Other patterns of exclusivity occur as well. In Nilgiri Langurs and Hamadryas Baboons, for instance, generally only the highest-ranking male in a group mates with females; remaining males, if they engage in sexual activity at all, are sometimes involved only in homosexual pursuits. In Nilgiri Langurs, cases of nonbreeding males having only same-sex interactions for at least four years have been documented. In Ruffs, there are several different categories of males, many of whom rarely, if ever, mate heterosexually; some of these individuals participate in homosexual activities and may do so over an extended period, perhaps even for life. Finally, in some species same-sex activity may be exclusive because it is incestuous, involving a parent and its nonbreeding offspring. In male White-handed Gibbons, for instance, father-son sexual relations may continue for several years; the son is not involved in concurrent heterosexual activity, and sometimes even his father may have little or no opposite-sex mating during this time. Red Fox daughters can remain with their family group for many years—sometimes they never leave—during which time they may be involved in occasional same-sex mounting with their mothers (or each other) but no heterosexual activity.18
Thus, while in many species documentation of exclusive long-term homosexuality (or heterosexuality, for that matter) is not directly available, exclusivity can be inferred from the general patterns of social organization in the species. For example, a system that involves large numbers of nonbreeders (including individuals who never mate heterosexually during their entire lives), combined with homosexual activities among at least a portion of these nonbreeding animals (sometimes in sex-segregated groups), will invariably entail some individuals whose only sexual contacts are with animals of the same sex. For some animals this period of exclusive homosexuality lasts no more than a few years; for others, it may extend considerably longer, even for the duration of their lives.
Shorter periods of exclusive or “preferential” homosexuality also occur. Sexual “friendships” in Stumptail Macaques and Rhesus Macaques, for example, and homosexual consortships in Japanese Macaques, last anywhere from a few days to several months, during which time there are no heterosexual involvements. During the seasonal aggregations of male Walruses and Gray Seals, same-sex activity usually occurs to the exclusion of opposite-sex behavior. Female Marmots forgo breeding for a couple of years but may still have sexual contact with other females. Same-sex pair bonds in King Penguins and homosexual associations in female Orang-utans are also exclusive for their duration. Of course, many of these animals are actually bisexual because they also engage in heterosexual pursuits at other times during their lives, but while they are involved in same-sex activity, they do not simultaneously engage in opposite-sex behavior. Thus, when considering various forms of exclusive homosexuality it is also necessary to understand the different types of nonexclusive homosexuality—that is, bisexuality.
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.