Evolutionary scientists have thought badly about same-sex sexual behavior.
This is the implication of a new study same-sex behavior in animals. Instead of asking why animals engage in same-sex behavior (SSB), researchers should ask, "Why not?" the authors said.
If they are right, same-sex sex may not have evolved independently in different animals for adpative reasons. Instead, same-sex sex may have appeared very early, and it can simply last because engaging in it costs a lot of animals, evolutionarily speaking.
"Usually, when evolutionary biologists see a trait that is really widespread in evolutionary lines, at least we consider the idea that this trait is ancestral and has been preserved in all these lines," said Julia Monk, PhD student at Yale University, who co-authored the new research. "So why didn't people take this SSB hypothesis into account?"
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In evolutionary science, same-sex sexual behavior has long been seen as a mystery: why do animals spend time and energy doing something sexual that will not pass through their genes to the next generation? Yet same-sex sexual behavior has been seen in at least 1,500 species, from fine-grained errors to humans.
(To avoid anthropomorphization, researchers do not use the terms "homosexual", "heterosexual", "homosexual" or "heterosexual" when referring to animal behavior.)
"We cannot attribute sexuality to animals – We try to learn the best about them by observing their behavior – said Monk Live Science. – And these behaviors should not be mapped to human cultural and social contexts.
The assumption that there must be an evolutionary reason for same-sex same-sex sex prompted researchers to look for possible benefits for same-sex behavior. For example, in humans, scientists have found that having a homosexual son or brother seems to be related to a woman having more offspring in total. Other studies have shown that same-sex sexual behavior is a side effect other genes which have reproductive benefits.
In evolutionary biology, an animal's ability to reproduce in its surroundings is called physical fitness. It is quite possible that in some species same-sex sex can have health benefits, wrote Monk and her colleagues in her article published on November 18 in the journal Ecology and evolution of nature. But these evolutionary benefits may not be required for same-sex sexual behavior.
Imagine that the earliest sexually reproducing animals just tried to associate with all members of their species – regardless of gender. Perhaps it was a logical evolution, because all the bells and whistles that distinguish men from women are vigorously expensive. So any effort put into associating the same sex would be offset by not spending energy on evolution and maintaining separation secondary sex characteristics, like different colors, smells and behaviors. The authors argued that these gender-distinctive features may have appeared later in the evolutionary chain.
In this formulation, same-sex and other-sex sexual behavior would start on an equal footing at an early stage in animal evolution. This may explain why same-sex sex is so common throughout the animal kingdom: it has not evolved many times independently, but instead has been part of the structure of animal evolution from the very beginning.
The new hypothesis undermines old assumptions about same-sex behavior, said Caitlin McDonough, a PhD student at Syracuse University and co-author of the study. Many studies on these sexual behaviors assume that same-sex sex is expensive for animals and that other-sex sex is not expensive, she said.
"You really need to go through these assumptions and test the costs and benefits of both behaviors in the system," said McDonough.
If same-sex behavior returns to the roots of animal evolution, then the fact that they are so common today makes sense, said Monk.
"If you assume that a feature like SSB is a new achievement and has high costs, it will really be difficult to understand how it can become more common at low starting frequencies," she said. "It would have to have really great fitness benefits or be otherwise resistant to natural selection, so that such a result is likely.
"On the other hand, if you assume that the trait is an ancestor and was originally common and has low costs, it is much more likely that it will remain widespread to this day, even if it does not seem to have a big impact on fitness. "
One evidence to support this hypothesis is that some echinoderms, including sea stars and sea urchins, are involved in same-sex sexual behavior. Echinoderms evolved in the early stages of life, probably in the pre-Cambrian period over 541 million years ago.
But other evidence is lean, mainly because scientists have not systematically studied same-sex sexual behavior in animals. Most observations were accidental, and biologists often considered sex between two same-sex animals irrelevant or inappropriate to record, said Monk. Sometimes researchers automatically assume that same-sex behavior is not really about sex, but rather about domination or bonding. And often, if two animals have been observed to have sex, they are assumed to be male and female without any supporting evidence, said McDonough.
"The learning we do is really informed and is influenced by cultural prejudices," she said.
Thinking about same-sex sexual behavior as a standard part of animal repertoire would change researchers' approach to studying the evolution of these behaviors. The next step, Monk said, would be to collect more data on the prevalence of same-sex behavior in animals. Researchers could then compare species from the tree of life to determine if all lines exhibit same-sex behavior. If so, this would reinforce the argument that same-sex sexuality was part of the lives of the ancestors of all sexually reproducing animals today.
Originally published on Live learning.