Which species has the highest proportion of homosexual individuals?

Are there more homosexual individuals in ducks, humans, or some other species?

Homosexual behavior have been observed more than 1500 species (according to Museum of Natural History of the University of Oslo). A full comparison between all known species is far beyond the scope of this site, but below I discuss three species of interest. Also, the details of what counts as an homosexual behavior, and also whether you compute incidence or prevalence makes the whole discussion semantically complicated.

  • According to Simon Levay and Aldo Polani, in domesticated sheep, 10% of the rams (males) prefer mating with other rams than with ewes (females).

  • About 25% of black swans are homosexuals (Braithwaite, 1981).

  • In lions, 8% of the mounting is in between two males (Bagemihl 2000)

In humans, around 3%-5% of the surveyed people claim being homosexual or bi-sexual (see wiki > demographics of sexual orientation#Modern_survey_results).

Homosexuality in the United States - Statistics & Facts

These shifting attitudes can be reflected in American politics. Since taking office, the Biden administration has prioritized the expansion of LGBT rights and protections, many of which had been rolled back during the preceding four years. In recent decades, the most significant steps forwards for the LGBT communities were taken under the Obama administration. Although originally opposing gay marriage, President Obama changed his stance to support same-sex marriage during his re-election campaign in 2012 while a majority of surveyed voters said this decision did not affect their opinion of him, almost half of Republican respondents stated their opinion of him had become less favorable. Today, 74 percent of young American adult support same-sex marriage.

Are there any homosexual animals?

During the winter mating season, competition is fierce for access to female Japanese macaques. But it's not for the reason you might think. Males don't just have to compete with other males for access to females: they have to compete with females too.

That's because in some populations, homosexual behaviour among females is not only common, it's the norm. One female will mount another, then stimulate her genitals by rubbing them against the other female. Some hold onto each other with their limbs using a "double foot clasp mount", while others sit on top of their mates in a sort of jockey-style position, says Paul Vasey of the University of Lethbridge in Alberta, Canada, who has been studying these macaques for over 20 years.

To our eyes these encounters look startlingly intimate. The females stare into each other's eyes while mating, which macaques hardly ever do outside of sexual contexts. The pairings can even last a whole week, mounting hundreds of times. When they're not mating, the females stay close together to sleep and groom, and defend each other from possible rivals.

That many humans are homosexual is well known but we also know the behaviour is extremely common across the animal kingdom, from insects to mammals. So what's really going on? Can these animals actually be called homosexual?

Animals have been observed engaging in same-sex matings for decades. But for most of that time, the documented cases were largely seen as anomalies or curiosities.

The turning point was Bruce Bagemihl's 1999 book Biological Exuberance, which outlined so many examples, from so many different species, that the topic moved to centre stage. Since then, scientists have studied these behaviours systematically.

On the face of it, homosexual behaviour by animals looks like a really bad idea

Despite Bagemihl's roster of examples, homosexual behaviour still seems to be a rarity. We have probably missed some examples, as in many species males and females look pretty much alike. But while hundreds of species have been documented doing it on isolated occasions, only a handful have made it a habitual part of their lives, says Vasey.

To many, that isn't surprising. On the face of it, homosexual behaviour by animals looks like a really bad idea. Darwin's theory of evolution by natural selection implies that genes have to get themselves passed on to the next generation, or they will die out. Any genes that make an animal more likely to engage in same-sex matings would be less likely to get passed on than genes pushing for heterosexual pairings, so homosexuality ought to quickly die out.

But that evidently isn't what's happening. For some animals, homosexual behaviour isn't an occasional event &ndash which we might put down to simple mistakes &ndash but a regular thing.

Take the macaques. When Vasey first observed the females mounting each other, he was "blown away" by how often they did it.

The females were simply seeking sexual pleasure

"So many females of the group are engaging in this behaviour and there are males sitting around twiddling their thumbs," he says. "There's got to be a reason for this. There is no way the behaviour can be evolutionarily irrelevant."

Vasey's team has found that females use a greater variety of positions and movements than males do. In a 2006 study, they proposed that the females were simply seeking sexual pleasure, and were using different movements to maximise the genital sensations. "She can do so in a homosexual context just as easily as in a heterosexual context, so the behaviour spills over," says Vasey.

But for all the homosexual pairings the females indulge in, Vasey is clear that they are not truly homosexual. A female may engage in female-female mounting, but that doesn't mean she isn't interested in males. Females often mount males, apparently to encourage them to mate more. Once they had evolved this behaviour, it was easy for them to apply it to other females as well.

In some cases, there is a fairly straightforward evolutionary reason why animals engage in homosexual behaviour.

Take male fruit flies. In their first 30 minutes of life, they will try to copulate with any other fly, male or female. After a while, they learn to recognise the smell of virgin females, and focus on them.

