Science about human same-sex and bi-sex

The boffins at the NIH, my dissertation research alma mater, have found data that might help explain why same bi-sex activities don't appear to fade out of the population, possibly due to fewer offspring. Their paper, Genetic variants underlying human bisexual behavior are reproductively advantageous, describes one possibility:

Because human same-sex sexual behavior (SSB) is heritable and leads to fewer offspring, how SSB-associated alleles have persisted and whether they will remain in human populations are of interest. Using the UK Biobank, we address these questions separately for bisexual behavior (BSB) and exclusive SSB (eSSB) after confirming their genetic distinction. We discover that male BSB is genetically positively correlated with the number of offspring. This unexpected phenomenon is attributable to the horizontal pleiotropy [having sex with more people] of male risk-taking behavior–associated alleles because male risk-taking behavior is genetically positively correlated with both BSB and the number of offspring and because genetically controlling male risk-taking behavior abolishes the genetic correlation between male BSB and the number of offspring. By contrast, eSSB is genetically negatively correlated with the number of offspring. Our results suggest that male BSB–associated alleles are likely reproductively advantageous, which may explain their past persistence and predict their future maintenance, and that eSSB-associated alleles are likely being selected against at present.   

Translated into American: Bisexuals have more partners and offspring units than people who are exclusively same-sex. So maybe evolutionary pressure disfavors eSSB, but favors the persistence of BSB possibly because BSB people are more risk-taking (with more offspring) than eSSB people. 

The Hill comments:

For generations, evolutionary biologists have seen homosexual or same-sex attraction as a hole in evolutionary theory, as it is an at least somewhat inherited trait that also leads those who have it to have fewer children.

“Those genes lead to fewer children, so that means they are being selected against in the population by natural selection. So gradually, they should just disappear from the population. So why do they still exist?”

That’s a question that goes far beyond the world of human sexuality. Same-sex attraction and behavior are widespread across the animal kingdom, from male gentoo penguins co-raising eggs, sex among all-male bachelor packs of gorillas and “seasonally bisexual” flying fox bats.

“It’s just wherever you look. I can give you papers on beetles, spiders, flies, fish, flamingos, geese, bison, deer, gibbons, bats,” said ecology and evolution researcher Jackson Clive in an interview with Imperial College London.  

Theories for a resolution to the paradox are widespread — and include arguments that there is no paradox at all. In the Nature Ecology and Evolution study, for example, the authors argue that scientists’ assumption that opposite-sex attraction is normal and ancestral “has not been rigorously examined.”

Instead, that team hypothesized “an ancestral condition of indiscriminate sexual behaviors directed towards all sexes.” 

In an essay in Scientific American, the Nature authors argued that the foundations of animal sex may have been laid down long before the relatively clear physical differences between males and females that the idea of the paradox takes for granted.

“It is unlikely that the other traits required to recognize a compatible mate — differences in size, shape, color or odor, for example — evolved at exactly the same time as sexual behaviors,” they wrote.

“Indeed, indiscriminate mating can be more beneficial than it is costly.”  

Or as a separate Scientific American piece about three “lesbian” capuchin monkeys in a Los Angeles sanctuary put it in September, “it’s clear being a little gay nearly every day helps primates get their way — in pleasure and in life.”

So in the early days of sex, male and female differences had not yet evolved. In that case, indiscriminate mating, which sounds exciting 😘, was the norm because all individuals looked alike and there were no boys and girls, just critters. Maybe SSB and BSB are not going to fade away due to evolutionary factors than maintains them in human populations.

 

Which is the boy and which is the girl?

Or are they same-sex?

By Germaine: Sex researcher