Typically, research into these behaviors has rested on two assumptions, the authors state. The first is that same-sex behavior has high costs because individuals spend time and energy on activities that have no potential for reproductive success. The other is that same-sex behaviors emerged independently in different animal lineages.
They argue that a combination of same-sex and different-sex sexual behaviors (DSBs) is an original condition for all sexually producing animals — and that these tendencies likely evolved in the earliest forms of sexual behavior.
They also dispute the assumption that because different-sex behaviors are essential for sexual reproduction selection — or the tendency of beneficial traits that promote increases in population, size, or resilience — will eliminate sexual behaviors that do not immediately result in reproduction. On the contrary, they suggest that SSB is not always — and maybe even seldom — very costly. This would suggest that this behavior is actually what evolutionary biologists call “neutral,” meaning that it has neither negative nor positive effects and therefore persists because there’s no reason for natural selection to weed it out.
Moreover, the authors suggest that not only are same-sex behaviors often “not costly,” but can be advantageous from a natural selection perspective because individuals are more likely to mate with more partners. Many species aren’t inherently monogamous but instead try to mate with more than one individual. In many species it can be difficult for individuals to even discern between different sexes.
“So, if you're too picky in targeting what you think is the opposite sex, you just mate with fewer individuals. On the other hand, if you’re less picky and engage in both SSB and DSB, you can mate with more individuals in general, including individuals of a different sex,” says co-author Max Lambert
’13 M.E.Sc. ’18 Ph.D., a postdoctoral fellow at the University of California-Berkeley’s Departmental of Environmental Science, Policy, and Management.
For example, scientists have found
that male burying beetles engage in increased same-sex behavior when they perceive a higher cost of missed mating opportunities with females. This suggests that engaging with different-sex behaviors exclusively is actually disadvantageous
because it reduces chances to display mating potential when mating opportunities are rare.
Such examples only hint at what scientists don’t know about same-sex behaviors in animals, Lambert said. There are thousands of examples of SSB in animals, he said, yet most of these observations occurred by chance and scientists rarely if ever actively study how often these behaviors occur compared with different-sex sexual behaviors.