The males are using homosexual behaviour as a roundabout way to fertilise more females

This trial-and-error approach may look rather inefficient, but actually it is a good strategy, says David Featherstone of the University of Illinois at Chicago, US. In the wild, flies in different habitats may have slightly different pheromone blends. "A male could be passing up an opportunity to have viable offspring if they are hardwired to only go for a certain smell," says Featherstone.

Male flour beetles use a distinctly sneaky trick. They often mount each other, and go so far as depositing sperm. If the male carrying this sperm mates with a female later, the sperm might get transferred &ndash so the male who produced it has fertilised a female without having to court her.

In both cases, the males are using homosexual behaviour as a roundabout way to fertilise more females. So it's clear how these behaviours could be favoured by evolution. But it's also clear that fruit flies and flour beetles are a long way from strictly homosexual.

Other animals really do seem to be lifelong homosexuals. One such species is the Laysan albatross, which nests in Hawaii, US.

Among these huge birds, pairs are usually "married" for life. It takes two parents working together to rear a chick successfully, and doing so repeatedly means that the parents can hone their skills together. But in one population on the island of Oahu, 31% of the pairings are made up of two unrelated females. What's more, they rear chicks, fathered by males that are already in a committed pair but which sneak matings with one or both of the females. Like male-female pairs, these female-female pairs can only rear one chick in a season.

Same-sex coupling is a response to a shortage of males

The female-female pairs are not as good at rearing chicks as female-male pairs, but are better than females that go it alone. So it makes sense for a female to pair up with another female, says Marlene Zuk of the University of Minnesota in Saint Paul, US. If she did not, she might manage to mate but would struggle to incubate her egg and find food. And once a female forms a pair-bond, the species' tendency towards monogamy means it becomes life-long.

There is even a subtle advantage for the females. The system means that they can get their eggs fertilised by the fittest male of the group, and pass his desirable traits on to her offspring, even if he is already paired with another female.

But once again, the female albatrosses are not inherently homosexual. The Oahu population has a surplus of females as a result of immigration, so some females cannot find males to pair with. Studies of other birds suggest that same-sex coupling is a response to a shortage of males, and is much rarer if the sex ratio is equal. In other words, the female Laysan albatrosses probably wouldn't choose to pair with other females if there were enough males to go round.

So perhaps we've been looking in the wrong place for examples of homosexual animals. Given that human beings are known to be homosexual, maybe we should look at our closest relatives, the apes.

Bonobo sex also cements social bonds

Bonobos are often described as our "over-sexed" relatives. They engage in an enormous amount of sex, so much so that it's often referred to as a "bonobo handshake", and that includes homosexual behaviour among both males and females.

Like the macaques, they seem to enjoy it, according to Frans de Waal of Emory University in Atlanta, Georgia, US. Writing in Scientific American in 1995, he described pairs of female bonobos rubbing their genitals together, and "emitting grins and squeals that probably reflect orgasmic experiences".

But bonobo sex also plays a deeper role: it cements social bonds. Junior bonobos may use sex to bond with more dominant group members, allowing them to climb the social ladder. Males that have had a fight sometimes perform genital-to-genital touching, known as "penis fencing", as a way of reducing tension. More rarely, they also kiss, perform fellatio and massage each other's genitals. Even the young comfort each other with hugs and sex.

Bonobos show that "sexual behaviour" can be about more than reproduction, says Zuk, and that includes homosexual behaviour. "There's a whole range of behaviours that fit in well with how evolution happens that now include homosexual behaviour." In fact, female bonobos still have sex when they are outside their reproductive period and can't get pregnant.

They don't show a consistent sexual orientation

Just like humans can use sex to gain all sorts of advantages, so can animals. For instance, among bottlenose dolphins, both females and males display homosexual behaviour. This helps members of the group form strong social bonds. But ultimately, all concerned will go on to have offspring with the opposite sex.

All these species might be best described as "bisexual". Like the Japanese macaques and the fruit flies, they switch easily between same-sex and opposite-sex behaviours. They don't show a consistent sexual orientation.

Only two species have been observed showing a same-sex preference for life, even when partners of the opposite sex are available. One is, of course, humans. The other is domestic sheep.

In flocks of sheep, up to 8% of the males prefer other males even when fertile females are around. In 1994, neuroscientists found that these males had slightly different brains to the rest. A part of their brain called the hypothalamus, which is known to control the release of sex hormones, was smaller in the homosexual males than in the heterosexual males.

That is in line with a much-discussed study by the neuroscientist Simon LeVay. In 1991, he described a similar difference in brain structure between gay and straight men.

How could this preference for other males be passed on to offspring?

This seems quite different from all the other cases of homosexual behaviour, because it is hard to see how it could possibly benefit the males. How could this preference for other males be passed on to offspring, if the males do not reproduce?

