a kangaroo as a pet.
How do Kangaroo mothers deliver the correct kind of milk to the correct joey? - Biology
All mammals share certain characteristics: milk-producing mammary glands, three bones in the middle ear and one in the lower jaw, fur or hair, heterodont dentition (different kinds of teeth), and both sebaceous (oil-producing) and sudoriferous (sweat) glands. What about placenta formation during embryonic development? This is a characteristic of humans, as we’ll explore in Chapter 3 of MCAT Biology Review, but there are two groups of mammals that birth their young a bit differently: prototherians and metatherians.
Prototherians (monotremes), which include the duckbilled platypus and echidna (spiny anteater), encase their developing embryos within hard-shelled amniotic eggs and lay them to be hatched, like reptiles. This method of development is referred to as oviparity. Metatherians (marsupials) include koalas and kangaroos. A typical metatherian fetus (joey) undergoes some development in its mother’s uterus and then climbs its way out of the birth canal and into her marsupium, or pouch. It might seem a bit strange that something as essential as reproduction could be so different between mammalian species, but the truth is that there is a wide variety of reproductive mechanisms in nature. Many organisms reproduce without a sexual partner. Others can reproduce sexually or asexually depending on environmental conditions. In the Chapter 1 of MCAT Biology Review, we explored how bacteria and viruses reproduce. In this chapter, we’ll explore how eukaryotic cells reproduce, as well as the male and female reproductive systems.
2.1 The Cell Cycle and Mitosis
In animals, autosomal cells are said to be diploid (2n), which means that they contain two copies of each chromosome. Germ cells, on the other hand, are haploid (n), containing only one copy of each chromosome. In humans, these numbers are 46 and 23, respectively we inherit 23 chromosomes from each parent. Eukaryotic cells replicate through the cell cycle, a specific series of phases during which a cell grows, synthesizes DNA, and divides. Derangements of the cell cycle can lead to unchecked cell division and may be responsible for the formation of cancer.
The cell cycle, shown in Figure 2.1, is a perennial MCAT favorite. For actively dividing cells, the cell cycle consists of four stages: G1, S, G2, and M. The first three stages (G1, S, and G2) are known collectively as interphase. Interphase is the longest part of the cell cycle even actively dividing cells spend about 90 percent of their time in interphase. Cells that do not divide spend all of their time in an offshoot of G1 called G0. During the G0 stage, the cell is simply living and serving its function, without any preparation for division.
Figure 2.1. The Cell Cycle
During interphase, individual chromosomes are not visible with light microscopy. Rather, they are in a less condensed form known as chromatin. This is because the DNA must be available to RNA polymerase so that genes can be transcribed. During mitosis, however, it is preferable to condense the DNA into tightly coiled chromosomes to avoid losing any genetic material during cell division.
G1 Stage: Presynthetic Gap
During the G1 stage, cells create organelles for energy and protein production (mitochondria, ribosomes, and endoplasmic reticulum), while also increasing their size. In addition, passage into the S (synthesis) stage is governed by a restriction point. Certain criteria, such as containing the proper complement of DNA, must be met for the cell to pass the restriction point and enter the synthesis stage.
S Stage: Synthesis of DNA
During the S stage, the cell replicates its genetic material so that each daughter cell will have identical copies. After replication, each chromosome consists of two identical chromatids that are bound together at a specialized region known as the centromere, as shown in Figure 2.2. Note that the ploidy of the cell does not change even though the number of chromatids has doubled. In other words, humans in this stage still only have 46 chromosomes, even though 92 chromatids are present. Cells entering G2 have twice as much DNA as cells in G1.
Figure 2.2. Chromosome Replication A single chromatid replicates to form two sister chromatids.
Each chromatid is composed of a complete, double-stranded molecule of DNA. Sister chromatids are identical copies of each other. The term chromosome may be used to refer to either a single chromatid before S phase or the pair of chromatids attached at the centromere after S phase.
G2 Stage: Postsynthetic Gap
During the G2 stage, the cell passes through another quality control checkpoint. DNA has already been duplicated, and the cell checks to ensure that there are enough organelles and cytoplasm to divide between two daughter cells. Furthermore, the cell checks to make sure that DNA replication proceeded correctly to avoid passing on an error to daughter cells that may further replicate the error in their progeny.
M Stage: Mitosis
The M stage consists of mitosis itself along with cytokinesis. Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. The features of each phase will be discussed in the next section. Cytokinesis is the splitting of the cytoplasm and organelles into two daughter cells.
In autosomal cells, division results in two genetically identical daughter cells. In germ cells, the daughter cells are not equivalent.
