Biological Exuberance: Animal Homosexuality and Natural Diversity

proportion of animals were exclusively homosexual. Most animal populations can and do support large numbers of nonbreeding individuals without suffering a decrease in numbers: indeed, in many species a majority of individuals do not reproduce without any adverse effects on the population as a whole. In Damaraland mole-rats, for example, 90–98 percent of all individuals never breed during their lifetime, yet the population sustains itself and even continues to grow. Scientists have also calculated that a stable Killer Whale population can include up to 30 percent nonreproducing females without experiencing any decline. A significant pool of nonbreeding individuals exists in many other species, and up to 90 percent or more of one sex may fail to mate and/or breed.¹⁴ Thus, exclusive homosexuality on a much more massive scale than that seen in any species would have to occur before homosexuality could even begin to impact on population growth and size.

A number of animals experience periodic and often dramatic fluctuations in their numbers, sometimes undergoing regular five- or ten-year cycles of population increase and decrease—for example, snowshoe hares, lemmings, voles, and some species of finches, sandpipers, falcons, and grouse.¹⁵ If homosexuality were correlated with population size, one might expect that it would feature prominently in such species. One might also predict that its occurrence would “shadow” or fluctuate along with the population cycles, becoming more prevalent when population size or growth rate reaches its maximum, and less prevalent or nonexistent when the population is at its ebb. In fact, homosexual behavior has not been reported for most such species, and in the few cases where it has—Scottish Crossbills, Kestrels, and Grouse, for example—it does not appear to be related to either the cyclic or the irregular population increases (“eruptions,” as they are sometimes known) that occur in these species.¹⁶

Similarly, if homosexuality actually resulted in a significant decrease in population growth, one might expect it to be disproportionately represented among animals that are suffering a severe decline in numbers, i.e., in endangered species. However, of the 2,203 mammals and birds in the world that are currently classified as threatened (either critically endangered, endangered, or vulnerable), homosexuality has been documented in just over 2 percent of these.¹⁷ Moreover, the distribution of homosexuality across different species clearly has nothing to do with their endangered status: there are examples of two closely related species, such as the Pukeko and the takahe—two birds of New Zealand—in which homosexuality only occurs in the nonendangered one (the Pukeko); or animals in which one subspecies is endangered (e.g., the Asiatic Lion) yet homosexuality is not restricted to this subspecies; or else cases in which one or more subspecies are threatened (e.g., the Baja California and Sonoran Pronghorns), yet homosexual behavior is found in the nonthreatened subspecies of the same animal (the American Pronghorn); or two closely related species, in one of which homosexuality is common yet the species is not endangered (Hanuman Langur), the other in which homosexuality is much less common but the species is threatened (Nilgiri Langur). Conversely, if homosexuality were a form of self-preservation for a species as a whole—a “safety-valve,” as it were, activated in times of overpopulation—one would not expect to find it at all in animals suffering severe population declines. Nevertheless, same-sex activity is reported in at least 50 endangered species. Perhaps the most dramatic example is the nearly extinct Black Stilt: less than 50 of these birds are left in the wild, yet some individuals still form lesbian pairs.¹⁸

Animals are perfectly capable of “regulating” their population size with far more efficient and effective strategies than homosexuality. A wide variety of mechanisms for reducing density and/or growth rates have been documented, including emigration, stress-induced hormonal changes that inhibit reproduction, decreased fertility, delayed maturation or slowed development, infanticide, and cannibalism (not to mention “outside” checks on population size such as predators).¹⁹ In summary, then, it appears that homosexuality is neither useful to the species as a way of controlling population growth, nor useful to individual families as a mechanism whereby breeding animals are supplied with nonbreeding “helpers.”

Bisexual Superiority and the Genetics of Homosexuality

In attempting to argue for the evolutionary value of homosexuality, scientists are confronted with an apparent paradox: if homosexuality is a valuable trait, it should have a genetic basis—yet how can a gene that doesn’t lead directly to reproduction continue to be passed on from one generation to the next? Perhaps, some have suggested, because the putative gene for homosexuality does not operate on its own, but rather is acting in tandem with another gene to promote reproduction. An often-cited analogy involves the genetics of sickle-cell anemia and malaria resistance in humans. People who receive a sickle-cell gene from one parent and a regular hemoglobin gene from the other parent are resistant to malaria; those who receive two sickle-cell genes (one from each parent) succumb to sickle-cell anemia, while those who receive two regular hemoglobin genes are more likely to succumb to malaria. Thus, genes that (on their own) can potentially decrease an individual’s reproductive capacity continue to be passed on because they are beneficial when combined with each other. Scientists have suggested that this might also be the case with homosexuality, as follows: Suppose there were one gene that predisposed an individual to homosexuality, and another that predisposed an individual to heterosexuality. Those individuals who receive two homosexual genes (one from each parent) would be exclusively homosexual; others would receive two heterosexual genes and be exclusively heterosexual; while those receiving one of each would be bisexual. If individuals who have one homosexual and one heterosexual gene were somehow more successful at reproducing, then the gene for homosexuality would confer an advantage and would continue to be passed on, even though it would sometimes result in individuals who do not reproduce (those who receive a homosexual gene from each parent).²⁰

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