Biological Exuberance: Animal Homosexuality and Natural Diversity

At first glance, this hypothesis seems counterintuitive: regardless of the genetic mechanism involved, why should bisexual individuals be superior at procreating or have a reproductive advantage? On the contrary, one would expect individuals with two heterosexual genes—those who are exclusively or “doubly” heterosexual, as it were—to be more successful breeders than bisexuals. Nevertheless, this hypothesis accords surprisingly well with a number of aspects of animal homosexuality that remain puzzling under other accounts. First of all, as noted previously, bisexuality is widespread in the animal kingdom. Unlike other theories about the evolutionary value of homosexuality, this hypothesis recognizes that many individuals who participate in homosexual activity may also be involved in heterosexual behavior, and therefore capable of reproducing and passing on their genes. Additionally, the incidence of bisexuality within populations is often high: in a number of animals such as Bonobos, Japanese Macaques, Bottlenose Dolphins, Mountain Sheep, Giraffe, and Kob, for instance, virtually all members of the species (or of one sex) participate in both same-sex and opposite-sex interactions (either concurrently or at different points in their life). Again, this hypothesis predicts that such situations should exist, since it argues for the maximization of bisexuality in a population—that is, if bisexual individuals are more successful breeders, they should tend to make up the majority of a population.

Even more startling, in a few species bisexual animals actually do appear to be more successful than exclusively heterosexual individuals at reproduction, heterosexual mating, and/or attracting members of the opposite sex. As we have already discussed, pairs of male Black Swans, who can father cygnets by associating temporarily with a female and then raise the resulting offspring on their own, are generally more successful parents than heterosexual pairs. In part, this is because such same-sex pairs are more aggressive than male-female pairs and are therefore able to acquire larger and better-quality territories, which are essential for successfully raising cygnets. They may also have an advantage because both males contribute to incubating the eggs, whereas in heterosexual pairs males may take part in less of the incubation duties. Over a three-year period, 80 percent of male pairs in one study were found to be successful parents, while only about 30 percent of heterosexual pairs successfully raised offspring (unsuccessful parents either deserted their clutches, lost them to predators or other hazards, or ended up having their cygnets die). Homosexual pairs constituted up to a quarter of all successful parents even though they made up only 13 percent of all breeding pairs or associations in the study population.²¹

Animals who participate in homosexual activity are also sometimes more successful at attracting members of the opposite sex, or participate more often in heterosexual mating. For example, male Ruffs who display with and mount male partners on their courtship territories attract females for mating more often than males who display by themselves. Because of their superior strength and courage, as well as their high rank in the flock, Greylag Geese in gander pairs or other homosexual associations are also sometimes attractive to the opposite sex. Females may associate themselves with a gander pair and eventually form a bisexual trio, mating with one or both of the males and raising their goslings together. In Pukeko, breeding groups in which homosexual interactions take place between males are also the groups in which the most intense heterosexual copulatory activity occurs. Adolescent Guianan Cock-of-the-Rock males who participate in the most visits to adult males’ display territories, during which homosexual courtship and mounting often occur, sometimes acquire their own territories at a younger age. With earlier access to heterosexual mating opportunities, this may give them a “head start” on breeding. Likewise, female Oystercatchers in bisexual (as well as heterosexual) trios may have an advantage in acquiring their own breeding territories and heterosexual mates in subsequent years.²²