Biological Exuberance: Animal Homosexuality and Natural Diversity

Often, the relevance of dominance to homosexuality contrasts sharply in two closely related species: Pukeko have a well-defined dominance hierarchy that some scientists believe impacts on the birds’ homosexual behavior, yet in the related Tasmanian Native Hen, same-sex mounting occurs in the absence of a dominance hierarchy. Male homosexual mounting has been claimed to correlate with dominance in Cattle Egrets, yet in Little Blue Herons this connection is expressly denied. And the white-browed sparrow weaver (and several other species of weaver birds) has an almost identical social organization and dominance system as the Gray-capped Social Weaver, yet mounting between males is only found in the latter species.⁷⁵ Not only cross-species but also cross-gender comparisons are relevant here. A particularly good example of the problematic relationship between dominance and same-sex activity becomes apparent when one looks at males and females within the same species. In many animals both sexes have their own dominance hierarchies, yet homosexuality occurs in only one sex—male but not female Wolves, for example, and female but not male Spotted Hyenas. A corollary to this is that in some species, only one sex exhibits a stable dominance hierarchy, yet homosexuality occurs among both males and females. In Squirrel Monkeys, for example, female interactions are not consistently organized around a dominance or rank system, yet same-sex mounting and genital displays are not limited to males. In Bottlenose Dolphins, stable dominance hierarchies (if they exist at all) are more prominent among females, yet homosexual activity occurs in both sexes.⁷⁶ Finally, homosexual mounting sometimes occurs between animals of different species. Although cross-species dominance relations have been documented (e.g., in birds), in the majority of the cases involving homosexual activity there is no well-established hierarchical relationship between the participating animals of different species.⁷⁷ Clearly, then, dominance cannot be the only factor involved in the occurrence of homosexuality in a given species.

Even in animals where there is a clear dominance hierarchy, same-sex mounting is often not correlated with an individual’s rank, and it rarely follows the idealized scenario of “dominant mounts subordinate, always and without exception.” In many species, there is simply no correlation between rank and mounting behavior, since subordinate animals frequently mount dominant ones. In Rhesus Macaques, for example, 36 percent of mounts between males are by subordinates on dominants, while 42 percent of all female Japanese Macaque homosexual mounts go “against” the hierarchy, as do 43 percent of mounts between male Common Chimpanzees. ⁷⁸ Both dominant-subordinate and subordinate-dominant mounting occur in Bonobos, Lion-tailed Macaques, Squirrel Monkeys, Gelada Baboons, and Ruffs, among others, while mounting of older, larger, and/or higher-ranking animals by younger, smaller, and/or subordinate individuals has also been reported for numerous species: Common Marmosets, Australian and New Zealand Sea Lions, Walruses, Bottlenose Dolphins, White-tailed Deer, Mule Deer, Père David’s Deer, Wapiti, Moose, Mountain Goats, Red Foxes, Spotted Hyenas, Whiptail Wallabies, Rufous Rat Kangaroos, Mocó, Préa, Guianan Cock-of-the-Rock, Emus, and Acorn Woodpeckers. Oftentimes, while a large proportion of mounts may seem to follow the dominance hierarchy in a particular species, mounting by subordinates on dominants also takes place in the same species. This is true for Hanuman Langurs, Bonnet Macaques, Musk-oxen, Bighorn and Thinhorn Sheep, Cattle Egrets, and Sociable Weavers.⁷⁹