The short answer is that it probably doesn't benefit the homosexual males themselves, but it might benefit their relatives, who may well carry the same genes and could pass them on. For that to happen, the genes that make some males homosexual would have to have another, useful effect in other sheep.

LeVay suggests that the same gene that promotes homosexual behaviour in male sheep could also make females more fertile, or increase their desire to mate. The female siblings of homosexual sheep could even produce more offspring than average. "If these genes are having such a beneficial effect in females, they outweigh the effect in males and then the gene is going to persist," says LeVay.

While male sheep do show lifelong homosexual preferences, this has only been seen in domesticated sheep. It's not clear whether the same thing happens in wild sheep, and if LeVay's explanation is right it probably doesn't. Domestic sheep have been carefully bred by farmers to produce females that reproduce as often as possible, which might have given rise to the homosexual males.

So LeVay and Vasey still say that humans are the only documented case of "true" homosexuality in wild animals. "It is not the case that you have lesbian bonobos or gay male bonobos," says Vasey. "What's been described is that many animals are happy to engage in sex with partners of either sex."

Homosexual behaviour doesn't challenge Darwin's ideas

The funny thing is, biologists should have predicted this. When Darwin was developing his theory of natural selection, one of the things that inspired him was the realisation that animals tend to have far more offspring than they seem to need. In theory a pair of animals need only have two offspring to replace themselves, but in practice they have as many as they possibly can &ndash because so many of their young will die before they manage to reproduce.

It seems obvious that this built-in need to keep reproducing would manifest itself in a powerful sex drive, one that might well spill over into mating while females are infertile, or same-sex matings. Victorian scientists saw animals having more offspring than seemed necessary: today we see animals having more sex than seems necessary.

"Homosexual behaviour doesn't challenge Darwin's ideas," says Zuk. Instead there are many ways it can evolve and be beneficial.

We may never find a wild animal that is strictly homosexual in the way some humans are. But we can expect to find many more animals that don't conform to traditional categories of sexual orientation. They are using sex to satisfy all sorts of needs, from simple pleasure to social advancement, and that means being flexible.

The Evolutionary Mystery of Homosexuality

C ritics claim that evolutionary biology is, at best, guesswork. The reality is otherwise. Evolutionists have nailed down how an enormous number of previously unexplained phenomena—in anatomy, physiology, embryology, behavior—have evolved. There are still mysteries, however, and one of the most prominent is the origins of homosexuality.

The mystery is simple enough. Its solution, however, has thus far eluded our best scientific minds.

The sine qua non for any trait to have evolved is for it to correlate positively with reproductive success, or, more precisely, with success in projecting genes relevant to that trait into the future. So, if homosexuality is in any sense a product of evolution—and it clearly is, for reasons to be explained—then genetic factors associated with same-sex preference must enjoy some sort of reproductive advantage. The problem should be obvious: If homosexuals reproduce less than heterosexuals—and they do—then why has natural selection not operated against it?

The paradox of homosexuality is especially pronounced for individuals whose homosexual preference is exclusive that is, who have no inclination toward heterosexuality. But the mystery persists even for those who are bisexual, since it is mathematically provable that even a tiny difference in reproductive outcome can drive substantial evolutionary change.

J.B.S. Haldane, one of the giants of evolutionary theory, imagined two alternative genes, one initially found in 99.9 percent of a population and the other in just 0.1 percent. He then calculated that if the rare gene had merely a 1-percent advantage (it produced 101 descendants each generation to the abundant gene’s 100), in just 4,000 generations—a mere instant in evolutionary terms—the situation would be reversed, with the formerly rare gene occurring in 99.9 percent of the population’s genetic pool. Such is the power of compound interest, acting via natural selection.

For our purposes, the implication is significant: Anything that diminishes, even slightly, the reproductive performance of any gene should (in evolutionary terms) be vigorously selected against. And homosexuality certainly seems like one of those things. Gay men, for example, have children at about 20 percent of the rate of heterosexual men. I haven’t seen reliable data for lesbians, but it seems likely that a similar pattern exists. And it seems more than likely that someone who is bisexual would have a lower reproductive output than someone whose romantic time and effort were devoted exclusively to the opposite sex.

Across cultures, the proportion of the population who are homosexual is roughly the same. What maintains the genetic propensity for the trait?

Nor can we solve the mystery by arguing that homosexuality is a “learned” behavior. That ship has sailed, and the consensus among scientists is that same-sex preference is rooted in our biology. Some of the evidence comes from the widespread distribution of homosexuality among animals in the wild. Moreover, witness its high and persistent cross-cultural existence in Homo sapiens.

In the early 1990s, a geneticist at the National Institutes of Health led a study that reported the existence of a specific allele, Xq28, located on the X chromosome, that predicted gay-versus-straight sexual orientation in men. Subsequent research has been confusing, showing that the situation is at least considerably more complicated than had been hoped by some (notably, most gay-rights advocates) and feared by others (who insist that sexual orientation is entirely a “lifestyle choice”).

