Further evidence that animals can differentiate between males and females that appear identical to us comes from the different frequencies of homosexual and heterosexual interactions in species with “indistinguishable” sexes. Animals often preferentially court, mate, or bond with individuals of one or the other sex. Male Mountain Goats. for example, court male yearlings more frequently than female yearlings even though they are supposedly “unable” to differentiate between the two. The opposite scenario occurs in Musk-oxen: although adult males court both yearling males and females, they interact with females more than with males. Likewise, Dwarf Cavy adult males court juvenile males more often than they do juvenile females (and even seek out specific male partners). In contrast, adult males in the closely related Aperea court only juvenile females and never males, even though in both of these species juvenile males and females are purportedly indistinguishable. Among Bighorn Sheep, rams are claimed to be sexually interested in other males in direct proportion to how closely the latter resemble females—yet yearling males, which resemble females the most, still receive far less sexual attention than do females, indicating that some form of sexual differentiation still occurs. Although male Common Murres are said to mount other males because they have difficulty distinguishing the sexes, females are still mounted at a much higher rate than males. Supposedly “indiscriminate” sexual chases by male Flamingos actually involve many more pursuits of females than males. Finally, adult male Pronghorns court and mount yearling and two-year-old males, both of whom superficially resemble females. However, adults actually direct more sexual behavior toward two-year-olds, who are as “femalelike” as (if not more “malelike” than) yearlings in terms of the size of their horns and black cheek patches.82
A related argument against sex misrecognition as a factor in precipitating homosexuality is that males and females are often behaviorally distinct even when they are physically identical. A male who “looks like” a female will frequently perform identifiably male behavior patterns during a homosexual interaction, seriously casting doubt on the notion that his partner has failed to recognize his actual sex. Male Antbirds “mistaken” for females actually initiate and reciprocate courtship feeding with their male partners (something females never do). Younger male Swallow-tailed Manakins and Regent Bowerbirds that participate in courtship with adult males may physically resemble females, but they exhibit distinctly “masculine” behaviors, displays, or vocalizations. Among male Greenshanks, both participants in homosexual copulations display in a typically male fashion prior to same-sex mounting, and both female partners in Jackdaw homosexual pairs preen each other (a typically female activity). There is hardly a more identifiably “male” activity than copulating with a female, yet male Laughing Gulls sometimes make sexual advances toward males who are in turn mating with their female partners (creating a three-bird “pile-up”). Nor are such homosexual mounts simply attempts by the topmost male to mate with the female, since he often remains mounted on the other male (or continues to remount him repeatedly) even after the female becomes “available” once her partner dismounts. Conversely, there is hardly a more definitively “female” activity for birds than laying eggs, yet male Black-headed Gulls have been observed bypassing females in the very act of laying an egg in order to try to copulate with her male partner!83 It seems highly unlikely that homosexual activity in a case such as this is due to faulty sex recognition (especially since heterosexual copulation attempts on laying or incubating females are fairly routine among Gulls), yet this is a prominent “explanation” for same-sex behavior in this species.
Deceptively Clear
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
Other species in which both transgender and homosexuality occur are particularly cogent examples of how ineffective sex misrecognition is in “explaining” homosexuality. Typically, the patterns of same-sex and opposite-sex interactions in these species do not follow the clear divisions that would be expected if individuals were simply “mistaking” their partners for the opposite sex. In Hooded Warblers, for example, some females have transvestite plumage, appearing almost identical to males because of their dark hoods (which are usually found only in males). Others have intermediate plumage, darker or more melanistic than most females but without the complete hood pattern of males, while others have no “malelike” head feathers at all. Males, though, are typically heavier and have longer wings than females, hooded or otherwise. It has been suggested that male homosexual pairs initially form in this species because of the visual resemblance between some females (transvestites) and males. Yet if males in homosexual pairs tended to confuse hooded females with males, one might expect them to pair with individuals whose sex is especially “blurred” or hard to decipher: darker, more malelike females and/or smaller, more femalelike males. However, at least one bisexual male chose just the opposite kinds of mates. His male partner did not have female body proportions but, on the contrary, was exceptionally “masculine” in this regard, exceeding the average weight and wing lengths of most males. Conversely, his heterosexual pairings involved “obviously” female partners, i.e., nontransvestite or only moderately melanistic individuals. Moreover, males that are supposedly mistaken for females in homosexual pairings do not develop brood patches (a distinctive bare patch of skin on the belly used for incubating eggs, characteristic only of females). So it is unlikely that such males are mistaken for hooded females.86 There is also evidence that male Hooded Warblers do not generally confuse transvestite or melanistic females with males. First of all, males are differentially aggressive toward other males, attacking them during territorial encounters more often and ignoring them less often than they do the darkest, most malelike females. Furthermore, “masculine-appearing” (melanistic) females are generally as successful as nontransvestite females in finding male partners and are as subject to promiscuous copulation attempts by males as are nontransvestite females.87 If males tended to confuse hooded females with males, they would probably avoid darker birds (including melanistic females) during heterosexual mating interactions (since such birds would more likely be other males), yet this does not appear to be the case.