Biological Exuberance: Animal Homosexuality and Natural Diversity

The fact that homosexual relations are usually prohibited between related individuals also points to the importance of nonbiological factors in the incest taboo. In most human cultures that “permit” some form of same-sex eroticism, from contemporary America to indigenous tribes of New Guinea, the choice of homosexual partners is subject to distinctions of “kin” and “nonkin.” This is in spite of the fact that no children, and hence no potentially harmful genetic effects, can result from such unions. Typically the same restrictions are applied to homosexual as to heterosexual relations. In a number of New Guinean societies, however, slightly different kinship constraints regulate the choice of same-sex and opposite-sex partners. In fact, homosexual partners in some tribes are actually required to be more distantly related than heterosexual ones—the exact opposite of what would be expected if incest taboos were based solely on biological factors.⁶⁶

This is significant, because most theories about the biological basis of the incest taboo focus on the potential for increased rates of birth defects and lower genetic variability as a result of inbreeding. Even for heterosexual relations, though, the evidence is not nearly as unequivocal as one would suppose: numerous studies of small populations that have practiced inbreeding for many generations reveal no deleterious effects, owing to the rapid elimination of genetic defects and subsequent stabilization of the gene pool.⁶⁷ To adduce further evidence for a biological basis to the prohibition, scientists often point to the existence of “incest taboos” in animals. Ironically, though, many animal species actually show evidence of a “cultural” or “social” dimension to their avoidance of sexual activity between relatives that parallels the human examples—most notably among primates, and most notably involving homosexuality.

There is a great diversity of incestuous activity among animals, not only in the frequency and types of relations that occur but also in the degree to which such activity is avoided or pursued, in both heterosexual and homosexual contexts. Even among primates, many different scenarios and versions of “taboos” are found. In Rhesus Macaques, for instance, incest of any sort is not common although mother-son, brother-sister, and brother-brother relations do occur (some males actually appear to prefer mating with their mothers). In Gibbons, heterosexual incest (both parental and sibling) is sometimes practiced and homosexual relations are almost always incestuous, while both heterosexual and homosexual activity between siblings (or half-siblings) occurs in Gorillas. Most strikingly, several species appear to have developed systematic homosexual “incest taboos,” each with its own socially defined set of “acceptable” and “unacceptable” partners. In some cases, these restrictions differ significantly from those governing the corresponding heterosexual relations (as in some human populations).

Homosexual consortships (pair-bonding with sexual activity) among Japanese Macaques, for example, virtually never occur between mothers and daughters or sisters. In contrast, heterosexual brother-sister or mother-son relations, while not common, are much more prevalent than homosexual incest in this species. Interestingly, aunts and nieces among Japanese Macaques do not generally recognize each other as kin—when intervening on behalf of individuals during aggressive encounters, for example, aunts do not assist their nieces any more often than do nonrelatives, and significantly less often than do mothers, grandmothers, and sisters. Consequently, some blood relatives are able to form consortships with one another: about a third of all aunt-niece dyads interact homosexually. In other words, Japanese Macaques have an overall pattern of incest avoidance unique to homosexual relations, within which apparently incestuous aunt-niece pairs are “permitted” because such partners do not count as kin in the larger social framework.

In Hanuman Langurs, both heterosexual and homosexual incest taboos are in effect, but with slightly different restrictions. Heterosexual incestuous relations of any kind are generally avoided; sexual activity between mothers and daughters is also “prohibited” (accounting for only about 1 percent of all homosexual mounting). In contrast, half sisters (females with the same mother but different fathers) are “allowed” to have sexual relations with one another—and in fact, more than a quarter of all mounts between females occur between half sisters. In Bonobos, incestuous relations between females also generally appear to be avoided: when females immigrate into new troops as adolescents, they are usually unrelated to most of the other troop members, but sexual activity is not practiced with those females to whom they are related.⁶⁸