Some studies have failed to confirm any role for Xq28 in gay behavior, while others have been supportive of the original research. It is also increasingly clear that whatever its impact on male homosexuality, this particular gene does not relate to lesbianism. Moreover, other research strongly suggests that there are regions on autosomal (nonsex) chromosomes, too, that influence sexual orientation in people.

So a reasonable summary is that, when it comes to male homosexuality, there is almost certainly a direct influence, although probably not strict control, by one or more alleles. Ditto for female homosexuality, although the genetic mechanism(s), and almost certainly the relevant genes themselves, differ between the sexes.

Beyond the suggestive but inconclusive search for DNA specific to sexual orientation, other genetic evidence has emerged. A welter of data on siblings and twins show that the role of genes in homosexual orientation is complicated and far from fully understood—but real. Among noteworthy findings: The concordance of homosexuality for adopted (hence genetically unrelated) siblings is lower than that for biological siblings, which in turn is lower than that for fraternal (nonidentical) twins, which is lower than that for identical twins.

Gay-lesbian differences in those outcomes further support the idea that the genetic influence upon homosexuality differs somewhat, somehow, between women and men. Other studies confirm that the tendency to be lesbian or gay has a substantial chance of being inherited.

Consider, too, that across cultures, the proportion of the population that is homosexual is roughly the same. We are left with an undeniable evolutionary puzzle: What maintains the underlying genetic propensity for homosexuality, whatever its specific manifestations? Unlike most mystery stories, in which the case is typically solved at the finish, this one has no ending: We simply do not know.

H ere are some promising possibilities.

Kin selection. Scientists speculate that altruism may be maintained if the genes producing it help a genetic relative and hence give an advantage to those altruistic genes. The same could be true of homosexuality. Insofar as homosexuals have been freed from investing time and energy in their own reproduction, perhaps they are able to help their relatives rear offspring, to the ultimate evolutionary benefit of any homosexuality-promoting genes present in those children.

Unfortunately, available evidence does not show that homosexuals spend an especially large amount of time helping their relatives, or even interacting with them. Not so fast, however: Those results are based on surveys they reveal opinions and attitudes rather than actual behavior. Moreover, they involve modern industrialized societies, which presumably are not especially representative of humanity’s ancestral situations.

Some recent research has focused on male homosexuals among a more traditional population on Samoa. Known as fa’afafine, these men do not reproduce, are fully accepted into Samoan society in general and into their kin-based families in particular, and lavish attention upon their nieces and nephews—with whom they share, on average, 25 percent of their genes.

Social prestige. Since there is some anthropological evidence that in preindustrial societies homosexual men are more than randomly likely to become priests or shamans, perhaps the additional social prestige conveyed to their heterosexual relatives might give a reproductive boost to those relatives, and thereby to any shared genes carrying a predisposition toward homosexuality. An appealing idea, but once again, sadly lacking in empirical support.

Group selection. Although the great majority of biologists maintain that natural selection occurs at the level of individuals and their genes rather than groups, it is at least possible that human beings are an exception that groups containing homosexuals might have done better than groups composed entirely of straights. It has recently been argued, most cogently by the anthropologist Sarah B. Hrdy, that for much of human evolutionary history, child-rearing was not the province of parents (especially mothers) alone. Rather, our ancestors engaged in a great deal of “allomothering,” whereby nonparents—other genetic relatives in particular—pitched in. It makes sense that such a system would have been derived by Homo sapiens, of all primate species the one whose infants are born the most helpless and require the largest investment of effort. If sufficient numbers of those assistants had been gay, their groups may have benefited disproportionately.

Alternatively, if some human ancestors with a same-sex preference reproduced less (or even not at all), that, in itself, could have freed up resources for their straight relatives, without necessarily requiring that the former were especially collaborative. Other group-level models have also been proposed, focusing on social interaction rather than resource exploitation: Homosexuality might correlate with greater sociality and social cooperation similarly, it might deter violent competition for females.

Balanced polymorphisms. Perhaps a genetic predisposition for homosexuality, even if a fitness liability, somehow conveys a compensating benefit when combined with one or more other genes, as with the famous case of sickle-cell disease, in which the gene causing the disease also helped prevent malaria in regions where it was epidemic. Although no precise candidate genes have been identified for homosexuality, the possibility cannot be excluded.

Sexually antagonistic selection. What if one or more genes that predispose toward homosexuality (and with it, reduced reproductive output) in one sex actually work in the opposite manner in the other sex? I prefer the phrase “sexually complementary selection": A fitness detriment when genes exist in one sex—say, gay males—could be more than compensated for by a fitness enhancement when they exist in another sex.

One study has found that female relatives of gay men have more children than do those of straight men. This suggests that genes for homosexuality, although disadvantageous for gay men and their male relatives, could have a reproductive benefit among straight women.

