Biological Exuberance: Animal Homosexuality and Natural Diversity

In some mammals and birds where homosexuality is attributed to the resemblance between younger males and adult females (e.g., Blackbucks, Manakins, Birds of Paradise), the two sexes are not necessarily identical. Rather, older adolescent and younger adult males exhibit physical characteristics that are actually intermediate between those of adult females and adult males, and they are often recognizably male.⁷⁹ Even in species where homosexuality is claimed to result from the identical appearance of males and females, there are often slight but noticeable physical differences between the sexes that may be discernible to individuals. These include body and horn size in Mountain Goats, wing length in Bishop Birds (with juvenile males distinct from adult females), iris color and other aspects of eye structure in Galahs, relative size and other body measurements in Humboldt and King Penguins, patterning of tail feathers in male and female (and between adult female and juvenile male) Superb Lyrebirds, wing and tail length (and, in some populations, wing feather notching) in Ocher-bellied Flycatchers, presence of a brown forehead patch and shorter wings in female Tree Swallows, and bill structure and tail coloration between adult female and juvenile male Anna’s Hummingbirds.⁸⁰

An adult male Blackbuck courting a younger male by “presenting the throat,” a stylized courtship display. Some scientists have suggested that homosexual activity in this species is triggered by the resemblance between younger males and adult females (e.g., their lighter coat color), yet younger bucks are clearly identifiable as male because of their horns and other anatomical features.

Are these (often subtle) differences actually perceptible to the animals themselves? Implicit in many scientists’ pronouncements of sex misrecognition is the assumption that just because males and females look alike to our eyes, they must be indistinguishable to the animals as well. Species differ widely in their visual acuity, color perception, and other sensory abilities, so each case needs to be evaluated individually before any conclusions can be made about animals’ sex recognition abilities—and this has most definitely not been systematically investigated for cases involving animal homosexuality. Nevertheless, one thing is certain: we are only beginning to understand many aspects of animal perception, including heretofore unimagined powers of visual, acoustic, and temporal recognition. Scientists recently discovered, for example, that a number of birds such as starlings, Zebra Finches, bluethroats, and Blue Tits use ultraviolet vision in distinguishing between individuals and between sexes. Birds that appear identical in ordinary light have different patterns under UV that are recognized and used by other members of their species to choose mates. Likewise, males and females of some butterfly species that are indistinguishable to us have radically different appearances in UV light. In the acoustic and temporal realms, analysis of tape recordings of Lyrebird vocal mimicry has revealed that their perception of time may be ten times greater than that of humans, giving them the extraordinary ability to imitate the calls of five different birds simultaneously.⁸¹ It is quite likely, then, that animals can perceive differences in appearance or other minute sensory cues that are distinguishable only to human measuring instruments and not to human eyes (or ears).