Biological Exuberance: Animal Homosexuality and Natural Diversity

There are further arguments against the bisexual-superiority hypothesis. If bisexual animals were more successful breeders, one would expect them to make up the majority of the population in any given species, with much smaller proportions being exclusively heterosexual or homosexual—yet the distribution of sexual orientations does not, in fact, typically follow this pattern. In Silver Gulls, heterosexual versus bisexual percentages are in accord with what we have just seen about their relative reproductive proficiencies: 79 percent of all females are exclusively heterosexual, 11 percent are bisexual, and 10 percent are exclusively lesbian. This pattern is characteristic of many other species for which we do not have information about the lifetime reproductive output of a cross-section of individuals: bisexual animals generally make up a much smaller percentage of the population, sometimes even less than the proportion of exclusively homosexual individuals. For example, the heterosexual-bisexual-homosexual proportions for male Black-headed Gulls are 63-15-22 percent, respectively, and for Galahs, 44-11-44 percent.³¹ In many other species the proportion of animals who engage in bisexual activity is even smaller.

Moreover, in some cases there do not appear to be any bisexual individuals at all in a population (i.e., same-sex activity occurs only in nonbreeding animals). For example, female homosexual pairs in Kittiwakes, Red-backed Shrikes, and Mute Swans, among others, appear to consistently lay infertile eggs (indicating that they do not mate with males); in Pied Kingfishers, homosexuality is typical of nonbreeding birds who are not likely to reproduce later in life; while male Ostriches who court other males do not appear to have heterosexual relations. Although longitudinal studies are needed in each case to verify that such individuals are not in fact sequentially bisexual, these patterns do not fit well with a bisexual-superiority hypothesis. More broadly, species in which homosexuality or bisexuality is only found in individuals of one sex—or in which all individuals are exclusively heterosexual—are extremely common and are further evidence against this hypothesis (since they are examples of bisexuality failing to be “maximized”).

What about species in which the majority of individuals are bisexual, i.e., the examples of maximization of bisexuality mentioned above? In all of the animals in which this is the case (Bonobos, Dolphins, Mountain Sheep, etc.), individuals differ significantly in the degree to which they are bisexual. Some animals participate very little in homosexual and/or heterosexual activity while others account for the majority of (one or both) such activities, and same-sex versus opposite-sex interactions make up varying proportions of each individual’s sexual encounters as well (as we saw in chapter 2). Thus, if bisexuality were related to reproductive success, one would expect animals to differ depending on “how bisexual” they are—successful breeders (i.e., those animals who are the most active heterosexually) should engage in a greater proportion of homosexual activity as well. Again, long-term studies of reproductive output are required to test this, but data on the sexual activity of individual animals in a number of species where bisexuality is widespread do not support this idea. If we take the number of heterosexual copulations that an animal participates in to be a rough measure of its reproductive prowess, then we do not generally find that there is a positive correlation between degree of bisexuality and breeding success.