To my knowledge, however, there is as yet no evidence for a reciprocal influence, whereby the male relatives of female homosexuals have a higher reproductive fitness than do male relatives of heterosexual women. And perhaps there never will be, given the accumulating evidence that female homosexuality and male homosexuality may be genetically underwritten in different ways.

A nonadaptive byproduct. Homosexual behavior might be neither adaptive nor maladaptive, but simply nonadaptive. That is, it might not have been selected for but persists instead as a byproduct of traits that presumably have been directly favored, such as yearning to form a pair bond, seeking emotional or physical gratification, etc. As to why such an inclination would exist at all—why human connections are perceived as pleasurable—the answer may well be that historically (and prehistorically), it has often been in the context of a continuing pair-bond that individuals were most likely to reproduce successfully.

There are lots of other hypotheses for the evolution of homosexuality, although they are not the “infinite cornucopia” that Leszek Kolakowski postulated could be argued for any given position. At this point, we know enough to know that we have a real mystery: Homosexuality does have biological roots, and the question is how the biological mechanism developed over evolutionary time.

Another question (also yet unanswered) is why should we bother to find out.

There is a chilling moment at the end of Ray Bradbury’s The Martian Chronicles, when a human family, having escaped to Mars to avoid impending nuclear war, looks eagerly into the “canals” of their new planetary home, expecting to see Martians. They do: their own reflections.

It wasn’t terribly long ago that reputable astronomers entertained the notion that there really were canals on Mars. From our current vantage, that is clearly fantasy. And yet, in important ways, we are still strangers to ourselves, often surprised when we glimpse our own images. Like Bradbury’s fictional family, we, too, could come to see humanity, reflected in all its wonderful diversity, and know ourselves at last for precisely what we are, if we simply looked hard enough.

Unlike the United States military, with its defunct “don’t ask, don’t tell” policy, many reputable investigators are therefore asking . not who is homosexual, but why are there homosexuals. We can be confident that eventually, nature will tell.

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When Homosexuality Stopped Being a Mental Disorder

5. Sexually antagonistic selection

A final possibility is that homosexuality genes might produce different effects for males versus females. It could be that when homosexual genes reside in male family members this would result in them having fewer offspring. Yet when these same genes reside in the female family line they could result in them getting more offspring to compensate for the loss of fitness in males. There is some support for this. One study found that the mothers of homosexuals had, on average, more children than mothers of heterosexual children. And the family members of the mothers’ line in homosexuals also sired more offspring. A review study just published in the Quarterly Review of Biology provides further support for this hypothesis. It suggests that particular (epigenetic) mechanisms that suppress androgens in female fetuses -- that enable them to grow into more feminine bodies with feminine brains -- also suppress androgens in male fetuses, which has the side effect of turning them into less masculine men. If these feminine women do better than the average female in getting more children such a mechanism could result in the propagation of homosexual genes.

These are some of the evolutionary hypotheses that are out there to support the relatively well-established scientific claim that homosexuality is a natural (normal) sexual orientation. More research is needed and hopefully in 2013 scientists will come closer to solving the mystery of homosexuality.

The biological science of homosexuality suggests that rather than discussing how we feel about homosexuals and how society should treat them, we should ask ourselves why homosexuality exists and what its functions are (or were).

1. Barash, D. P. (2012). Homo Mysterious: Evolutionary Puzzles of Human Nature. Oxford University Press.


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Self-fertilization, fusion of male and female gametes (sex cells) produced by the same individual. Self-fertilization occurs in bisexual organisms, including most flowering plants, numerous protozoans, and many invertebrates. Autogamy, the production of gametes by the division of a single parent cell, is frequently found in unicellular organisms such as the protozoan Paramecium. These organisms, however, may also reproduce by means of conjugation, in which cross-fertilization is achieved by the exchange of genetic material across a cytoplasmic bridge between two individuals. Likewise, among higher plants, most of which are monoecious (i.e., bisexual—male and female gametes being produced by the same individual), most self-pollinating species are also capable of cross-fertilization, and even those that are obligate self-fertilizers are occasionally cross-pollinated by accident. Hermaphroditic animals (those in which both male and female gonads are borne on one individual) are rarely capable of self-fertilization, since many such species have adaptations encouraging cross-fertilization.

As an evolutionary and reproductive mechanism, self-fertilization allows an isolated individual to create a local population and stabilizes desirable genetic strains, but it fails to provide a significant degree of variability within a population and thereby limits the possibilities for adaptation to environmental change.


Data collection

The data for this study were collected from 2007 to 2020 on a population of wild Sumatran orangutans (Pongo abelii) at Suaq Balimbing, South Aceh, Indonesia and from and 2003 to 2018 on a population of wild Bornean orangutans (Pongo pygmaeus wurmbii) at Tuanan, Central Kalimantan, Indonesia. Data were collected during focal animal follows, wherein the activity and associations of the focal individual were assessed through scans at 2-minute intervals.

