Biological Exuberance: Animal Homosexuality and Natural Diversity

The shortage hypothesis is not only suspect on theoretical grounds, it is often applied to particular cases in a hasty or inconsistent fashion. Skewed sex ratios in animals exhibiting same-sex activity are often presumed without adequate supporting evidence, or else questionable “explanations” are proposed for the origin of such skewed ratios.⁵⁸ This is best illustrated by the species in which the shortage explanation is most prominent: Gulls. In the late 1970s and early 1980s scientists noticed that high levels of DDT and other environmental contaminants seemed to be associated with some populations of Western and Herring Gulls where nests contained supernormal clutches (often belonging to lesbian pairs). The following chain of “causation” was proposed to explain the apparent correlation: toxins (such as DDT) cause “feminization” of male Gull embryos, which in turn leads to female-biased sex ratios, which in turn results in lesbian pairs, who then attempt to breed, ultimately laying supernormal clutches.⁵⁹ Let’s set aside for the moment the fact that this explanation is only of limited applicability—homosexual pairing is not associated with environmental toxins in over 70 other bird species, including several Gulls (e.g., Ring-billed Gulls, Common Gulls, and Kittiwakes).⁶⁰ Let’s also set aside the fact that it is only of limited explanatory value—even if it could be shown conclusively that same-sex pairing results from skewed sex ratios that in turn result from toxins, the fact that only some species (and only some individuals in each species) respond to such conditions with homosexuality would still need to be addressed. Even for the Gull species where this explanation is supposedly relevant, however, each link in the overall chain is weak.

First, although laboratory experiments have shown that some toxins may cause male bird embryos to develop some ovarian tissue, no “feminized” male chicks or adults have actually been found in the wild among Western Gulls (or other species) living in contaminated areas.⁶¹ Second, it is unlikely that toxin-induced feminization of males would result in populations with more breeding females than males (since it is most definitely not the case that toxins actually “convert” male embryos into female birds with fully functional ovaries). It would have to act either directly on the health of males, causing more deaths, or indirectly, by resulting in behavioral changes in males that would prevent them from mating with females. But there is no direct evidence that toxins cause anything beyond some physiological alterations in the reproductive organs of Gulls.⁶² A higher mortality rate among males exposed to toxins has never been demonstrated, nor have behavioral differences among such males been observed that might lead them to forgo mating or otherwise to be “unavailable” as mates.⁶³ It has been suggested that “chemically sterilized” males simply fail to join the breeding colonies, or that such males are “no longer interested” in copulating heterosexually. Yet this begs the question of how or why sterility (or other physiological modifications) causes such males to exempt themselves from reproductive activities, or what exactly prevents them from pairing (or even copulating) with females even if they are sterile. Just because an animal is intersexed or transgendered (e.g., “feminized”) does not necessarily mean that it is asexual or that its reproductive organs or behavior are “dysfunctional.” “Masculinized” female Deer, Bears, and Spotted Hyenas, for example, regularly mate with males, give birth, and raise offspring—even though such individuals often have highly modified reproductive anatomies and hormonal profiles. Even when they are sterile, transgendered and intersexual animals in other species engage in courtship, copulation, and/or pair-bonding. Thus, it is overly simplistic to equate changes in reproductive physiology with an absence of sexual, pairing, or even procreative abilities. It should also be pointed out that a skewed sex ratio is not necessarily an “unnatural” result of environmental contamination: many Gull populations are in fact “naturally” biased in favor of females independently of the effects of toxins, owing to the overall higher survival rate of females (among other factors).⁶⁴