Biological Exuberance: Animal Homosexuality and Natural Diversity

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