Peering was defined as directly looking at the action of another individual sustained for at least 5 seconds and at a close enough range to enable the peering individual to observe the relevant details of the action (within 0 to 5 meters). The peering individual had to be facing the target individual and show signs of following the actions performed by the target individual (e.g., by head movements). We focused on immatures between 2 years and 8 years old because this age class shows the highest peering rates [68]. Before the age of 2 years, immature orangutans are mostly carried by their mothers and cannot approach association partners on their own [64]. Eight years marks the approximate end of the constant association of immatures with their mothers and thus the end of dependency on the mother, although this varies between individuals and sites [69]. After the age of 8, peering rates decrease significantly [6] and, because locally born males start to leave the research area at around 10 years old, sample sizes of older independent juveniles are female biased.

The peering data were collected during focal follows conducted by 64 experienced observers who recorded peering events of focal individual at 2-minute intervals (scan data set), as well as all peering events of any individual in sight between the scans (all occurrence data set). At both sites, interobserver reliability between experienced and new incoming observers was assessed via simultaneous follows of the same focal animal. Every observer whose data was included in the data sets of this study had reached a Cohen’s kappa [70] of k ≥ 0.8 with an experienced observer. For each peering event, the identity of the peerer, the identity of the peering target, and details about the context were noted. The total, unrestricted ad libitum data set of peering by immatures (0 to 8 years) comprised 2,543 peering events by 18 immatures (6 females and 12 males) at Suaq and 567 peering events of 17 immatures (4 females and 13 males) at Tuanan. The difference in sample size in the peering data between the 2 study sites resulted from a difference in overall peering rates between the populations (see [70]). Peering in the feeding context was defined as peering at an individual that was searching for, processing, or ingesting food.

To calculate overall hourly peering rates, we used the scan data set (i.e., only peering events that were recorded on the 2-minute intervals of the focal follows). We calculated a peering rate for each immature individual for each year, and we only included data points which comprised at least 25 follow hours. To assess the development of peering preferences, we used the all-occurrence peering data set to look at the proportion of all peering events directed at the mother of the focal animal versus at other association partners. We calculated peering proportions for each individual per year and only included data points that comprised a minimum of 5 peering events (range = 5 to 231, mean = 42.6) per individual per year. To analyze detailed non-mother peering target selection, we used all non-mother peering events for each immature individual and compared the proportion of peering targeted at (i) adult males (i.e., unflanged males and flanged males) versus adult females and (ii) residents (adult females and resident independent juveniles) versus immigrants (unflanged males, flanged males, and nonresident independent juveniles, with independent juveniles being defined as independently ranging, but not yet adult sized and not yet sexually active, individuals). Whereas we used all data points in these statistical analyses, to visualize those patterns, we computed peering proportions at each class of peering target based on a minimum total of 5 peering events (range = 5 to 80, mean = 21.5) per individual.

To calculate overall hourly association rates and proportions of association time spent in close proximity to particular classes of conspecifics, we used the association compositions and interindividual distances that were recorded on the 2-minute intervals of the focal follows. Orangutans were defined as being in association when they were within 50 meters of each other and in close proximity when they were within 2 meters of each other. The association data from Suaq contained a total of 4,149 association hours of 18 immatures (5 females and 13 males) and 3,163 association hours of 13 mothers, and from Tuanan, 11,538 association hours of 26 immatures (8 females and 18 males) and 4,443 association hours of 16 mothers. Overall association rates and proportions of association time spent in close proximity were calculated for each individual for each year. For association rates, we only included data points that comprised a minimum of 25 follow hours (range = 25.5 to 345.8, mean = 139.5) per individual. For the proportion of association time spent in close proximity to non-mother association partners, we only included data points that were based on a minimum of 10 association hours (range = 21.6 to 171.0, mean = 47.8) with non-mother, adult, and independent juvenile association partners.

To calculate dietary overlap between immatures and their mothers, we compared the repertoires of all food items recorded as eaten by each immature and its mother from when an immature was born until it was 8 years old. These data were collected during simultaneous focal follows of the immatures and their mothers and comprised a total of 670,748 scans at which the focal individuals were feeding (196,636 scans of the immatures and 474,112 of their mothers). A single food item was defined as the combination of the species and the part of the species that was eaten. For plant parts, we differentiated between bark, flowers, fruits, leaves, pith, and other vegetative items, and for insect parts, we differentiated between the insect itself and its different products (i.e., honey or eggs). Additional food items included bird eggs and the meat of small mammals. Each food item requires a distinct sequence of processing steps before ingestion [72]. For each food item in the mother’s diet, we checked whether or not it was eaten by the offspring, and vice versa. Since repertoire overlaps may depend on the follow effort, we only included data that were collected during simultaneous follows of the immatures and their mothers and that comprised at least 1,000 feeding scans (range = 2,821 to 43,211, mean = 10,489) of the offspring (collected at 2-minute intervals, when the immature was between 0 and 8 years old). We also included follow effort as a factor in our analyses (see below).

