One of the most prevalent myths about animal homosexuality is that it is invariably caused by a shortage of members of the opposite sex. This is typically attributed to skewed sex ratios in the population (more males than females, or vice versa), or the unavailability of opposite-sex partners due to sex segregation, hostility or indifference on the part of potential mates, or other factors. This belief is widespread among nonscientists and is also the most common “explanation” that biologists have proposed for the occurrence of homosexual behavior in animals. In more than 65 species of mammals and birds, for example, same-sex activity is claimed by zoologists to result from individuals being “unable” to mate heterosexually. Sometimes this is attributed to a predominance of one sex over the other in wild or captive populations: the formation of lesbian pairs in Australian Shelducks and Ring-billed Gulls, for instance, is supposedly “caused” by an excess number of females (65 percent females in Shelduck populations, 55 percent females in Ring-billed Gulls).
Homosexual pairs in Mute Swans occurring in populations with unbalanced sex ratios are said to be “examples of the lengths to which deprived creatures will go to satisfy their natural urge to reproduce.” In some cases, homosexual behavior is labeled a “substitute” for heterosexuality or “redirected” heterosexual behavior, resulting from a variety of factors. For example, it is claimed that individuals are “prevented” from mating with (or otherwise having access to) the opposite sex by other (often higher-ranking) animals, or by the overall social organization (e.g., in Mountain Sheep, Bottlenose Dolphins, or Killer Whales). Alternatively, it has been suggested that individuals resort to homosexuality when their heterosexual advances are met with refusal or disinterest (e.g., in White-handed Gibbons, West Indian Manatees, Asiatic Elephants). In a few cases (e.g., Hanuman Langurs, Lions, Sage Grouse) scientists have even suggested that females turn to one another because they have not been “satisfied” or received enough attention from male partners—a version of the widespread stereotype about the “cause” of lesbianism among people.34
The line of reasoning in “explanations” such as these is curious, since it implies that unless there is an adequate supply of the opposite sex, homosexuality will inevitably ensue. This is actually an unintentional assertion of the relative strength of the homosexual urge, or correspondingly, the relative weakness of the heterosexual imperative—for the stronghold of heterosexuality must be tenuous indeed if such factors are capable of upsetting the balance. Besides this, however, unavailability of the opposite sex—what we will call the shortage hypothesis—is simply incompatible with the facts.
Surplus Homosexuality
The shortage hypothesis cannot be a universal explanation for animal homosexuality because of the many examples of animals engaging in same-sex activity when opposite-sex partners are freely available.35 In Orang-utans, Japanese Macaques, Stumptail Macaques, Rhesus Macaques, Common Gulls, Black-headed Gulls, King Penguins, Galahs, and more than 40 other species, scientists have documented individuals either ignoring opposite-sex partners and seeking out same-sex partners instead, or else engaging in homosexual activity more or less concurrently with heterosexual activity (i.e., even when opposite-sex partners are accessible, as already mentioned in the discussion of simultaneous bisexuality).36 In fact, in a surprisingly large number of species, homosexual activity is positively correlated with heterosexual activity: same-sex interactions actually increase as animals gain access to opposite-sex partners and decrease in their absence. This is the exact reverse of what would be expected if homosexuality resulted from a lack of access to heterosexual mating opportunities.
Homosexual activity among male Bottlenose Dolphins in captivity, for instance, actually declined when females were removed from their tank, while aggressive interactions between the males increased. Conversely, female Squirrel Monkeys in one study engaged in virtually no homosexual activity when kept in same-sex groups, yet showed significant rates of homosexual mounting and other activities (along with heterosexual behaviors) when males were introduced into their group. Another study of this species found that females with the most attention from heterosexual partners also engaged in the most homosexual pursuits. In Bonobos, Stumptail Macaques, Savanna (Yellow) Baboons, and West Indian Manatees, same-sex activity is often stimulated by opposite-sex activity (and vice versa), with the result that sessions may involve both heterosexual and homosexual encounters among multiple participants. Homosexual mounting in Pukeko is most prevalent in breeding groups that have the greatest amount of heterosexual activity, while homosexual mounts in Common Murres become more common as promiscuous heterosexual mounts also increase in frequency (although the latter may, ironically, result from a decrease in available females). In some species, individuals that participate in the most heterosexual matings may also engage in the most homosexual ones, as in Sociable Weavers and Bonnet Macaques. Conversely, animals that are the least active heterosexually are often the least active homosexually. In Ruffs, for example, the class of males who do not generally mate with females (known as marginal males) also rarely participate in homosexual matings, while in one study of Japanese Macaques, the only female who did not consort with any other females also failed to consort with any males.37 Finally, in a number of species such as Swallows, Laysan Albatrosses, and Herons, same-sex mounting occurs primarily among breeding individuals (i.e., those who already have heterosexual mates) and is largely absent among nonbreeders.
A number of species do have skewed sex ratios, but (like dominance) this is neither a sufficient nor a necessary prerequisite for the occurrence of homosexuality in a population. Male homosexuality is not reported for red-winged blackbirds or giant cowbirds, for example, even though some populations are 80-84 percent male, nor for pintail duck populations with two-thirds males, or kiwis with 58 percent males, or purple finches with 57 percent males. Likewise, female homosexuality is absent in boat-tailed grackles even though males may comprise only a third of the population, and in sparrow hawks, where there is also a “surplus” of females (less than 40 percent males). In contrast, homosexuality occurs in numerous species or populations that have equal (or nearly equal) sex ratios, including Bonobos, Bonnet Macaques, West Indian Manatees, Snow Geese, California Gulls, and Pukeko.38 Moreover, closely related species or different populations of the same species that have identical (or similar) sex ratios and forms of social organization often exhibit strikingly different patterns of homosexuality. Many Seals and Sea Lions with polygamous mating systems, for instance, have strongly female-biased sex ratios (three to five females for every male) and social systems that often include sex segregation and/or exclusion of large numbers of males from breeding opportunities. Some of these species exhibit male homosexuality (e.g., Gray Seals, Northern Elephant Seals, Walruses), others have female homosexuality (e.g., Northern Fur Seals), some have both (e.g., Australian Sea Lions), while the majority have no homosexuality at all (e.g., California sea lions, southern fur seals). Likewise, lunulated antbirds, salvin’s antbirds, Bicolored Antbirds, and Ocellated Antbirds all live in populations that have an excess of males, yet homosexual pairing is only found in the latter two species.39
In many animals that have skewed sex ratios, homosexuality only occurs (or is more common) in the sex that is in shorter supply rather than in the “surplus” sex. In some populations of Crab-eating Macaques, for example, females outnumber males by more than two to one, yet same-sex activity only occurs among males. Female homosexuality accounts for more than 80 percent of same-sex activity in Pukeko even though some populations are more than 70 percent male. The reverse is true for Rhesus Macaques: in some populations females outnumber males nearly three to one, yet the majority of same-sex activity (over 80 percent) is between males. Tree Swallow populations often have a surplus of females, but only male homosexuality occurs. Likewise, female pairs have formed in captive populations of Galahs and Scarlet Ibises that have an excess of males, while male pairs of Flamingos are reported from populations that have more females than males. In Nilgiri Langurs, there is a female-biased sex ratio in the overall population (and individuals live in groups with more females than males), yet only male same-sex activity is reported. Finally, in Little Egrets and Little Blue Herons there is a “surplus” of unpaired males, yet same-sex mounting occurs almost exclusively among paired males rather than in the population of birds that are unable to find heterosexual mates.40