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.
剛剛發表于《Nature Ecology and Evolution 》的一篇論文提供一種不同的方法。
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.