Biological Exuberance: Animal Homosexuality and Natural Diversity

consort with any males.³⁷ Finally, in a number of species such as Swallows, Laysan Albatrosses, and Herons, same-sex mounting occurs primarily among breeding individuals (i.e., those who already have heterosexual mates) and is largely absent among nonbreeders.

A number of species do have skewed sex ratios, but (like dominance) this is neither a sufficient nor a necessary prerequisite for the occurrence of homosexuality in a population. Male homosexuality is not reported for red-winged blackbirds or giant cowbirds, for example, even though some populations are 80-84 percent male, nor for pintail duck populations with two-thirds males, or kiwis with 58 percent males, or purple finches with 57 percent males. Likewise, female homosexuality is absent in boat-tailed grackles even though males may comprise only a third of the population, and in sparrow hawks, where there is also a “surplus” of females (less than 40 percent males). In contrast, homosexuality occurs in numerous species or populations that have equal (or nearly equal) sex ratios, including Bonobos, Bonnet Macaques, West Indian Manatees, Snow Geese, California Gulls, and Pukeko.³⁸ Moreover, closely related species or different populations of the same species that have identical (or similar) sex ratios and forms of social organization often exhibit strikingly different patterns of homosexuality. Many Seals and Sea Lions with polygamous mating systems, for instance, have strongly female- biased sex ratios (three to five females for every male) and social systems that often include sex segregation and/or exclusion of large numbers of males from breeding opportunities. Some of these species exhibit male homosexuality (e.g., Gray Seals, Northern Elephant Seals, Walruses), others have female homosexuality (e.g., Northern Fur Seals), some have both (e.g., Australian Sea Lions), while the majority have no homosexuality at all (e.g., California sea lions, southern fur seals). Likewise, lunulated antbirds, salvin’s antbirds, Bicolored Antbirds, and Ocellated Antbirds all live in populations that have an excess of males, yet homosexual pairing is only found in the latter two species.³⁹

In many animals that have skewed sex ratios, homosexuality only occurs (or is more common) in the sex that is in shorter supply rather than in the “surplus” sex. In some populations of Crab-eating Macaques, for example, females outnumber males by more than two to one, yet same-sex activity only occurs among males. Female homosexuality accounts for more than 80 percent of same-sex activity in Pukeko even though some populations are more than 70 percent male. The reverse is true for Rhesus Macaques: in some populations females outnumber males nearly three to one, yet the majority of same-sex activity (over 80 percent) is between males. Tree Swallow populations often have a surplus of females, but only male homosexuality occurs. Likewise, female pairs have formed in captive populations of Galahs and Scarlet Ibises that have an excess of males, while male pairs of Flamingos are reported from populations that have more females than males. In Nilgiri Langurs, there is a female-biased sex ratio in the overall population (and individuals live in groups with more females than males), yet only male same-sex activity is reported. Finally, in Little Egrets and Little Blue Herons there is a “surplus” of unpaired males, yet same-sex mounting occurs almost exclusively among paired males rather than in the population of birds that are unable to find heterosexual mates.⁴⁰

While homosexual activity in some species may appear to be associated with an unavailability of the opposite sex, the patterns of its occurrence are often far more complex than a shortage explanation would indicate. Although lesbian pairs in Black Stilts, for example, generally do occur in populations where the sex ratio is biased toward females, in other populations of the same species with a surplus of males, no male homosexual pairs have formed. The same is true, in reverse, for captive Humboldt Penguins: male pairs form when there is a surplus of males but female pairs do not form when there is a surplus of females. Among some populations of Savanna (Yellow) Baboons, the sex ratio becomes skewed among older juveniles, where males outnumber females two to one—and indeed, 10 percent of such animals’ mounting is homosexual. However, the sex ratio is equal among adults and younger juveniles, and the prevalence of homosexual mounting in these segments of the population is the exact opposite of what the shortage hypothesis would predict: 17-24 percent of their mounting is same-sex. In other words, older juvenile males actually exhibit the lowest proportion of homosexual activity and the greatest participation in heterosexual mounting of any segment of the population (accounting for more than half of all male- female mounts), even though their age group contains the greatest surplus of males. Sex ratios in wild Mallard Ducks fluctuate during the breeding season, with fewer females being present in some months than others. Although male pairs sometimes form at these times, during other months when there is also an excess of males in the population, there are no male pairs.⁴¹

