How Sexual Selection Drove The Emergence Of Homosexuality

It can be a sore point for evolutionary biologists who study sexual selection. In the popular coverage of evolution, mate choice too often gets overlooked, in the shadow of natural selection. Yale biologist Richard O. Prum's new book responds to this imbalance.

Prum is William Robertson Coe Professor of Ornithology at Yale. Over the years he has conducted detailed field studies of multiple bird species and their mating habits all around the world. This has given him a broad perspective on sexual selection.

And in his book, due out this week, The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World and Us, Prum outlines his own theory of what he calls aesthetic evolution, driven by male and (mostly) female preferences. In tackling the question surrounding a much broader range of species--including our own, Prum offers some provocative and convincing hypotheses on how and why homosexuality evolved. (I imagine, too, that this has started some lively arguments amongst his colleagues who focus exclusively on primates and humans.)

So, at the risk of disappointing readers more interested in birds and ducks, I'm going to focus on his discussion of Homo sapiens.

Sexual diversity poses distinct challenges to evolutionary explanation, according to Prum. How can evolution explain sexual behavior that is not directly related to reproduction?

"One of the most exciting aspects of my emerging theory of aesthetic evolution is the possibility that it sheds light on this enduring mystery of variation in human sexual desire."

First and foremost, he points out, this requires setting aside conceptual categories of sexual identity. Categories like heterosexuality, homosexuality, bisexuality, etc. "The idea that sexual behavior is a marker or definition of a person's identity is actually a quite modern, cultural invention--perhaps only 150 years old. Because we live in a society that is accustomed to conceiving of sexual behavior in terms of sexual identity, we tend to think that sexual identity categories are biologically real and, therefore, require scientific explanation."

Prum pushes back against this tendency and notes its prevalence even in the scientific field. "Sure enough, an ample scientific literature on 'the evolution of homosexuality' gets this issue mostly wrong and has undermined itself as a result."

The problem with 'the evolution of homosexuality', he writes, is that it starts with the assumption that there is an evolutionary conundrum to be solved in the first place. But before the concepts of sexual identity emerged, it was not at all clear, Prum argues, that same-sex preferences were associated with lowered reproductive success.

"Humans have evolved to engage in sex more frequently, for greater duration, with greater pleasure, and in a greater variety of ways than did our ape ancestors," he writes, "and many of the resulting sexual behaviors do not contribute to reproduction directly, yet they are perfectly consistent with reproductive success."

Prum proposes that human same sex-behavior might have evolved through female mate choice as a mechanism to advance female sexual autonomy and to reduce sexual conflict over fertilization and parental care. According to his aesthetic hypothesis, he writes, the existence of same-sex behavior in humans is another evolutionary response to the persistent primate problem of male sexual coercion, a trait that is widespread in other species.

"Although I think that all human same-sex behavior might have evolved to provide females with greater autonomy and freedom of sexual choice, I address the evolution of female same-sex behavior and male same-sex behavior separately because I believe that their evolutionary mechanisms differ substantially in detail."

The social and sexual behavior of primates is greatly influenced by which sex leaves the social group into which it is born when it reaches the age of sexual maturity. The movement of young adults out of one social group into another is necessary to prevent genetic inbreeding, he points out.

With most primate species, it is the male that moves out in search of a female from another group, while the females stay at home.

But African apes and a few of the old world monkey species evolved the opposite pattern--female dispersal among social groups, Prum notes. And this is the ancestral condition for humans. A consequence is that all primate females within such female-dispersal based societies begin their sexual lives at a disadvantage, writes Prum, "because of the lack of social support of developed social networks  to help them resist male sexual coercion and social intimidation."

As a result, females needed to organize a natural defense by selecting mates and friends most willing to protect their autonomy.

Even when females stay in their natal social groups, Prum points out, they must create protective social networks, and primatologists have noted that even male friends in primates (like baboons) help protect females' offspring from males who would otherwise kill them. Female-female friendships contribute to protection of each other's offspring against infanticide and other threats, he writes.

Based on this mutually supportive network, Prum believes that female same-sex behavior in humans evolved as a way to construct and strengthen new female-female social alliances "and make up for the ones that were lost when the females left their original, natal social groups."

In a similar way, he argues, male same-sex behavior in humans might also have evolved to advance female sexual autonomy, but by a different mechanism.

In aesthetic evolution, Prum's proposed hypothesis, female mate choice has acted over time not only on the selection of preferred male physical features, but also on male social traits, "in such a way as to remodel male behavior and, secondarily, to transform male-male social relationships."

"In other words," he writes, "selection for the aesthetic, pro-social personality features that females preferred in their mates also contributed, incidentally, to the evolution of broader male sexual desires, including male same-sex preferences and behavior."

So, once male same-sex behavior emerged within a population, according to Prum's hypothesis, it would advance female sexual autonomy in a number of ways. "I suggest first that even if relatively few males within a social group had same-sex attractions, this could result in substantial changes in the social environment."

As some males evolved same-sex sexual preferences, the increased breadth of male sexual outlets could lessen the intensity of male interest, and investment, in sexual and social control over females and diminish the ferocity of male-male sexual competition. Because male sexual competitors might also be sexual partners, this could further minimize their competitiveness with each other without necessarily producing any loss in their reproductive success.

In fact, Prum adds, he is proposing that the evolutionary changes in male sexual preferences occurred specifically because males with traits that are associated with same-sex preferences were preferred as mates by females.

The upshot of this is that in a sense all of these desired traits passed into the male population, regardless of whether the individual turns out to be heterosexual or homosexual in practice.

The aesthetic theory of the evolution of male same-sex behavior does not imply that men with a predominantly same-sex orientation have any physical or social personality traits that differ from those of other males. Exactly the contrary, in fact. The hypothesis maintains that there is nothing distinctive about such men, because the features that evolved along with same-sex preferences have become a typical component of human maleness in general. Therefore, individuals with exclusively same-sex sexual preferences are distinctive only in the exclusivity, not in the existence, of their same-sex desires.

It will be interesting to see how Prum's hypothesis fares amongst his colleagues who specialize in primatology and the other disciplines.

I highly recommend The Evolution of Beauty.

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