UC Berkeley study finds same-sex sexual behavior in the wild may not be ‘Darwinian paradox’

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Animal displays of same-sex sexual behavior, or SSB, may be the evolutionary norm, not the exception, according to a study co-authored by UC Berkeley researchers in the department of environmental science, policy and management.

Published by Nature Ecology and Evolution, head researcher Julia Monk and co-authors Max Lambert and Ambika Kamath challenge the longstanding idea that SSB among animals is abnormal.

The study proposes that historically, animals have mated indiscriminately in regards to sex, noting that SSB has been documented in more than 1,500 animal species, including many species of mammals, birds, snakes and even nematode worms, according to the study.

“Sexual diversity is part of larger biodiversity,” Lambert said. “We really would hope … to systematically look at the data to see how common this is … to flesh out the tree of life.”

While SSB is extensively documented in the field of evolutionary biology, it has also been long regarded as a “Darwinian paradox,” according to the study. Kamath added that SSB behavior was presumed to imply costs to reproduction.

Previous studies addressing the “paradox” of SSB have generally focused on a single species of interest that implies these patterns have independently evolved in many species, according to the study. Studies examining different traits occurring at a similar frequency generally assume a prevalent trait evolved one or more times in a common ancestor, rather than independently evolving many times.

“Evolving new things is very complex. … Evolving (the trait) once and having it persist is a much more likely scenario,” Kamath said. “We think it’s weird that the parsimony explanation has not even been considered.”

Previous theories in evolutionary biology have placed different-sex sexual behavior, or DSB, as the baseline, according to the study. The reigning ideology suggested that SSB in animal species was a detriment to reproduction, a key component of evolutionary species survival.

While one of the arguments against SSB as an evolutionary strategy is that it doesn’t directly lead to reproduction, exclusive DSB can also be “disadvantageous” for certain species, according to the study. Models also assume that DSB is highly efficient for reproduction, but the paper argues that this is not the case, pointing to factors that also contribute to its costs.

“It is undeniably true that the way we ask questions … is influenced by who we are,” Kamath said. “We need to be really cognizant of that balance of our identities influencing the question … the natural world isn’t necessarily going to conform.”

Sasha Langholz is a research and ideas reporter. Contact her at [email protected] and follow her on Twitter at @LangholzSasha‏.