Biological Exuberance: Animal Homosexuality and Natural Diversity

For example, in Kob antelopes—in which virtually all females engage in both same-sex and opposite-sex mounting—there is generally an inverse relationship between an individual’s heterosexual and homosexual activity. One study revealed that a female who had the most homosexual mounts had the fewest heterosexual ones and vice versa, while individuals who ranked in the upper quarter or third of the population in terms of heterosexual activity often ranked much lower in their homosexual participation. Furthermore, the female whose heterosexual and homosexual activities were most equal—i.e., the most “bisexual” individual—actu—ally participated in the fewest total number of heterosexual matings. Similarly, all Bonobo females interact sexually with both males and females, but differ widely in the extent of their bisexuality. In one troop, three females participated in the most heterosexual copulations—two-thirds of all mating activity—yet these same females accounted for less than one-third of all homosexual activity, and one had among the fewest same-sex encounters of any of the females. Nor were these females necessarily “balanced” in terms of their individual proportions of same-sex and opposite-sex activity. One had fairly equal ratios of homosexual and heterosexual interactions, but the other two were less “proportional” bisexuals, with the majority (two-thirds) of their sexual encounters skewed toward opposite-sex partners. Likewise, those female Japanese Macaques who were the most involved in homosexual activity in each of four mating seasons (the top two in terms of the proportion of time they spent) were rarely as involved in heterosexual interactions and were often among the least heterosexually active members of their troop. Another pattern was revealed in a study of Pig-tailed Macaques. Although all the males in one troop mounted both females and other males, they had roughly the same number of homosexual encounters regardless of their participation in heterosexual activity (which varied enormously). The male who was the most heterosexually active was also the least “bisexual” individual (same-sex mounting made up only 8 percent of his sexual activity, compared to an average of 48 percent for the other males).³²

Of course, heterosexual activity (i.e., number of opposite-sex matings) is not necessarily an accurate measure of reproductive success, and none of these studies tracked individual animals and the number of offspring they produced throughout their entire lives.³³ Nevertheless, there does not appear to be the sort of connection between homosexual and heterosexual activity that would be expected if bisexuality contributed to an animal’s reproductive prowess or success. Moreover, in most of the species where bisexuality seems to be “maximized,” it is usually the case that one sex participates in homosexual activity to a greater extent than the other: females in Kob, Bonobos, and Japanese Macaques, males in Mountain Sheep and Bottlenose Dolphins, for example. Even if bisexuality were somehow an advantageous reproductive strategy, it would remain to be explained why there should be a gender difference in its “efficacy” (and why it should pertain to different genders in different species).

Finally, most of the specific cases mentioned above (e.g., Black Swans, Pukeko, Ruffs) that seem to support some sort of connection between bisexuality and reproductive prowess are not as convincing as they initially appear. In each instance, closer investigation reveals that the connection is doubtful, if not completely spurious. ³⁴ For example, although male pairs in Black Swans tend to be more successful parents, such couples are not necessarily made up of bisexual birds, nor do they always raise their own offspring. Same-sex pairs in this species often “adopt” cygnets by taking over or stealing nests from heterosexual pairs (rather than mating with a female)—thus many successful male pairs need not have been involved in any heterosexual activity at all and may be exclusively homosexual rather than bisexual. Moreover, even if such individuals prove to be bisexual over their entire lives (e.g., by subsequently pairing with females), much of their parenting success involves raising offspring that are not related to them (by virtue of having been “adopted”). This situation is inimical to the rationale behind the bisexual-superiority hypothesis, which depends on bisexual individuals being more successful at passing on their own genes, not other animals’.