Homosexual behavior is not limited to humans. Biologists have reported homosexual behavior in many species of wild animal, ranging from bats and birds to dolphins and primates.
Homosexual behavior is not limited to humans. Biologists have reported homosexual behavior in many species of wild animal, ranging from bats and birds to dolphins and primates.
When animals engage in homosexual behavior, one might assume that they invest less time and energy on reproduction. This suggests that there may be strong reproductive costs associated with such behavior, such as having fewer offspring. So it raises the question of how homosexual behavior manages to evolve and continue to exist within a population.
The underlying presumption is that there is not only a cost associated with engaging in homosexual activity, but also that variation in such behavior is passed down from one generation to the next. Called heritability, this is essential for any evolution by natural selection to occur.
We set out to investigate these issues by studying 236 male rhesus macaques living freely in a colony of 1,700 monkeys on the tropical island of Cayo Santiago, Puerto Rico. We observed these monkeys for three years and found that male same-sex sexual behavior (SSB) was widespread. In fact, 72% of the males we observed mounted other males, while only 46% mounted females.
Critically, male SSB is not unique to this population of macaques. We saw similar behavior in wild rhesus macaque populations in northern Thailand. And there have been previous reports of SSB in this species from India, too.
We also had access to pedigree records that traced the parentage of each macaque back to 1956. This allowed us to explore the effect of relatedness (heritability) on their behavior, taking into account other factors that could influence the results, such as age and social group structure.
