A group of monkeys is challenging humans' views on sexuality by showing that same-sex behavior among males strengthens their social networks and may even help them father more offspring.
The findings, reported this month in the journal Nature Ecology and Evolution, suggest that same-sex behavior is not only natural in the animal kingdom, it can be socially advantageous.
The study "puts to rest the doubts of same-sex behavior occurring naturally in nature," said senior author Vincent Savolainen, a biology researcher at Imperial College London.
Scientists have observed same-sex sexual behavior among more than 1,500 animal species, including penguins, giraffes and elephants. But it was unclear whether this behavior was widespread, if it was influenced by genetics and to what degree it might affect the reproductive success of the population as a whole.
"We suspected same-sex behavior occurred in these macaques, but we didn't know how common it was, or the partnership that it develops," Savolainen said.
To find out, he and his colleagues visited a colony of about 1,700 free-range rhesus macaques living on a wildlife preserve in Puerto Rico. The colony has been monitored for the last 67 years, providing researchers with a comprehensive family tree of the primates.
The researchers defined same-sex behavior as the act of mounting because it was the most frequent — and most identifiable — form of sexual contact. Although it occurs in both male and female macaques, it is much more frequent in males.
Over three years, the study team observed 236 males who belonged to two distinct social groups within the colony. During that time, they documented 1,739 instances of mounting — 722 involving male-female pairs and 1,017 involving same-sex pairs.
The research team had expected to catch some same-sex couples in action, and Savolainen said he wasn't surprised that their pairings outnumbered those of male-female couples.
Among male macaques, same-sex sexual behavior isn't necessarily about sex, but more about social interaction. Male macaques mounted each other after grooming, eating, fighting, playing and resting as well as while traveling, according to the study. The activity could be a way to strengthen bonds between males, making them more likely to form alliances and ultimately gain access to more females, the researchers said.
It is important to note that the same-sex socio-sexual behavior observed in the study is distinct from homosexual behavior because its motivation and purpose are social, said Jean-Baptiste Leca, who studies primate behavior at the University of Lethbridge in Canada and was not involved in the new research. To be classified as true homosexual behavior, the form, motivation and function would all have to be sexual in nature, he said. (As far as the researchers could tell, only one of the 236 macaques they studied engaged exclusively with other males.)
The younger a macaque, the more likely he was to engage in same-sex encounters, the study authors found. That could be a sign that in some cases, the couplings "could partially function as 'practice' for future reproductive activity," they wrote.
Indeed, engaging in same-sex mounting did not negatively affect a macaque's overall reproductive success. After examining the offspring count for all 236 males, the researchers found that the more times a monkey paired up with a fellow male, the higher his offspring count tended to be.
The trend wasn't statistically significant, but it was enough to confirm that same-sex behavior didn't have a reproductive cost — something that Savolainen said he was surprised to find. Perhaps the stronger social bonds formed during these sexual encounters strengthened their coalitions and ultimately gave participants greater access to females, the researchers wrote.
By examining the behaviors of related macaques over multiple generations, the study authors determined that about 6% of same-sex sexual behavior could be explained by genetics. Though this may not seem like a lot, it's comparable to the genetic component of complex behaviors in humans and other primates, such as grooming or alloparenting, which is care provided by individuals other than parents, Savolainen said.
Savolainen and his colleagues warned that the behaviors they observed in the Puerto Rico macaques might be specific to that population. Regardless, they said, the findings challenge the assumption that same-sex sexual behavior is rare in non-human animals and that it results in fewer offspring Others agreed.
"This study contributes to our understanding of animal behavior," said Rachna Reddy, a primatologist at the University of Utah who was not involved in the study. "Now we're seeing that [same-sex] behavior is really frequent, occurs in lots of species, may have many functions, and that it's not always costly."
