The Dominant Paradigm
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
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
The precise opposite of the “standard” dominance-based system of mounting is often found as welclass="underline" mountings by subordinates on dominants occur more frequently than the reverse in many species. In Crested Black Macaques, for example, 60—95 percent of mounts are subordinate on dominant, while nearly two-thirds of Bison male homosexual mounts are by subordinates on dominants. To complicate things further, this is often combined with a gender difference in the relationship between mounting and dominance, with female mounts “following” the hierarchy and male mounts going “against” it. For instance, in Pig-tailed Macaques mounting between females is usually by a dominant individual on a subordinate one, but more than three-quarters of mounts between males are just the opposite. Similarly, in both Red Deer and Pukeko, females tend to mount lower-ranking animals while males tend to mount higher-ranking ones. There are often individual or geographic differences as welclass="underline" in some consortships between female Japanese Macaques, all mounting is done by the lower-ranking individual on the higher-ranking partner, while in some populations of Bighorn Sheep, mounting of dominant rams by subordinates is much more prevalent than in other populations.80 Furthermore, homosexual mounting in many species is reciprocal, which means that partners exchange positions—mounter becomes mountee, and vice versa—either in the same mounting session or in alternation over longer periods of time. This behavior, which is found in at least 30 different species, is potent evidence of the irrelevance of dominance for homosexual interactions, since mounting should only occur unidirectionally if it strictly followed the rank of the participating individuals.81 Finally, in some species mounting can also occur between individuals of the same or close ranks—for example, in Common Chimpanzees, White-faced Capuchins, Musk-oxen, Blackbucks, Cavies, and Gray-capped Social Weavers.82