To make our immature focal animals more comparable to one another, we excluded their semi-dependent older siblings from the non-mother peering target category for all analyses. A semi-dependent older sibling ranges in close association with its mother-offspring pair and shows high interaction rates with the dependent offspring, i.e., its younger sibling [71]. The presence of a semi-dependent older sibling, however, depends on birth order and the length of the mother’s interbirth interval. A total of 20 of the 31 individuals in our peering data set had no, or very limited, opportunities to peer at an older sibling because they were either firstborns or born after the older sibling had left their mothers. Only 11 individuals were exposed to an older sibling for an extended period of time (i.e., they were born when their older sibling was still ranging with their mother). Only 5 individuals were still exposed to an older sibling at the age of 2 years or older, i.e., during the time they were included in our data set.

Ethics statement

As a strictly observational study on wild animals, we did not interact with our study animals in any way. All our research protocols were approved by the Ministry of Research, Technology and Higher Education (RISTEKDIKTI Research Permit No.: 152/SIP/ FRP/SM/V/2012 and following) and adhered to the legal requirements of Indonesia.

Statistical analyses

All statistical analyses and graphs were performed/made using the R programming Language [71]. To compare overall peering and association rates of the 2 sexes, we used generalized linear mixed models (GLMM, as implemented in the lme4 package in R [73]) with a Gaussian family distribution, using a full model approach. Aside from the main factor sex, we (here and throughout) included the factors site and age into the model because of their effects on peering and association rates found in previous studies [68]. We assessed the P values of the factors with the cftest function implemented in the multcomp package in R [74].

To examine sex-specific age effects on peering allocations and proportion of time immatures spent in close proximity, we used GLMMs with a Gaussian family distribution. A visual assessment of the data showed that these age effects were not linear and differed for the 2 sexes. Therefore, we fitted the models for female and male data separately. We used forward model selection to find the best fitting age effect by including age as a linear, quadratic, and linear and quadratic effect (see S4 Table for more detailed information about the model fits), using likelihood ratio test [75]. As a last step of this forward model selection process, we included site. The P values of the model selection process are summarized in S4 Table. The P values of the resulting final GLMMs were assessed with the cftest function implemented in the multcomp package in R [74].

To assess the effects of the immatures’ sex on their choices of peering targets (i.e., comparing the likelihood of peering at adult females versus adult males and resident versus immigrant individuals), we used GLMMs with a binomial family distribution, using a full model approach and including the factors sex, site, and age. For the analysis on peering at residents versus immigrants, the interaction between age and sex significantly improved the model fit, which is why we included it in the model.

In all GLMMs, we included the ID of the individual as a random factor, to account for the fact that individuals occur multiple times in the data set. Throughout all the abovementioned models, age was standardized through a Z-transformation.

To compare immature females’ and males’ dietary overlap with their mothers at the end of the dependency period (at the age of 8 years), we used generalized linear models (GLMs) with a Gaussian family distribution. Since dietary overlap is likely to be dependent on follow effort, we controlled for differences in follow effort by including the total number of 2-minute scans during which the focal was feeding as a factor in the model. To investigate sex differences in peering outside the feeding context, we used a GLM with a Gaussian family distribution and analyzed the proportion of total peering events that each immature individual directed at non-mother individuals engaging in activities other than feeding.

All model fits were examined visually to assess whether they satisfied model assumptions and to check for the presence of influential observations [76]. We assessed the stability of all our mixed models on the level of the random effects by excluding individuals one at a time. We found that the directions of these effects were consistent in all of the supported mixed models. The maximum of the variance inflation factors (VIFs, computed with the vif function in the car package in R [77]) of our independent variables across all models was 1.176, suggesting that our independent variables were not correlated with each other. For the binomial models, we tested for overdispersion and zero inflation using the testDispersion and testZeroInflation function in the DHARMa package in R [78]. The dispersion parameters of our models ranged from 0.9863 to 0.9996 and the ratio of observed to predicted zeros from 1.0016 to 1.0033.

Which species has the highest proportion of homosexual individuals? - Biology


There is not one gene, trait, or characteristic that distinguishes all members of one race from all members of another. We can map any number of traits and none would match our idea of race. This is because modern humans haven't been around long enough to evolve into different subspecies and we've always moved, mated, and mixed our genes. Beneath the skin, we are one of the most genetically similar of all species.

Lots of animals are divided into subspecies. Why doesn't it make sense to group humans the same way?

