Biological Exuberance: Animal Homosexuality and Natural Diversity

In mammals, cases of long-term, exclusively homosexual pairing are indeed rare. One example is male Bottlenose Dolphins: the majority of males in some populations form lifelong homosexual pairs, specific examples of which have been verified as lasting for more than ten years and continuing until death. Although the sexual involvements (both same- and opposite-sex) of such individuals have not in all cases been exhaustively tracked, it is quite likely that at least some of these animals have little or no sexual contact with females (since breeding rates tend to be low in Bottlenose communities, with many individuals not participating in reproduction each year and, by extension, possibly throughout their lives).¹² Absolute verification in this species, however, may not be forthcoming, since it is virtually impossible to continuously monitor the sexual behavior of all individuals within a given population of an oceangoing species. Bottlenose Dolphins are exceptional, however, in that the homosexual pattern in this species is distinct from the heterosexual one: opposite-sex pair-bonding does not occur among Bottlenose Dolphins. In most other species, homosexual and heterosexual activities tend to follow the same basic patterns, whether this means pair-bonding, polygamy, promiscuity, or some other arrangement.¹³ Lifetime homosexual couples are not prevalent among mammals, therefore, for the same reason that lifetime heterosexual couples are not: monogamous pair-bonding is simply not a common type of mating system in mammals (it is found in only about 5 percent of all mammalian species).¹⁴

Nevertheless, long periods of exclusive homosexuality among mammals have been documented for other social contexts besides pair-bonding. In many species, significant portions of the population do not engage in breeding or heterosexual pursuits for at least a part of their lives. Because some of these animals continue to engage in same-sex interactions, however, they are exclusively homosexual for at least that time, which can be considerable. Among Gorillas, for example, males often live in sex-segregated groups where homosexual activity takes place. The average length of stay in a male-only group is more than six years, although some males remain in such exclusively homosexual environments for much longer. One individual lived in an all-male group for ten years, staying until his death, and nearly a third of the males who joined the group over a thirteen-year study period were still with the group at the end of that time. Likewise, Hanuman Langur males may spend upward of five years in male-only bands in which homosexual activity takes place, and some individuals live their entire adult lives in such groups.¹⁵ In a number of hoofed mammals, a similar form of exclusivity based on sex segregation occurs: only a few individual males participate in heterosexual mating, while the remainder live in “bachelor herds” where homosexual activity often takes place.¹⁶ Among Mountain Zebras, for example, males stay an average of three years in such groups before joining breeding groups, and some remain their whole lives without ever mating heterosexually. Analogous patterns occur in a number of other species where only a relatively small percentage of males ever breed: antelopes and gazelles, including Blackbuck, Pronghorn, and Grant’s and Thomson’s Gazelles; Giraffe; Red Deer; Mountain Sheep; seals such as Northern Elephant Seals and Australian and New Zealand Sea Lions; and birds such as Ruffed Grouse, Long-tailed Hermit Hummingbirds, and Guianan Cock-of-the-Rock. In some hoofed mammals such as American Bison, a related age-based pattern is found. Males generally do not participate in heterosexuality until they are five to six years old; prior to that time, many engage in homosexual activities, entailing a period of exclusively same-sex activity of up to five years for some individuals.¹⁷