CONTROL OF THE CELL CYCLE
The cell cycle is controlled by checkpoints, most notably between the G1 and S phase, and the G2 and M phase. At the G1/S checkpoint, the cell determines if the DNA is in good enough condition for synthesis. As mentioned above, this checkpoint is also known as the restriction point. If there has been damage to the DNA, the cell cycle goes into arrest until the DNA has been repaired. The main protein in control of this is known as p53.
At the G2/M checkpoint, the cell is mainly concerned with ensuring that the cell has achieved adequate size and the organelles have been properly replicated to support two daughter cells. p53 also plays a role in the G2/M checkpoint.
The molecules responsible for the cell cycle are known as cyclins and cyclin-dependent kinases (CDK). In order to be activated, CDKs require the presence of the right cyclins. During the cell cycle, concentrations of the various cyclins increase and decrease during specific stages. These cyclins bind to CDKs, creating an activated CDK–cyclin complex. This complex can then phosphorylate transcription factors. Transcription factors then promote transcription of genes required for the next stage of the cell cycle.
Cell cycle control is essential to ensure that cells that are damaged or inadequately sized do not divide. When cell cycle control becomes deranged, and damaged cells are allowed to undergo mitosis, cancer may result. One of the most common mutations found in cancer is mutation of the gene that produces p53, called TP53. When this gene is mutated, the cell cycle is not stopped to repair damaged DNA. This allows for mutations to accumulate, eventually resulting in a cancerous cell that divides continuously and without regard to the quality or quantity of the new cells produced. Often, cancer cells undergo rapid cell division, creating tumors. Eventually, if the cell begins to produce the right factors (such as proteases that can digest basement membranes or factors that encourage blood vessel formation), the damaged cells are then able to reach other tissues. This may include both local invasion as well as distant spread of cancerous cells through the bloodstream or lymphatic systems. This latter result is known as metastasis.
Cancer-causing genes can often be classified into oncogenes (genes that, when mutated, actively promote cell division) and tumor suppressor genes (genes that, when mutated, lose their ability to regulate or pause the cell cycle). Different cancer types are often associated with specific mutations in either oncogenes or tumor suppressor genes, or both. The biochemistry of these genes is discussed in Chapter 6 of MCAT Biochemistry Review.
Mitosis, shown in Figure 2.3, is the process by which two identical daughter cells are created from a single cell. Mitosis consists of four distinct phases&mdashprophase, metaphase, anaphase, and telophase&mdashand occurs in somatic cells, or cells that are not involved in sexual reproduction.
Figure 2.3. Mitosis Mitosis results in two identical daughter cells.
Prophase is the first phase in mitosis. The first step in prophase involves condensation of the chromatin into chromosomes. Also, the centriole pairs separate and move toward opposite poles of the cell. These paired cylindrical organelles, shown in Figure 2.4, are located outside the nucleus in a region known as the centrosome and are responsible for the correct division of DNA. Once the centrioles migrate to opposite poles of the cell, they begin to form spindle fibers, which are made of microtubules. Each of the fibers radiates outward from the centrioles. Some microtubules form asters that anchor the centrioles to the cell membrane. Others extend toward the middle of the cell. The nuclear membrane dissolves during prophase, allowing these spindle fibers to contact the chromosomes. The nucleoli become less distinct, and may disappear completely.Kinetochores appear at the centromere. Kinetochores are protein structures located on the centromeres that serve as attachment points for specific fibers of the spindle apparatus appropriately called kinetochore fibers.
Figure 2.4. The Centrosome Each centrosome contains two tubulin-based centrioles responsible for proper movement of the chromosomes during mitosis.
·&emspProphase&mdashchromosomes condense, spindle forms
·&emspAnaphase&mdashsister chromatids separate
·&emspTelophase&mdashnew nuclear membranes form
In metaphase, the centriole pairs are now at opposite ends of the cell. The kinetochore fibers interact with the fibers of the spindle apparatus to align the chromosomes at the metaphase plate (equatorial plate), which is equidistant between the two poles of the cell.
During anaphase, the centromeres split so that each chromatid has its own distinct centromere, thus allowing the sister chromatids to separate. The sister chromatids are pulled toward the opposite poles of the cell by the shortening of the kinetochore fibers.
Telophase and Cytokinesis
Telophase is essentially the reverse of prophase. The spindle apparatus disappears. A nuclear membrane reforms around each set of chromosomes, and the nucleoli reappear. The chromosomes uncoil, resuming their interphase form. Each of the two new nuclei has received a complete copy of the genome identical to the original genome and to each other.
At the end of telophase, cytokinesis is the separation of the cytoplasm and organelles so that each daughter cell has sufficient supplies to survive on its own. Each cell undergoes a finite number of divisions before programmed death for human somatic cells, this is usually between 20 and 50. After that, the cell can no longer divide continuously.
MCAT Concept Check 2.1:
Before you move on, assess your understanding of the material with these questions.
1. What are the five stages of the cell cycle? What happens in each stage?