Study shows same-sex sexual behavior is widespread and heritable in
macaque monkeys

Date:
July 10, 2023
Source:
Imperial College London
Summary:
Observations of a wild colony of macaques over three years show
same-sex sexual behavior among males is widespread and may be
beneficial.


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FULL STORY ========================================================================== Observations of a wild colony of macaques over three years show same-sex
sexual behaviour among males is widespread and may be beneficial.

The results, published today in Nature Ecology and Evolution, suggest
same-sex sexual behaviour (SSB) has evolved and may be a common feature
of primate reproduction.

Conducted by researchers at Imperial College London, the observations
and genetic data form the first long-term study of SSB in males within
one species.

Their study challenges the beliefs of some that SSB is a rare behaviour
in non- human animals or solely the product of unusual environmental conditions.

First author Jackson Clive, from the Georgina Mace Centre for the
Living Planet at Imperial, said: "We found most males were behaviourally bisexual, and that variation in same-sex activity was heritable. This
means that the behaviour can have an evolutionary underpinning; for
example, we also found that males that mounted each other were also more
likely to back each other up in conflicts - - perhaps this could be one
of many social benefits to same-sex sexual activity." "Our research
therefore shows that same-sex sexual behaviours can be common amongst
animals and can evolve. I hope our results encourage further discoveries
in this area.'' Origins of same-sex behaviours The team studied 236
males within a colony of 1,700 rhesus macaques living freely on the
tropical island of Cayo Santiago, Puerto Rico. As well as observing
their behaviour and conducting genetic analyses, the team had access to pedigree records, which detail parentage of each individual back to 1956.

The researchers recorded all social 'mountings' for the 236 males,
both of males-on-males (same-sex behaviour, SSB) and males-on-females (different-sex behaviour, DSB). They found that male same-sex mounting
was widespread: 72% of sample males engaged in same-sex mounting, in
comparison with 46% for different-sex mounting.

SSB has been observed in thousands of different animals, ranging from
insects to penguins, leading many to theorise about why it happens. These theories include ideas about establishing dominance in groups, shortages
of different- sex partners, and reducing tension following aggression,
but little data is available to support any theory.

The team investigated several of these theories with their data, finding
that, for this colony of macaques, SSB in males was strongly correlated
with 'coalitionary bonds'. This means male pairs that regularly engage
in SSB were more likely to back each other up in conflicts, providing
them with an advantage in the group.

Heritable behaviours The researchers also investigated whether SSB led to
any fitness cost -- a reduction in the amount of offspring they have. In
fact, they found the opposite -- males that engaged in SSB may be more successful in reproducing, potentially due to the benefits provided by
more coalitionary bonds.

In line with this, the team also analysed whether SSB was heritable. Using
the pedigree data, they found SSB in males was 6.4% heritable, which
provides the first evidence of a genetic link to primate SSB outside of
humans. This figure is similar to other heritable behaviours in primates,
such as grooming and sociality.

These findings support counter-arguments to the idea that SBB 'defies
nature and evolution' (the so-called 'Darwinian Paradox').

The team also found some genetic correlation between males that were more
often 'mounters' or 'mountees' when engaging in SSB, suggesting these sub-behaviours may have a common basis. In addition, whether individuals
were more likely to be mounters or mountees did not corelate with their
social position, suggesting asserting their place in the hierarchy is
not an important factor for SSB in this species.

Macaques and humans Though the researchers caution against direct
comparisons to humans, they say their study challenges the beliefs of
some that SSB is a rare behaviour in non- human animals or solely the
product of unusual environmental conditions.

Instead, the results suggest some degree of SSB can evolve adaptively, depending on the context, and so may be a common feature of primate reproductive ecology. There are many examples of other primate groups
engaging in different forms of SSB, so further in-depth genetic studies
could strengthen this conclusion.

Lead researcher Professor Vincent Savolainen, Director of the Georgina
Mace Centre for the Living Planet at Imperial, said: "Unfortunately
there is still a belief amongst some people that same-sex behaviour is 'unnatural,' and some countries sadly still enforce the death penalty
for homosexuality. Our research shows that same-sex behaviour is in fact widespread amongst non-human animals.

"Our mission is to advance scientific understanding of same-sex
behaviour, including exploring the benefits it brings to nature and
within animal societies. Amongst the macaques we looked at in this study,
more than two- thirds displayed same-sex behaviour and this behaviour strengthened the bonds within the community." The research was funded by
the Natural Environment Research Council (NERC), the American Institute
of Bisexuality, and the Evolution Education Trust.

* RELATED_TOPICS
o Plants_&_Animals
# Behavioral_Science # Mating_and_Breeding #
Evolutionary_Biology # Nature # Monkeys #
Animal_Learning_and_Intelligence # Biology # Life_Sciences
* RELATED_TERMS
o Sex_linkage o Rhesus_Macaque o Green_Iguana o Wild_Cat o
Spaying_and_neutering o Seahorse o Tortoise o Wild_Horse

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Story Source: Materials provided by Imperial_College_London. Original
written by Hayley Dunning. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Jackson Clive, Ewan Flintham, Vincent Savolainen. Same-sex
sociosexual
behaviour is widespread and heritable in male rhesus
macaques. Nature Ecology & Evolution, 2023; DOI:
10.1038/s41559-023-02111-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230710113924.htm

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