Biological Exuberance: Animal Homosexuality and Natural Diversity

Mills’s final results were conclusive: “Females which were bisexual during their life produced 14 percent fewer chicks than females in exclusively male-female pairings.” ²⁵ Furthermore, fewer of those chicks went on to join the breeding population as adults: exclusively heterosexual birds raised chicks who survived to breed at a rate that was more than one and a third times higher than that of bisexual females. Nor was the lower overall reproductive output of bisexual females due to their participation in (potentially less productive) homosexual pairings at some point in their life: such females also “tended to be less successful breeders even with male partners.”²⁶ It would be difficult to find a more definitive or better-documented refutation of the bisexual-superiority hypothesis. Not only do bisexual females hatch and raise fewer chicks than heterosexual females, they also contribute fewer offspring to the pool of breeding individuals in the population, and their decreased reproductive output appears to be independent of whether they happen to be breeding with a male or a female partner.

One criticism that has been leveled at the bisexual-superiority hypothesis is that it is so difficult to test, and a number of scientists have even remarked that they cannot imagine a relevant experiment or study that could possibly verify or falsify its claims.²⁷ Amazingly, although it has all of the elements needed to evaluate the bisexual-superiority hypothesis, Mills’s study was not specifically designed to test this proposal, nor even to focus on the reproductive performance of bisexual animals in particular. Indeed, it is doubtful that Mills was even aware of this hypothesis—it had yet to be formulated at the time he initiated his project in 1958, and it was not widely known or discussed in the scientific community even after it had been published and revised in various forms over the next 30 some years.²⁸ Nevertheless, the procedures and analyses Mills used were almost tailor-made to assess the validity of this hypothesis, and it is a testament to his expertise that his results should prove useful for a line of inquiry so far removed from their original purpose.

Unfortunately, studies of a similar scale and quality have yet to be undertaken for most other relevant species. Nevertheless, although it is possible that different patterns of reproductive performance across sexual orientations may be revealed in other animals, this is unlikely. Most reports of same-sex parenting and/or breeding in other species appear to be in line with the Silver Gull results.²⁹ Notwithstanding the Black Swan case (to which we’ll return shortly), animals in homosexual pairs who also reproduce are generally only as successful or less successful than heterosexual parents in raising offspring, not more successful. Moreover, in a number of instances homosexual activity on the part of breeding animals actually interferes with their reproductive performance: in female Jackdaws, Oystercatchers, Canada Geese, and Calfbirds, for example, homosexual associations may in fact be detrimental to the successful raising of offspring, often by interfering with incubation (these examples will be discussed more fully later in this chapter). Same-sex activity in Buff-breasted Sandpipers often discourages heterosexual mating and breeding opportunities, while male Cheetahs living in bonded pairs or trios often disrupt, compete with, or prevent their companions from mating heterosexually (and thereby reduce their reproductive output).³⁰ Although differential breeding success can be associated with sexual variance in some species, typically transgendered rather than bisexual (or homosexual) individuals are more reproductively successful (as in the examples of Northern Elephant Seals, Red Deer, Black-headed Gulls, and Common Garter Snakes discussed in the preceding chapter).