Bruce Bagemihl - Biological Exuberance

However, one scientist—James A. Mills—has conducted exactly this sort of long-term, comprehensive study on the Silver (Red-billed) Gull in New Zealand, a species in which there is extensive bisexuality and homosexuality. His results show that bisexual individuals are in fact significantly less successful breeders than heterosexual ones. Over more than 30 years, Dr. Mills and his colleagues banded over 80,000 individual gulls, tabulating detailed lifetime reproductive and sexual profiles of more than 5,000 of these. Because of the enormity of this project, special computer programs had to be developed to analyze and keep track of all the data. The Silver Gull is an ideal species in which to test this hypothesis, because the sexual orientation of females (in terms of their pairing behavior) falls into three clear-cut categories: some form only homosexual pairs during their entire lifetimes and hence are exclusively lesbian, while others have both same-sex and opposite-sex partners during their lives and are therefore unequivocally bisexual, while other females only pair with male partners and thus are exclusively heterosexual. 24Moreover, Mills and his team looked not only at how many chicks were hatched and raised by heterosexual versus bisexual (and homosexual) individuals, but also at how many of those chicks survived to adulthood and became breeders themselves—the true measure of whether an individual is actually passing on his or her genes.

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.” 25Furthermore, 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.” 26It 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. 27Amazingly, 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. 28Nevertheless, 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. 29Notwithstanding 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). 30Although 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).

There are further arguments against the bisexual-superiority hypothesis. If bisexual animals were more successful breeders, one would expect them to make up the majority of the population in any given species, with much smaller proportions being exclusively heterosexual or homosexual—yet the distribution of sexual orientations does not, in fact, typically follow this pattern. In Silver Gulls, heterosexual versus bisexual percentages are in accord with what we have just seen about their relative reproductive proficiencies: 79 percent of all females are exclusively heterosexual, 11 percent are bisexual, and 10 percent are exclusively lesbian. This pattern is characteristic of many other species for which we do not have information about the lifetime reproductive output of a cross-section of individuals: bisexual animals generally make up a much smaller percentage of the population, sometimes even less than the proportion of exclusively homosexual individuals. For example, the heterosexual-bisexual-homosexual proportions for male Black-headed Gulls are 63-15-22 percent, respectively, and for Galahs, 44-11-44 percent. 31In many other species the proportion of animals who engage in bisexual activity is even smaller.

Moreover, in some cases there do not appear to be any bisexual individuals at all in a population (i.e., same-sex activity occurs only in nonbreeding animals). For example, female homosexual pairs in Kittiwakes, Red-backed Shrikes, and Mute Swans, among others, appear to consistently lay infertile eggs (indicating that they do not mate with males); in Pied Kingfishers, homosexuality is typical of nonbreeding birds who are not likely to reproduce later in life; while male Ostriches who court other males do not appear to have heterosexual relations. Although longitudinal studies are needed in each case to verify that such individuals are not in fact sequentially bisexual, these patterns do not fit well with a bisexual-superiority hypothesis. More broadly, species in which homosexuality or bisexuality is only found in individuals of one sex—or in which all individuals are exclusively heterosexual—are extremely common and are further evidence against this hypothesis (since they are examples of bisexuality failing to be “maximized”).

What about species in which the majority of individuals are bisexual, i.e., the examples of maximization of bisexuality mentioned above? In all of the animals in which this is the case (Bonobos, Dolphins, Mountain Sheep, etc.), individuals differ significantly in the degree to which they are bisexual. Some animals participate very little in homosexual and/or heterosexual activity while others account for the majority of (one or both) such activities, and same-sex versus opposite-sex interactions make up varying proportions of each individual’s sexual encounters as well (as we saw in chapter 2). Thus, if bisexuality were related to reproductive success, one would expect animals to differ depending on “how bisexual” they are—successful breeders (i.e., those animals who are the most active heterosexually) should engage in a greater proportion of homosexual activity as well. Again, long-term studies of reproductive output are required to test this, but data on the sexual activity of individual animals in a number of species where bisexuality is widespread do not support this idea. If we take the number of heterosexual copulations that an animal participates in to be a rough measure of its reproductive prowess, then we do not generally find that there is a positive correlation between degree of bisexuality and breeding success.