Biological Exuberance: Animal Homosexuality and Natural Diversity

In many animal societies, individuals can be ranked with respect to each other on the basis of a number of factors—aggression, access to food or heterosexual mating opportunities, age and/or size, and so on. The resulting hierarchy of individuals and their interaction within this system is often subsumed under the term dominance. Many scientists have suggested that mounting and other sexual behaviors between animals of the same sex are not in fact sexual behavior at all, but rather express dominance relations between the two individuals. The usual interpretation is that the “dominant” partner mounts the “subordinate” one and thereby asserts or solidifies his or her ranking relative to that individual. This “explanation” of homosexual behavior is firmly entrenched within the scientific establishment: one of the earliest statements of this position is a 1914 description of same-sex mounting in Rhesus Macaques, and since then dominance factors have regularly been invoked in discussions of animal homosexuality.⁷² Most scientists have appealed to dominance as an explanation for animal homosexuality only in relation to the particular species (or at most, animal subgrouping) that they are studying—and sometimes only for one sex within that species—without regard for a broader range of considerations. Once the full panoply of animal types, behaviors, and forms of social organization is taken into account, however, it becomes quite clear that dominance has little, if any, explanatory power. While dominance may be relevant in a few specific cases, it cannot account for the full range of homosexual interactions found throughout the natural world. Moreover, even in particular instances where dominance seems to be important, mitigating factors usually render its influence suspect, if not irrelevant.

At the most basic level, dominance is neither a sufficient nor a necessary condition for the occurrence of homosexual behavior in a species. Just because an animal has a dominance-based or ranked form of social organization does not mean that it exhibits homosexuality, and just because homosexual behavior occurs in a species does not mean that it has a dominance hierarchy. For example, many animals with dominance hierarchies have never been reported to engage in homosexual mounting. Dominance systems are found in “the vast majority of mammal species forming groups with any degree of social complexity”—most primates, seals, hoofed mammals, kangaroos, and rodents, for instance—yet only a fraction of these participate in same-sex mounting. Specific examples of birds with dominance hierarchies but no reported homosexuality include curlews, silvereyes, Harris’s sparrows, European jays, black-capped chickadees, marabou storks, white-crowned sparrows, and Steller’s jays.⁷³ Conversely, homosexuality is found in many animals that do not have a dominance hierarchy or in which the relative ranking of individuals plays only a minor role in their social system: for example, some populations of Gorillas, Savanna (Olive) Baboons, Bottlenose Dolphins, Mountain and Plains Zebras, Musk-oxen, Koalas, Buff-breasted Sandpipers, and Tree Swallows.⁷⁴