Biological Exuberance: Animal Homosexuality and Natural Diversity

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.⁶⁵ Even 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).⁶⁶ Apparently 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.⁶⁷ Significantly, this interpretation has not generally been afforded homosexual pairing.

Fourth, the evidence for homosexual pairing as a breeding strategy is slim. According to scientists, females bond with same-sex partners in order to raise young that result from copulation (but not pairing) with males (since two-parent care is generally required in these species). However, only a relatively small proportion of females in homosexual pairs actually mate with males and lay fertile eggs: 0–15 percent of Western Gull eggs belonging to female pairs are fertile, while only 4–30 percent of Herring Gulls’ are fertilized, indicating that few such females are actually breeding.⁶⁸ Most importantly, females that could potentially benefit from same-sex pairing—were it a reproductive strategy—do not generally “avail” themselves of this option. Researchers found that unpaired Herring Gull females that copulate with males do not in fact go on to form homosexual pairs in order to raise any resulting offspring, nor do they even try to form such pairs. Likewise, Ring-billed Gull, Western Gull, and Roseate Tern mothers that have lost their male partners (and are otherwise unable to find another male) do not establish same-sex pair-bonds with available females, even though they supposedly need to find a new mate to assist them with parenting. In addition, some unpaired and homosexually paired Ring-billed females may actually lay eggs in the nests of other (heterosexual) pairs. This shows that: (a) single females need not seek pair-bonding (with birds of either sex) in order to have their young raised by two parents, and (b) at least some females in homosexual pairs lay eggs that they have no intention of caring for themselves.⁶⁹