Biological Exuberance: Animal Homosexuality and Natural Diversity

One particularly common version of the shortage explanation is that males turn to homosexuality after being prevented from mating with females, due to the active interference or inhibiting presence of higher-ranking males. This is often suggested for mammals with polygamous or promiscuous mating systems in which only relatively few males ever get to mate heterosexually. While this may contribute to same-sex activity in some species (e.g., Mountain Sheep, Northern Elephant Seals), there is considerable evidence that this explanation is overly simplistic. Homosexual mounting in American Bison, for instance, is especially common among younger males that do not breed (ages one to six), but this cannot be attributed solely to their being “denied” access to females by older, higher-ranking males. Same-sex activity is much less common among bulls four to six years old than it is in males one to three years old, even though both groups are “prevented” (or abstain) from engaging in breeding activities. Also, studies of populations in which older bulls have been removed (in order to give younger males more “access” to females) show that although heterosexual activity increases among the younger bulls in the absence of the higher-ranking males, so does homosexual activity. In fact, more than 55 percent of mounts in such groups are still between males even though there are no older bulls to “prevent” the males from mating heterosexually (and even though females outnumbered males in such populations). Likewise, as they are growing up, male Bonobos experience a sharp drop in their access to heterosexual partners that is not reflected in a corresponding increase in their homosexual activity. As infants and juveniles they are very sexually active with mature females, but once they reach adolescence, they are generally prevented by older males from interacting sexually with females. However, their participation in same-sex activity increases steadily throughout this period rather than rising sharply when female partners become “unavailable” (or dropping when they are once again available). In fact, homosexual activity reaches its maximum level during adulthood when heterosexual activity also peaks.⁵¹

Although younger male Musk-oxen may be excluded from heterosexual mating opportunities by older males, this does not “cause” the homosexual activity that occurs in this species. Courtship and mounting activity between males generally takes place in the breeding (harem) herds and is initiated by adult bulls toward yearlings—in other words, by the males who do have access to females. Most other adult males are excluded from breeding as well, yet homosexual behavior is not characteristic of the all-male herds where such bulls often spend much of their time. Additionally, in captive groups lacking older males to “prevent” heterosexual mating, homosexual mounting still occurs at a fairly high rate alongside opposite-sex mounting. Likewise, homosexual behaviors in male Asiatic Elephants occur in both breeding and nonbreeding individuals. When older or higher-ranking males (who usually monopolize female partners) are prevented from mating heterosexually in captive groups, younger or lower-ranking males are then able to copulate with females. However, same-sex activities continue in both age/rank groups regardless of whether they have access to female partners or not. In fact, males who are heterosexually active may exhibit higher levels of homosexual activity than those who do not mate with females. In New Zealand Sea Lions, most younger males are excluded from breeding, but homosexual activity also occurs among adult breeding males. Lower-ranking male Wolves are prevented from mating with the highest-ranking female in the pack, but rather than mating with the available lower-ranking females, such males often mount each other. Finally, although homosexual activity in adolescent Killer Whales is attributed to their exclusion from heterosexual mating opportunities, adult males who have access to females also sometimes participate in same-sex activity. In addition, adolescents spend roughly the same proportion of their time as do adults engaging in sexual and related social behaviors with females.⁵²