Subspecies are animal groups that are related, can interbreed, and yet have characteristics that make them distinct from one another. Two basic ingredients are critical to the development of separate subspecies: isolation and time. Unlike most animals, humans are a relatively young species and we are extremely mobile, so we simply haven't evolved into different subspecies.

The earliest hominids evolved from apes about 5 million years ago, but modern humans (Homo sapien sapiens) didn't emerge until 150,000-200,000 years ago in eastern Africa, where we spent most of our evolution together as a species. Our species first left Africa only about 50,000-100,000 years ago and quickly spread across the entire world. All of us are descended from these recent African ancestors.

Many other animal species have been around much longer or they have shorter life spans, so they've had many more opportunities to accumulate genetic variants. Penguins, for example, have twice as much genetic diversity as humans. Fruit flies have 10 times as much. Even our closest living relative, the chimpanzee, has been around at least several million years. There's more genetic diversity within a group of chimps on a single hillside in Gomba than in the entire human species.

Domesticated animals such as dogs also have a lot of genetic diversity, but this is mostly due to selective breeding under controlled conditions. Humans, on the other hand, have always mixed freely and widely. As a result, we're all mongrels: Eighty-five percent of all human variation can be found in any local population, whether they be Kurds, Icelanders, Papua New Guineans, or Mongolians. Ninety-four percent can be found on any continent.

Animals are also limited by habitat and geographical features such as rivers and canyons, so it is easy for groups to become isolated and genetically distinct from one another. Humans, on the other hand, are much more adaptable and have not been limited by geography in the same way. Early on, we could ford rivers, cross canyons, move great distances over a relatively short time, and modify our environment to fit our needs. We are also extremely mobile as a species. Even the remotest island tribe in the Pacific originally came from elsewhere and maintained some contact with neighboring groups.

We may think global migration is a recent phenomenon, but it has characterized most of human history. Whether we're moving halfway around the world or from one village to another, the passage of genes takes place under many circumstances, large scale and small: migration, wars, trade, slave-taking, rape, and exogamous marriage (marriage with "outsiders").

It takes a long time to accumulate a lot of genetic variation, because new variants arise only through mutation - copying errors from one generation to the next. On the other hand, it takes just a very small amount of migration - one individual in each generation moving from one village to another and reproducing - to prevent groups from becoming genetically distinct or isolated. Humans just haven't evolved into distinct subgroups.

But I can see obvious differences between people - don't those translate into deeper differences, like propensity for certain diseases?

The visual differences we are attuned to don't tell us anything about what's beneath the skin. This is because human variation is highly non-concordant. Most traits are influenced by different genes, so they're inherited independently, not grouped into the few packages we call races. In other words, the presence of one trait doesn't guarantee the presence of another. Can you tell a person's eye color from their height? What about their blood type from the size of their head? What about subtler things like a person's ability to play sports or their mathematical skills? It doesn't make sense to talk about group racial characteristics, whether external or internal.

Genetic differences do exist between people, but it is more accurate to speak of ancestry, rather than race, as the root of inherited diseases or conditions. Not everyone who looks alike or lives in the same region shares a common ancestry, so using "race" as a shorthand for ancestry can be misleading. Sickle cell, for example, often thought of as a "racial" disease afflicting Africans, is actually a gene that confers resistance to malaria, so it occurs in areas such as central and western Africa, the Mediterranean, and Arabia, but not in southern Africa. In medicine, a simplistic view can lead to misdiagnoses, with fatal consequences. Racial "profiling" isn't appropriate on the New Jersey Turnpike or in the doctor's office. As evolutionary biologist Joseph Graves reminds us, medicine should treat individuals, not groups.

On the other hand, the social reality of race can have biological effects. Native Americans have the highest rates of diabetes and African American men die of heart disease five times more often than white men. But is this a product of biology or social conditions? How do you measure this relationship or even determine who is Native American or African American on a genetic level? Access to medical care, health insurance, and safe living conditions can certainly affect medical outcomes. So can the stress of racism. But the reasons aren't innate or genetic.

Believing in race as biology allows us to overlook the social factors that contribute to inequality. Understanding that race is socially constructed is the first step in addressing those factors and giving everyone a fair chance in life.

The Resources section of this Web site contains a wealth of information about issues related to race. There you'll find detailed information about books, organizations, film/videos, and other Web sites. For more about this topic, search under "human variation," "evolution," "genetics" and "biology." Explore the HUMAN DIVERSITY interactivities in the LEARN MORE section of this Web site.


The benefits of rabbits are few. Warrens are utilised by a number of native species including echidnas, goannas and brushtail possums to name a few. Rabbits are also a valuable food source for birds of prey, particularly where the introduction of the rabbit has led to a decline in native fauna. Rabbits were also a valuable resource during the Great Depression, feeding families and providing an income through the sale of skins and meat. And of course, rabbits are the most important ingredient in that Aussie icon, the Akubra.

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