How a recent theory about evolution may overturn millennia of assumptions
Homosexuality has been called The Great Evolutionary Puzzle. Ever since Darwin, those who think deeply about such matters have been trying to understand why humans exhibit homosexual (same-gender attraction) behaviors.
As organisms are essentially bags of genes and as the genes that do the best job of ensuring they get passed forward into future generations are the ones we end up with, and as mating is the way in which animals accomplish this passing forward, same-sex mating appears on the surface to be counter-intuitive. Because obviously same-sex mating for most animals (flies and a very small number of other creatures being the exception) doesn’t result in genes being passed forward.
So why does homosexual behavior persist?
A couple of decades ago a team of researchers hypothesized that if a gene conferred significant reproductive advantage to one gender such that it outweighed the reproductive cost in the other gender, this could explain the persistence of homosexual behavior. Candidate genes were duly identified in humans.
Unfortunately for this theory, homosexual behavior is seen in such a wide range of animals involving such a wide diversity of genes that it seems implausible such benefit-cost models can apply to the vast majority of cases. To date, more than 1,500 species of animal have been seen exhibiting same-sex mating behaviors in the wild.
So what’s going on?
A team led by Julia Monk at Yale University has just published a paper in Nature Ecology and Evolution that upends the traditional way of looking at the puzzle and proposes a radical new interpretation.
By running a daring thought experiment, Monk and her team imagined the first animals to emerge from the primordial soup. As reproduction machines, they’d be hardwired to mate. Sensory apparatus is, however, complex and takes time to evolve and will only do so in circumstances in which such apparatus provides significant advantage relative to the ecosystem the organism is inhabiting.
Initially therefore a primeval organism’s best strategy would be to mate with anything that more or less resembled itself because this would require very little in the way of sensory apparatus and would maximize the probability of its genes being passed forward into future generations.
We already know that in the wild most animals indulge in far more mating behavior than is strictly necessary to ensure reproduction. This implies the cost of such activity is relatively modest for most animals, so there’s little downside to over-mating. This is especially true when we remember that in the wild most animals mate very quickly in order to reduce their moment of vulnerability to predators.
If our ancestral organisms were essentially indiscriminate in mating preference this would achieve passing on their genes at the cost of merely a little wasted effort. Evolutionary theory would then suggest that purely heterosexual preferences would arise only if environmental pressures were strong enough.
So, for example, birds that raise a single chick each year and rely on pair-bonds to incubate and then feed the hatchling would exhibit strong heterosexuality but most animals would continue to display a wide range of sexual behaviors because there is little or no selection pressure to force adjustment.
Thus we see clusters of male flying foxes licking one another’s penises; we see female snow macaques forming lesbian trysts, and of course we often see male dogs indiscriminately trying to mate with pretty much anything including their owner’s leg.
If this hypothesis is correct it not only answers the question of why homosexual mating occurs but also opens up a new way of understanding what is normal across the animal kingdom, of which we humans are a small part.
The hypothesis can be tested in a couple of different ways. The first is to amass more observational evidence from species in the wild, to see if indeed such sexual fluidity is normal in the absence of distinct selection pressures to favor strict heterosexuality. The second is to use computer modeling to determine if the range of behaviors we see and the baseline incidence of homosexual preferences in species we’re familiar with (including our own) are compatible with the “mate with everything” primal ancestor starting-point.
Personally I’m looking forward to keeping up with research in this area as, whatever happens to be the case, it will shed important new light on the important topic of animal sexual behaviors and by extension lead to a greater understanding of our own species.
An alternative hypothesis for the evolution of same-sex sexual behaviour in animals: https://www.nature.com/articles/s41559-019-1019-7