Biological Exuberance: Animal Homosexuality and Natural Diversity

The participation of an individual in both homosexual and heterosexual activities is widespread among animals: bisexuality occurs in more than half of the mammal and bird species in which same-sex activity is found. Nevertheless, there are many different forms and degrees of bisexuality, and these must be carefully distinguished when discussing sexual orientation in animals. A useful differentiation to start with is sequential as opposed to simultaneous bisexuality, a distinction that hinges on the temporal or chronological separation between homosexual and heterosexual pursuits. In sequential or serial bisexuality, periods of exclusively same-sex activity alternate with periods of exclusively opposite-sex activity. In simultaneous bisexuality, homosexual and heterosexual activities co-occur or are interspersed within a relatively short period (say, within the same mating season). Thus, many of the “shorter” periods of exclusive homosexuality that we have been considering actually fall into a larger pattern of sequential bisexuality, which itself forms a continuum in which same-sex activity may occupy anywhere from several months to several decades of an animal’s life. Moreover, the “sequentiality” of bisexual experience assumes many different forms: a seasonal pattern (for example, in Walruses, who engage in homosexuality primarily outside of the breeding season, or in Gray Whales, during migration and summering); an age-based pattern (e.g., in Bison or Giraffe, where same-sex activity is more characteristic of younger animals, or in which the earlier years of an animal’s life are occupied largely with homosexual pursuits, to be followed by heterosexual activity in later years—or the reverse, as in some African Elephants); onetime “switches,” in which individuals change over from heterosexual to homosexual activity at a specific point in time (e.g., Herring Gulls, Humboldt Penguins), or from homosexual to heterosexual (e.g., Great Cormorants); as well as less structured sequencing, in which several periods of same-and opposite-sex activity of varying lengths may alternate with each other (e.g., Gorillas, Silver Gulls, King Penguins, Bicolored Antbirds).¹⁹

A group of male Walruses off the coast of Round Island (Alaska). Pairs of males are participating in courtship and other activities with each other while floating in the water. Male Walruses are often seasonally bisexual, engaging in homosexual pursuits outside the breeding season.

Simultaneous bisexuality also assumes many guises. At one extreme, sexual activity with same-sex and opposite-sex partners takes place at literally the same time: “pile-up” copulations, for example, in which a male mounts another male who is mounting a female (e.g., Wolves, Laughing Gulls, Little Blue Herons), or group sexual activity in which some or all participants are interacting with both males and females (e.g., Bonobos, West Indian Manatees, Common Murres, Sage Grouse). At the other extreme, individuals court or mate with both sexes separately, over short but relatively distinct spans of time, as in Crab-eating Macaques, Mountain Goats, Redshanks, and Anna’s Hummingbirds. In between these extremes are other patterns, such as ongoing bisexual trios and quartets, in which both same-sex and opposite-sex partners are bonded to one another concurrently (e.g., Greylag Geese, Oystercatchers, Jackdaws). Another form of simultaneity involves an animal in a pair-bond with a member of the opposite sex who has occasional courtship and/or sexual encounters with a member of the same sex (or vice versa). For example, male Herring and Laughing Gulls, Herons, Swallows, and Common Murres who have female partners, and female Mallard Ducks who have male partners, sometimes mount birds of the same sex. Conversely, female Snow Geese, Western Gulls, and Caspian Terns and male Humboldt Penguins and Laughing Gulls who have same-sex partners sometimes mate with opposite-sex partners. Still another variation is found in Lesser Flamingos: males in homosexual pairs sometimes try to mate with females who are themselves in homosexual pairs. And in some animals such as Bottlenose Dolphins, Black-headed Gulls, and Galahs, the combinations are even more varied: different forms of sequential and simultaneous bisexuality, as well as exclusive homosexuality (and heterosexuality) are found in different individuals within the same species and may even combine in the same individual at different points in time.