[But the researchers cautioned that the work could not shed much
light on sexual orientation in humans. “We’re trying to steer way
from one explanation to rule them all.”]
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SAME-SEX BEHAVIOR EVOLVED IN MANY MAMMALS TO REDUCE CONFLICT, STUDY
SUGGESTS  
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Carl Zimmer
October 3, 2023
New York Times
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_ But the researchers cautioned that the work could not shed much
light on sexual orientation in humans. “We’re trying to steer way
from one explanation to rule them all.” _

Same-sex sexual behaviour has been reported in 261 mammalian species,
and males and females were equally likely to be observed engaging in
it, J.M. Gomez et al./Nat. Commun.

 

In more than 1,500 animal species, from crickets and sea urchins to
bottlenose dolphins and bonobos, scientists have observed sexual
encounters between members of the same sex.

Some researchers have proposed that this behavior has existed since
the dawn of the animal kingdom
[[link removed]].
But the authors of a new study
[[link removed]] of thousands of
mammalian species paint a different picture, arguing that same-sex
sexual behavior evolved when mammals started living in social groups.
Although the behavior does not produce offspring to carry on the
animals’ genes, it could offer other evolutionary advantages, such
as smoothing over conflicts, the researchers proposed.

“It may contribute to establishing and maintaining positive social
relationships,” said José Gómez
[[link removed]], an evolutionary biologist at the
Experimental Station of Arid Zones in Almería, Spain, and an author
of the new study.

But Dr. Gómez cautioned that the study, published on Tuesday in the
journal Nature Communications, could not shed much light on sexual
orientation in humans. “The type of same-sex sexual behavior we have
used in our analysis is so different from that observed in humans that
our study is unable to provide an explanation for its expression
today,” he said.

 

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Previous studies of same-sex sexual behavior have typically involved
careful observations of a single species, or a small group of them.
Dr. Gómez and his colleagues instead looked for the big evolutionary
patterns that gave rise to the behavior in some species but not
others.

The researchers surveyed the 6,649 species of living mammals
[[link removed]] that arose from reptilelike
ancestors starting roughly 250 million years ago. Looking over the
scientific literature, they noted which of them had been seen carrying
out same-sex sexual behaviors — defined as anything from courtships
and mating to forming long-term bonds.

The researchers ended up with a list of 261 species, or about 4
percent of all mammalian species, that exhibited these same-sex
behaviors.

Males and females were about equally likely to be observed carrying
out same-sex sexual behavior, the analysis showed. In some species,
only one sex did. But in still others — including cheetahs and
white-tailed deer — both males and females engaged in same-sex
sexual behavior.

The researchers then investigated how the behavior arose in mammals.
Looking at an evolutionary tree, they found that species engaging in
it were scattered across the tree’s branches, suggesting that the
behavior independently arose in each lineage.

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“With the current data available, it seems that it has evolved
multiple times,” Dr. Gómez said.

The researchers concluded that the earliest members of major groups of
living mammals, such as primates or cats, probably didn’t engage in
same-sex sexual behavior. As new lineages evolved, some of them
started exhibiting the behavior.

Apes, for example, branched off from other primates about 25 million
years ago [[link removed]]. Since then,
they evolved a much higher rate of same-sex sexual behavior than
species on older branches of primates, such as lemurs.

Dr. Gómez and his colleagues then looked for traits that these
same-sex branches had in common. A statistical analysis of the
evolutionary tree revealed that they tended to be social species
instead of solitary ones.

Paul Vasey, a primatologist at the University of Lethbridge in Canada
who was not involved in the study, said that a number of researchers
who have studied same-sex sexual behavior have hypothesized that the
evolution of social groups had favored it. But they were looking at
individual species, rather than across the tree of life.

“For anyone familiar with the literature, I don’t think it is a
huge surprise to see that same-sex sexual behavior is related to
sociality,” Dr. Vasey said. “It is nice to see this conclusion
supported by the methods used by the authors.”

Living in a social group offers a lot of benefits to mammals, such as
better protection from predators. But it also creates new challenges.
Mammalian societies may form hierarchies, for example, in which
top-ranking animals keep lower-ranking ones in line with violence. But
these conflicts can cause a group to fracture, which is bad for
everyone.

Dr. Gómez said that same-sex sexual behavior might be one of the ways
that mammals can manage their unstable social worlds. It may be a way
for mammals to form bonds and alliances, to reconcile after a fight or
to divert aggression into courtship.

But Dieter Lukas, an evolutionary biologist at the Max Planck
Institute for Evolutionary Anthropology in Leipzig, Germany, who was
not involved in the new study, was skeptical of this conclusion.
“Taken together, the findings in this study have not convinced me
that there is a single explanation for the occurrence of same-sex
behavior,” he said.

His skepticism came in part from the data on which the scientists
based their study. The challenges of observing animals in the wild may
mean that same-sex behavior in some species goes overlooked. “It
will be much easier to observe whether the behavior occurs if
individuals are on open ground and active during daytime,” Dr. Lukas
said.

Marlene Zuk, an evolutionary biologist at the University of Minnesota
who was not involved in the study, commended the researchers for
focusing their study on mammals alone, rather than the entire animal
kingdom. “We’re trying to steer way from one explanation to rule
them all,” she said.

In April, Dr. Zuk and Jon Richardson, a postdoctoral researcher in her
lab, put forward a different explanation for same-sex behavior based
on an experiment
[[link removed]] on
crickets. They showed that male crickets will sometimes produce
courtship songs and try to mate with other males and with juveniles.

Since the crickets don’t live in social groups, that can’t explain
the behavior Dr. Zuk and Dr. Richardson documented. Instead, crickets
and perhaps many other species may engage in same-sex sexual behavior
as part of a strategy to take advantage of as many opportunities to
mate as possible.

Dr. Zuk likened the strategy to a smoke detector. “You want a smoke
detector that is sensitive enough to detect all fires,” she said.
“And if it does that, occasionally it’s going to go off when you
burn your toast.”

_Carl Zimmer [[link removed]] writes
the “Origins” [[link removed]] column.
He is the author of fourteen books, including “Life's Edge: The
Search For What It Means To Be Alive.” More about Carl Zimmer
[[link removed]]_

_Science Times Newsletter - The New York Times
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capture the wonders of nature, the cosmos and the human body. _

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