Same-sex behavior can be found in almost all species in the animal kingdom - from worms to birds - making the practice nearly universal among animals, according to a new review of existing research.
"It's clear that same-sex sexual behavior extends far beyond the well-known examples that dominate both the scientific and popular literature: for example, bonobos, dolphins, penguins and fruit flies," said Nathan Bailey, the first author of the review paper and a postdoctoral researcher in the Department of Biology at UC Riverside.
However, the review also reports that same-sex behaviors in different species are not all equivalent.
"For example, male fruit flies may court other males because they are lacking a gene that enables them to discriminate between the sexes. But that is very different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or female Laysan Albatross that can remain pair-bonded for life and cooperatively rear young," Bailey said.
The review also found a gap in the literature. While many studies have tried to understand why same-sex coupling exists and why it might make sense in terms of evolution, few have looked at what the evolutionary consequences of this behavior might be.
"Same-sex behaviors-courtship, mounting or parenting-are traits that may have been shaped by natural selection, a basic mechanism of evolution that occurs over successive generations. But our review of studies also suggests that these same-sex behaviors might act as selective forces in and of themselves," Bailey said.
A selective force, which is a sudden or gradual stress placed on a population, affects the reproductive success of individuals in the population.
"When we think of selective forces, we tend to think of things like weather, temperature, or geographic features, but we can think of the social circumstances in a population of animals as a selective force, too. Same-sex behavior radically changes those social circumstances, for example, by removing some individuals from the pool of animals available for mating," Bailey said.
Bailey noted that researchers in the field have made significant strides in the past two and a half decades studying the genetic and neural mechanisms that produce same-sex behaviors in individuals, and the ultimate reasons for their existence in populations.
"But like any other behavior that doesn't lead directly to reproduction-such as aggression or altruism-same-sex behavior can have evolutionary consequences that are just now beginning to be considered," Bailey said.
"For example, male-male copulations in locusts can be costly for the mounted male, and this cost may in turn increase selection pressure for males' tendency to release a chemical called panacetylnitrile, which dissuades other males from mounting them," he added.
In their future research, Bailey and Marlene Zuk, a biology professor at UCR, plan to try and address questions about the evolutionary outcomes of same-sex couplings, focusing on the Laysan Albatrosses.
The review article was published June 16 in the journal Trends in Ecology and Evolution.