Bruce Bagemihl - Biological Exuberance

Of all the scientists who have advocated a shortage explanation for homosexuality, not one has ever specified a critical sex ratio that will consistently “induce” homosexuality, or a crucial threshold of members of the opposite sex that must be present in order to unfailingly prevent individuals from “resorting” to homosexuality. Is a mere 5 percent surplus of one sex enough to tip the scales? Apparently, since a population of Ring-billed Gulls with only 55 percent females is claimed to have enough of a skew to “cause” homosexual pairing. Yet a 5 percent excess of males in other species such as Greylag Geese is apparently not sufficient to “precipitate” homosexual pairing. 56In fact, it is highly unlikely that a single critical sex ratio could ever be specified, because the proportion that “causes” homosexuality in one species (or population) has no such effect at all in other species, even where enormous “surpluses” (of, say, 80 percent or more of one sex) are concerned. More broadly, the underlying assumption behind the shortage hypothesis—that sex ratios actually determine a species’ mating habits and social systems—has already been shown to be false for other types of mating behaviors. Scientists now recognize that there is not a clear, one-way causal relationship between how many males or females are available in a population, and the form that their mating system takes (e.g., polygamy as opposed to monogamy). Rather, a complex interplay of many factors is at work. 57Unfortunately, the subtlety of this interaction is generally only recognized where heterosexual mating systems are concerned.

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. 58This 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. 59Let’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). 60Let’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. 61Second, 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. 62A 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. 63It 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

Regarding the third link in this proposed chain: skewed sex ratios do not in fact automatically result in homosexual pairs—some populations of Western and Herring Gulls with a disproportionate number of females, for example, have few (if any) same-sex pairs. 65Even in populations that do have a surplus of females, only a subset of the birds actually form homosexual pairs: most unpaired female Herring Gulls remain single (lesbian pairs constitute less than 3 percent of all pairs, and sometimes as few as 1 in 350), and some males remain unpaired even in populations with more females than males (indicating that some “extra” females are bypassing heterosexual mates). Granted, scientists were able to “induce” the formation of female pairs in a population of Ring-billed Gulls by removing males. However, this simply demonstrates that many females in this species have a latent bisexual capacity that manifests itself when males are in short supply—not that all same-sex pairs in this (or any other species) result from a shortage of the opposite sex. Moreover, the sex ratio that was required to “trigger” homosexual pairing (77 percent females) was much higher than the proportion of females in naturally occurring populations with homosexual pairs (55 percent females). 66Apparently not all bird species have this latent bisexual capacity either (or at least not to the same extent), since homosexual pairing does not occur in most species that have their sex ratios experimentally manipulated. Removing males (or adding females) to populations of willow ptarmigans, bufflehead ducks, Pied Flycatchers, great tits, Brown-headed Cowbirds, and song, seaside, and savanna sparrows, for example, results in females mating polygamously with males or remaining single (among other strategies) rather than forming same-sex pairs. When alternate heterosexual behaviors such as polygamy are “induced” in usually monogamous species such as these, scientists do not interpret this as evidence that the behavior is somehow “artificial” or that its occurrence is due solely to the experimentally triggered demographic changes. Rather, it is taken to indicate that the species possesses an inherent capacity for polygamy (and more broadly, a flexibility of mating behavior), perhaps expressed at relatively low levels in most populations but manifesting itself on a larger scale under the appropriate conditions. 67Significantly, this interpretation has not generally been afforded homosexual pairing.