Biological Exuberance: Animal Homosexuality and Natural Diversity

A number of studies have also shown that animals that are the most active heterosexually are sometimes also the most active homosexually. In specific populations of Sociable Weavers, Bonnet Macaques, and Asiatic Elephants, for example, the top two males in terms of heterosexual mountings and other behaviors also participated in the most homosexual activities. Some of the most complete male homosexual behavior in Japanese Macaques, including full copulations with ejaculation, was exhibited by “one of the most vigorously heterosexual males in the troop,” while in another study the one female in a troop who failed to form any homosexual consortships also did not participate in any heterosexual consortships.²³ And as mentioned in the preceding chapter, in a number of birds such as Common Murres, Laysan Albatrosses, and Swallows, most individuals who participate in homosexual copulations are in fact breeders who have heterosexual mates, rather than nonbreeders who are heterosexually inactive.

In spite of these rather unexpected confirmations, however, the bulk of the evidence does not actually favor this hypothesis and in fact disconfirms many of its predictions. Most of the examples cited above that seem to support the idea of “bisexual superiority” are misleading because they are based on anecdotal, rather than quantitative, information, and because they only look at a few individuals at a single point in time (or, at most, over the span of a few breeding seasons). To assess whether bisexual animals are more successful at reproducing, what is actually needed is a long-term study of large numbers of individuals that tracks them over their entire lifetimes, comparing the total number of offspring produced by bisexual animals to the total number produced by heterosexual individuals. Needless to say, this would be a huge and difficult undertaking, complicated by the logistics of keeping track of hundreds or even thousands of animals over many years and potentially large geographic areas, tabulating not only the reproductive output of each individual but also his or her entire sexual history to determine which animals are bisexual and which are exclusively heterosexual. Not surprisingly, few longitudinal studies of this type have been conducted, and those that have rarely involve species in which homosexual or bisexual activity is prominent (or else they do not take into account such behavior when it is present).

However, one scientist—James A. Mills—has conducted exactly this sort of long-term, comprehensive study on the Silver (Red-billed) Gull in New Zealand, a species in which there is extensive bisexuality and homosexuality. His results show that bisexual individuals are in fact significantly less successful breeders than heterosexual ones. Over more than 30 years, Dr. Mills and his colleagues banded over 80,000 individual gulls, tabulating detailed lifetime reproductive and sexual profiles of more than 5,000 of these. Because of the enormity of this project, special computer programs had to be developed to analyze and keep track of all the data. The Silver Gull is an ideal species in which to test this hypothesis, because the sexual orientation of females (in terms of their pairing behavior) falls into three clear-cut categories: some form only homosexual pairs during their entire lifetimes and hence are exclusively lesbian, while others have both same-sex and opposite-sex partners during their lives and are therefore unequivocally bisexual, while other females only pair with male partners and thus are exclusively heterosexual.²⁴ Moreover, Mills and his team looked not only at how many chicks were hatched and raised by heterosexual versus bisexual (and homosexual) individuals, but also at how many of those chicks survived to adulthood and became breeders themselves—the true measure of whether an individual is actually passing on his or her genes.