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 portion of the “surplus” individuals form same-sex bonds include Flamingos, Laughing Gulls, Humboldt Penguins, Gentoo Penguins, Pied Kingfishers, Peach-faced Lovebirds, Galahs, and Bicolored Antbirds.47
Although homosexual involvements in such species may be the “result” of skewed sex ratios, any “explanation” of homosexuality that relies on this factor alone needs to address why only some individuals “choose” this strategy, and why this strategy rather than another. For in addition to remaining single or forming same-sex pair-bonds, a wide variety of other behavioral responses occur among animals in populations that have a surplus of one sex, or in situations where the opposite sex is “unavailable.” For example, in many otherwise monogamous species that have more females than males (or vice versa), some individuals form polygamous heterosexual trios (so-called “bigamy”) or even quartets (“trigamy”). These options occur alongside homosexual pairings in Flamingos and Humboldt Penguins, and instead of same-sex pairing in Cattle Egrets, emperor penguins, and dippers (among many others). Individuals in the same population may also adopt different strategies or combine these strategies, to varying degrees: in Oystercatcher communities, for instance, with large surpluses of nonbreeding birds unable to find heterosexual mates or breeding territories of their own, only a small fraction of these birds form polygamous trios (most remain unmated); and only a portion of these in turn go on to develop homosexual bonds within their trio. Australian noisy miners (a bird species with a heavily male-biased sex ratio) have developed a complex, specieswide system of communal breeding that involves, among other arrangements, polyandry (several males associating with each female, without any same-sex bonding). The reverse situation occurs in spotted sandpipers, in which “surplus” birds actually participate in monogamy rather than polygamy. In this species, females usually mate with several males and generally leave the parenting duties to them; females unable to find polygamous mates, however, often “revert” to monogamous (heterosexual) pairing and parenting, helping one male partner with incubation and brooding.48