Biological Exuberance: Animal Homosexuality and Natural Diversity

In many animals where only a subset of the population resembles the opposite sex (or is transgendered), the occurrence of homosexuality is often directly counter to what would be expected if confusion between the sexes were “causing” homosexual behavior. For example, adolescent male Scottish Crossbills resemble females in their plumage coloration, yet homosexual pairs in this species form between adult males, not between adult and juvenile males. In Ruffs, some males resemble females in that they lack the elaborate neck feathers and other distinctive plumage characteristics of other males, yet homosexuality in this species is not limited to these “naked-nape” males. Males who do not resemble females also court and mount each other, while “femalelike” males often mount more “masculine” males. Tree Swallows are unusual among North American perching birds in that females retain the drab gray-brown plumage of adolescence during the first year that they breed, making them resemble adolescent males more than adult females. Thus, one would expect that either (a) adult males would be more apt to “mistake” brown-plumaged females for males, perhaps responding more aggressively to them (i.e., as if they were males); or (b) homosexuality in this species would manifest itself as an age-based system, with males pursuing only younger brown-plumaged males because they “mistake” them for first-year breeding females. Neither of these scenarios is true, however: males have no trouble recognizing the sex of brown-plumaged females (and in fact are significantly less aggressive toward them), and homosexuality in this species involves adult males interacting with each other, not adults being “confused” by brown-plumaged males.⁸⁴

Black-headed Gull males and females are nearly identical in appearance, except that males have, on average, slightly longer heads and bills than females. However, some males are more “femalelike” in that they have shorter head and bill lengths than average. If sex misrecognition were operative in this species, one would predict that smaller males (i.e., birds who more closely resemble females in size) would be more likely to form homosexual pairs (since males would “mistake” them for females) and less likely to form heterosexual pairs (since females would “mistake” them for other females). On the contrary, scientists studying sex recognition in this species found that female-resembling males are just as likely to form heterosexual as homosexual pair-bonds. In fact, smaller males are more successful at maintaining long-lasting heterosexual bonds and fathering chicks than more “masculine”-appearing males—paralleling other cases of greater heterosexual prowess in some transgendered animals.⁸⁵