June 17, 2009 — Same-sex behavior is a nearly universal phenomenon in the animal
kingdom, common across species, from worms to frogs to birds, concludes
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.
There is a caveat, however. The review also reports that same-sex
behaviors are not the same across species, and that researchers may be
calling qualitatively different phenomena by the same name.
"For example, male fruit flies may court other males because they
are lacking a gene that enables them to discriminate between the
sexes," Bailey said. "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."
Published June 16 in the journal Trends in Ecology & Evolution,
the review by Bailey and Marlene Zuk, a professor of biology at UCR,
also finds that although many studies are performed in the context of
understanding the evolutionary origins of same-sex sexual behavior,
almost none have considered its evolutionary consequences.
"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," Bailey said. "But
our review of studies also suggests that these same-sex behaviors might
act as selective forces in and of themselves."
A selective force, which is a sudden or gradual stress placed on a
population, affects the reproductive success of individuals in the
"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," Bailey said. "Same-sex behavior radically changes those social
circumstances, for example, by removing some individuals from the pool
of animals available for mating."
Bailey, who works in Zuk’s lab, 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
"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," he 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."
The review paper:
Examines work done to test hypotheses about the origins of same-sex behavior in animals.
Provides a framework for categorizing same-sex behavior, for example, is it adaptive, not adaptive, occurs often, infrequently?
Discusses what has been discovered about the genetics of same-sex
behavior, especially in the model organism, the fruit fly Drosophila,
and in human beings.
Examines connections between human sexual orientation research, and
research on non-human animals, and highlights promising avenues of
research in non-human systems.
The reviewers expected the research papers they read for their
article would give them a better understanding of the degree to which
same-sex behaviors are heritable in animals.
"How important are genes to the expression of these behaviors,
compared to environmental factors?" Bailey said. "This is still
unknown. Knowing this information would help us better understand how
the behaviors evolve, and how they affect the evolution of other
traits. It could also help us understand whether they are something
that all individuals of a species are capable of, but only some
Bailey recommends that fellow evolutionary biologists studying
same-sex behavior in animals adopt some of the research approaches that
have been successful in human studies.
"We have estimates, for example, of the heritability of sexual
orientation in humans, but none that I know of in other animals," he
said. "Scientists have also targeted locations on the human genome that
may contribute to sexual orientation, but aside from the fruit fly, we
have no such detailed knowledge of the genetic architecture of same-sex
behavior in other animals."
Next in their research, Bailey and Zuk plan to begin experimentally addressing some of the many issues raised in their review.
Said Bailey, "We want to get at this question: what are the
evolutionary consequences of these behaviors? Are they important in the
evolution of mating behavior, or do they just add extra ‘background
noise’? We are pursuing work on the Laysan Albatross, in which females
form same-sex pairs and rear young together. Same-sex behavior in this
species may not be aberrant, but instead can arise as an alternative
The UCR Academic Senate funded the one-year study.
Reference: Bailey et al. Same-sex sexual behavior and evolution. Trends in Ecology & Evolution, June 16, 2009.