Tuesday, June 16, 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
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," 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 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," 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:
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 actually express."
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 reproductive strategy."
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University of California - RiversideThanks to University of
California - Riverside for this article.