Rhesus macaques are one of the most widely distributed and well-studied primates in the world. They are known for their complex social behavior, including grooming, vocalizing, and mating.
They are also known for engaging in homosexual behavior, especially among males.
Male rhesus macaques often mount and ejaculate on other males, sometimes more frequently than on females.
A recent study by a team of researchers from Imperial College London, the University of Lausanne, and the University of Puerto Rico, published in Nature Communications, has shed new light on the evolutionary and genetic basis of male same-sex sexual behavior (SSB) in rhesus macaques.
The study found that male SSB is widespread and heritable in a population of rhesus macaques living on Cayo Santiago, a small island off the coast of Puerto Rico.
The study also found that male SSB is influenced by environmental factors, such as temperature and rainfall.
How male SSB varies among rhesus macaques
(Photo : SAJJAD HUSSAIN/AFP via Getty Images)
For three years, the researchers observed 236 male rhesus macaques from a colony of 1,700 monkeys on Cayo Santiago.
They recorded the number of times each male mounted and ejaculated on other males and females. They also collected genetic samples from each male and used pedigree records to trace their ancestry back to 1956.
The researchers found that 72% of the males mounted other males, while only 46% mounted females, as per Phys.org.
They also found that the frequency of male SSB varied widely among individuals, ranging from zero to 1,000 mounts per year. The average number of mounts per year was 41 for males and 11 for females.
The researchers used statistical models to estimate the heritability and evolvability of male SSB, which measure the degree to which genetics contribute to the variation and evolution of the trait.
They found that the heritability of male SSB was 6.4%, meaning that genetics account for a small but significant proportion of the variation in male SSB.
They also found that the evolvability of male SSB was 14.9%, meaning that the trait has the potential to evolve rapidly in response to natural selection.
Also Read: Male Monkeys In Puerto Rico Engage More In Copulation With Other Males Than Females, Study Says
How environmental factors affect male SSB
The researchers also examined how environmental factors affect male SSB. They found that temperature and rainfall had significant effects on male SSB. Higher temperatures increased male SSB, while higher rainfall decreased it, as per Live Science.
It is speculated by the team of experts that temperature may affect the hormonal levels and sexual arousal of males, while rainfall may affect their activity levels and visibility.
They also found that male SSB was correlated with the number of times a male was mounted by other males, suggesting that there is a social component to male SSB.
Male SSB may serve various functions, such as establishing dominance, reducing aggression, enhancing social bonds, or increasing reproductive success, according to the study.
The researchers concluded that male SSB in rhesus macaques is a complex trait influenced by genetic and environmental factors.
They argued that male SSB is not maladaptive or wasteful, but rather an adaptive response to changing environmental conditions and social dynamics.
Despite the results, the research team called for more research on male SSB's ecological and evolutionary consequences for rhesus macaques and other primates.
Related article: Florida Airport Monkeys Are From the Lineage of Zoo Escapees in 1948
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