Biological Exuberance: Animal Homosexuality and Natural Diversity

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