If access to heterosexual mates were the only factor involved in the occurrence of homosexuality, both males and females in sex-segregated populations should exhibit the same degree of homosexual activity. However, in the majority of species that have some form of sex segregation, homosexual activity is found in only one sex (e.g., Walruses, Gray Seals, Warthogs, American Bison) or is much more common in one sex (usually males) than the other (e.g., Giraffes, Blackbucks, Mountain Sheep, Australian Sea Lions). Conversely, in some species that have unbalanced sex ratios (in wild or captive contexts), such as Pig-tailed Macaques, Bottlenose Dolphins, Cheetahs, Koalas, Canada Geese, and Flamingos, homosexuality occurs in both sexes (although it may be more common in the “surplus” sex). This indicates that more is involved than simply a “shortage” of available heterosexual partners.⁴² Likewise, where populations of the same species vary in their sex ratios, homosexuality is sometimes less common in nonskewed populations, but it is still present. In Japanese Macaques, Giraffes, and Greylag Geese, for example, same-sex activity may increase in populations with an excess of one sex, but it still occurs at a fairly steady rate in other circumstances regardless of the sex ratio and may even be present in the “limiting” sex (e.g., in Giraffe populations with more than 60 percent females, male homosexuality still occurs). Even in populations of Japanese Macaques with highly skewed sex ratios, most individuals still manage to participate in both heterosexual and homosexual activities, indicating that they are not turning to same-sex partners as a result of being completely “deprived” of opposite-sex partners.⁴³

Similarly, in a number of species where homosexuality sometimes occurs in the absence of opposite-sex partners (due to sex segregation, heterosexual refusal, captive situations, etc.), same-sex activity is not limited to these contexts, but also occurs in mixed-sex groups (e.g., Gorillas, Hanuman Langurs, Crested Black Macaques, Squirrel Monkeys, Walruses, Lions, Mallard Ducks, Black-headed Gulls) or in contexts where it is not a response to the refusal or unavailability of the opposite sex (e.g., West Indian Manatees, Cheetahs).⁴⁴ If same- sex activity were due entirely to an absence of the opposite sex, it should disappear completely once opposite-sex partners are available, yet these examples show that it does not. Conversely, homosexuality does not arise automatically or immediately when animals are deprived of opposite-sex partners, nor does heterosexuality necessarily ensue once they have access to such partners. Homosexual activity in a captive group of female Squirrel Monkeys, for instance, did not develop until fully one year after they had been sequestered away from males, while female Long-eared Hedgehogs that were homosexually involved with each other in the absence of males did not participate in heterosexual mating for more than two years after they were given access to males.⁴⁵

Multiple Possibilities

Even if homosexuality in some species only occurs in populations where there is more of one sex than the other, this is, at the very least, evidence of a “latent” bisexual capacity among some individuals. Moreover, the skewed sex ratio is probably only a contributing factor rather than a determining “cause” of same-sex interactions in such cases. Typically only a portion of the “surplus” sex in these populations actually participates in homosexuality, and sometimes “available” opposite-sex partners are even passed over. This is most obvious in Silver Gulls, where nearly half of all females are “unable” to find a male partner each year, yet lesbian pairs constitute only about 6 percent of the population—in other words, the vast majority of “surplus” females remain single rather than forming homosexual pairs. Furthermore, about 14 percent of all males are unpaired, which means that females who form same-sex bonds do so in spite of the presence of single males in the population. Likewise, some female Mallard Ducks remain unpaired even in populations with more males than females. In one semi-wild population of Canada Geese with an excess of males, some of the unpaired males failed to form homosexual pairs; furthermore, some females also remained unpaired or formed homosexual bonds even though opposite-sex birds were “available.” While approximately 10 percent of widowed Jackdaws form homosexual pair-bonds, the majority of widowed birds who do not find male partners actually remain single rather than pairing with female partners. Lesser Scaup Duck populations generally consist of 60-80 percent males, yet only a fraction of these individuals engage in homosexual mounting (and none form homosexual pair-bonds). Similarly, herds of Caribou may contain only 30-40 percent males, yet same-sex activity among females is not overwhelming.⁴⁶ Other species in which only a