Student Writes Biologists Should Update Views on Same-Sex Behavior in Animals

Over the years, scientists have recorded same-sex sexual behavior in more than 1,500 animal species, from snow geese to common toads. And for just as long evolutionary biologists studying these behaviors have grappled with what has come to be known as a "Darwinian paradox": How can these behaviors be so persistent when they offer no opportunity to produce offspring?

Examples of species with documented same-sex sexual behaviors (SSB) demonstrate the widespread distribution of SSB in animals: a. Variegated sea urchin, b. Bonin flying fox, c. Common slipper shell, d. Humboldt squid, e. Garter snake, f. Snow goose, g. Damselfly, h. Laysan albatross, i. Red flour beetle, j. Field cricket, k. Domestic cow, l. Sea star, m. Japanese macaque, n. Chinstrap penguin, o. Common toad, p. Rat gastrointestinal roundworm, q. Bluestreak cleaner wrasse, r. Box crab.

Writing in the journal Nature Ecology & Evolution, researchers from Yale University, The University of California-Berkeley, Syracuse University and The University of Texas at Austin make the case that it's time to reframe the question from "why do animals engage in same sex behavior (SSB)" to "why not?" In a Perspectives piece, the authors suggest that these behaviors may actually have been part of the original, ancestral condition in animals and have persisted because they have few — if any — costs and perhaps some important benefits.

"We propose a shift in our thinking on the sexual behaviors of animals," says Julia Monk, lead author from Yale's School of Forestry and Environmental Studies. "We're excited to see how relaxing traditional constraints on evolutionary theory of these behaviors will allow for a more complete understanding of the complexity of animal sexual behaviors."

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.

"When you look at the sheer diversity of species that exhibit these behaviors — mammals, birds, reptiles, amphibians, fish, insects, crustaceans, nematodes, sea urchins — it becomes evident that same-sex behaviors have probably been a part of sex since the evolution of sex itself," adds Erin Giglio, a graduate student in the lab of Steve Phelps and UT Austin's Ecology, Evolution and Behavior program, who was a coauthor of the paper.

They also dispute the assumption that because different-sex behaviors are essential for sexual reproduction selection — or the emergence of beneficial traits that promote increases in population, size, or resilience — sexual behaviors that do not immediately result in reproduction will be eliminated. 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, a postdoctoral fellow at UC Berkeley's Department of Environmental Science.

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.

Other coauthors include Ambika Kamath from UC Berkeley and Caitlin McDonough from the Center for Reproductive Evolution at Syracuse University.

This post is based on a press release by Yale University.