Biological Exuberance: Animal Homosexuality and Natural Diversity

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, Pere David’s Deer, Wapiti, Moose, Mountain Goats, Red Foxes, Spotted Hyenas, Whiptail Wallabies, Rufous Rat Kangaroos, Moco, Prea, 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 well: 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 well: 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

In a dominance-based view of homosexual mounting, it is often assumed that the animal being mounted is somehow a less willing participant in the interaction, “submitting” to the will of the more dominant individual, who thereby asserts his or her “superiority.” In fact, in more than 30 species the mounted animal actually initiates the interaction, “presenting” its hindquarters to the other individual as an invitation to mount, sometimes even actively facilitating anal penetration (among males) or other aspects of the interaction. Where the presenting animal is subordinate, this could be interpreted as simply a reinforcement of the dominance system, but in a number of species it is actually the more dominant individual who presents and actively encourages the lower-ranking animal to mount.⁸³ In addition, dominance “explanations” often ignore the clear differences between consensual and nonconsensual mounts (or rapes), as well as evidence for sexual arousal and even enjoyment on the part of mounted animals.⁸⁴

The relationship between sexuality and dominance is complex and multifaceted, differing greatly from the frequent simplistic equating of homosexual mounting with nonsexual rank-based or aggressive behavior. In many species a gradation or continuum exists between sexual mounts and dominance mounts, with one type “blending” into the other so that any distinction between the two is essentially arbitrary. Thus, same-sex mounting can have an unmistakable sexual component even when it still follows a dominance pattern. Among Hanuman Langurs, for example, usually only dominant females mount subordinate ones, yet so inextricably linked are signs of sexual excitement with this behavior that scientists have concluded, “It seems virtually impossible to separate ‘sexual mounting’ from ‘dominance mounting.’ … Sexual arousal and dominance are obviously not mutually exclusive in langur females, since mounting between females is related to both dominance and sexuality.”⁸⁵ At the other end of the spectrum, in some species a sharp distinction does in fact exist between two types of mounting, both of which occur between same-sex partners: a nonsexual form associated with dominance and/or aggression, and a clearly sexual form that occurs in other contexts (often within a homosexual pair-bond or consortship). This is true for female Japanese Macaques, Rhesus Macaques, and Black- winged Stilts, and male Greylag Geese, among others.⁸⁶

Finally, in some animals dominance and mounting are entirely separate, with social rank being expressed through obviously nonsexual activities. For example, male Walrus dominance interactions involve fighting and tusk displays that usually occur during the breeding season and often involve younger animals. Male homosexual mounting is not associated with either of these activities and usually takes place in the nonbreeding season among males of all age groups (a similar pattern is also seen in Gray Seals). Oystercatchers use a special ritualized “piping display” (neck arched, bill pointed downward, accompanied by shrill piping notes) to negotiate their dominance interactions, while same-sex mounting and courtship occur in other contexts.⁸⁷ Dominance in many other animals is expressed through fighting and aggressive encounters, access to food or feeding frequency, body size or age, physical displacement (causing another individual to move off through posture, threats, staring, or other activities), access to heterosexual mating opportunities, or a combination of these or other factors, and specifically does not involve mounting or the other homosexual interactions that occur in these species. Savanna (Yellow) Baboons, (female) Hamadryas Baboons, Bottlenose Dolphins, Killer Whales, Caribou, Blackbucks, Wolves, Bush Dogs, Spotted Hyenas, Grizzly Bears, Black Bears, Red-necked Wallabies, Canada Geese, Scottish Crossbills, Black-billed Magpies, Jackdaws, Acorn Woodpeckers, and Galahs are all species in which this is the case.⁸⁸

Another limitation in looking at homosexual interactions from the perspective of dominance is that only mounting behavior lends itself to such an interpretation. A whole host of other homosexual activities do not fit neatly into the dominance paradigm—either because, by their very nature, they are reciprocal activities, or because neither participant can be assigned a clearly “dominant” or “subordinate” status on the basis of what “position” it assumes during the activity. For example, mutual genital rubbing—in which two animals rub their genitals on each other without any penetration—often occurs with neither participant “mounting” the other. Gibbon and Bonobo males frequently engage in this activity when hanging suspended from a branch, facing each other in a more