Biological Exuberance: Animal Homosexuality and Natural Diversity

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