Biological Exuberance: Animal Homosexuality and Natural Diversity

Homosexual activity in birds also generally fails to be correlated with patterns of exclusion from (or nonparticipation in) heterosexual mating. In Ruffs, for instance, mountings between males are actually more common when females are present on the breeding grounds than when they are absent. Even though males sometimes do try to prevent each other from mating heterosexually, same-sex mounts are not simply “redirected” or “substitute” heterosexual copulations. Homosexual mounts occur among a variety of different classes of males even when they are not directly “prevented” from mating with females (such as individuals known as satellites and naked-nape males). Conversely, one class of males—so-called marginal males—that is routinely excluded from opposite-sex interactions (by direct attacks from other males) does not usually engage in homosexual activity either. In Pukeko, a significant portion of the population assists other birds in raising their young rather than breeding themselves; however, homosexual activity in this species is characteristic of breeding individuals rather than such nonbreeding “helpers.” In Ocher-bellied Flycatcher and Ruffed Grouse populations that have a “surplus” of nonbreeding birds, breeding territories often go unused, indicating that nonbreeders are not being “prevented” from mating heterosexually (or at least are choosing to remain nonbreeders until better territories become available). Moreover, only a portion of such nonbreeders ever participate in same-sex activity, which in these species generally involves at least one partner that does have his own breeding territory. Likewise, the incidence of homosexual bonding among Oystercatchers (in the form of bisexual trios) does not increase significantly under higher population densities, when many individuals are unable to acquire their own opposite-sex mates and breeding territories. Brown-headed Cowbird populations generally have a significant surplus of males, and higher-ranking males also actively prevent lower-ranking males from courting and pairing with females. Yet the large number of birds thereby “excluded” from heterosexual mating opportunities—half to two-thirds of all males—do not regularly court or mate with one another. In fact, the only homosexual activity observed in this species involves male Cowbirds occasionally being mounted by males of another species, House Sparrows. Finally, Guianan Cock-of-the-Rock exhibit a pattern of homosexual and heterosexual participation that is strikingly similar to that of American Bison. Both yearling and two-year-old males are generally “excluded” from breeding, yet extensive homosexual activity only occurs in the former age group—and almost always with adult males as partners, many of whom are not excluded from heterosexual mating.⁵³