Mating strategies

(Retrieved from my “to post” files from some considerable time ago.)

From the Economist issue of 11/28/19 (on-line), in the Science & technology section, header “Mating strategies”, title “A new theory argues same-sex sexual behaviour is an evolutionary norm: Unless it is essential to know a partner’s sex, why bother?”:

When it comes to sexual behaviour, the animal kingdom is a broad church. Its members indulge in a wide variety of activities, including with creatures of the same sex. Flying foxes gather in all-male clusters to lick each other’s erect penises. Male Humboldt squid have been found with sperm-containing sacs implanted in and around their sexual organs in similar quantities to female squid. Female snow macaques often pair off to form temporary sexual relationships that includes mounting and pelvic thrusting. Same-sex sexual behaviour has been recorded in some 1,500 animal species.

The mainstream explanations in evolutionary biology for these behaviours are many and varied. Yet they all contain a common assumption: that sexual behaviours involving members of the same sex are a paradox that does indeed need explaining. Reproduction requires mating with a creature of the opposite sex, so why does same-sex mating happen at all?

A paper just published in Nature Ecology and Evolution offers a different approach. Instead of regarding same-sex behaviour as an evolutionary oddity emerging from a normal baseline of different-sex behaviour, the authors suggest that it has been a norm since the first animals came into being. The common ancestor of all animals alive today, humans included, did not, they posit, have the biological equipment needed to discern the sex of others of its species. Rather, it would have exhibited indiscriminate sexual behaviour — and this would have been good enough to transmit its genes to the next generation.


The group of young researchers from institutions across America who wrote the paper, led by Julia Monk, a graduate student at Yale, argue that conventional models of sexual behaviour’s evolution take two things for granted that they should not. The first is that the cost of same-sex behaviour is high because energy and time spent engaged in it do not contribute to reproductive success. If that were true it would indeed mean that maintenance of same-sex behaviour over the generations requires some exotic explanation whereby such activity confers benefits that outweigh the disadvantage. The second assumption is that same-sex activity evolved separately in every species that exhibits it, from an ancestral population that engaged exclusively in different-sex behaviour.

Detailed discussion of these two points in the Economist piece. With a note there on doing science:

Although the idea that same-sex behaviour has always been a norm is scientifically intriguing, the paper’s authors are also making a broader point about human beings’ pursuit of knowledge. Ms Monk says that the paper’s authors met through a Twitter account which promotes the work of lgbt scientists. This was a serendipitous encounter which gave them space to explore an idea that might have been dismissed at first sight in a more conventional setting. The group includes people with a range of sexual orientations, so naturally they had an incentive to ask whether mainstream evolutionary biology’s view of sexual orientation is correct.

The conventional view. From Wikipedia:

Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include same-sex sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Various forms of this are found in every major geographic region and every major animal group. The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from social species.

Scientists perceive homosexual behavior in animals to different degrees. The motivations for and implications of these behaviors have yet to be fully understood. According to Bruce Bagemihl, the animal kingdom engages in homosexual behavior “with much greater sexual diversity – including homosexual, bisexual and nonreproductive sex – than the scientific community and society at large have previously been willing to accept.”

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