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Male Rhesus Macaques Often Have Sex With Each Other, A Trait They Have Inherited In Part From Their Parents
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.
From one generation to the nextWe 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.
We found that the heritability of male SSB was 6.4%, meaning genetics do account for a small proportion of SSB—the rest is environmental.
We calculated "evolvability" to be 14.9%, giving the potential rate at which the trait can evolve per generation through natural selection. Evolvability is thought to be a more reliable indicator than heritability of the degree to which genetics can respond to evolutionary pressure, and provides us with further evidence that SSB can evolve through selection.
Our estimates align with what we would expect for a behavioral trait that is probably influenced by multiple genetic factors and environmental effects. They are also consistent with heritability values reported in studies of other social behavior in primate species, including social grooming in baboons and social proximity in capuchins.
We also found a genetic correlation between the number of times a male was observed mounting another male and the number of times he was mounted by other males. This suggests that different forms of SSB in these monkeys share a common genetic basis.
What underpins this behavior?Previous studies on the heritability of SSB have primarily focused on humans. However, these studies often rely on self-reported data, which can introduce complications. The cultural stigma surrounding homosexuality, for instance, could lead to the underreporting of homosexual activity.
Heritability of SSB has also been found in some invertebrate species, including seed beetles and fruit flies. However, the pathways through which SSB develops in these species are thought to be different from those observed in social vertebrates like primates. For example, factors such as imperfect sex recognition are believed to influence the development of SSB in invertebrates.
Demonstrating that SSB is heritable and its potential for evolutionary response to natural selection is an important first step towards understanding the factors that influence variation in this behavior.
Many evolutionary theories for SSB in animals exist. But they all depend on the behavior showing a degree of heritability.
One theory suggests that in some species, animals may engage in SSB because it serves a beneficial social function. For example, it may strengthen the bonds between males, ultimately benefiting them during competition for mates and food.
In support of this theory, our research found that male rhesus macaques involved in SSB partnerships were more likely to support each other in conflicts with other individuals. This effect could be a way in which SSB benefits a macaque and its chances of producing offspring, thereby allowing the behavior and the genes associated with it to persist within a population.
Learning from primatesSo what can we learn from these findings about SSB across primate species, including humans?
A previous study examining SSB heritability in humans found significant reproductive costs associated with this behavior. In contrast, we found no such costs in macaques.
This suggests that the costs associated with human SSB might arise from specific social factors unique to humans. However, more research is needed to explore this idea further.
Today, some people still believe that SSB is rare or the product of extreme and unusual environmental conditions, and selectively look to examples in nature to validate their view. Our results may help to challenge these beliefs and combat prejudice against homosexuality and bisexuality. However, society's moral obligation to strive for more inclusivity and acceptance of different sexual orientations ultimately does not rely on observations from the natural world.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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Scientists Studying Rhesus Monkeys Find Surprising Benefits To Same-sex Relationships
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.
Rhesus macaques monkeys grooming each other. A new study examining same-sex behavior among a colony of rhesus macaques is challenging humans' views on sexuality.
Dreamstime/TNSScientists 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.
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"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 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."
Meet the Japanese macaques of Jigokudani Monkey Park IntroThe Japanese macaques in Jigokudani Monkey Park begin most days with a relaxing dip in their own private hot tub.
Located in Joshinetsu Kogen National Park in Japan's Nagano prefecture, the primate-only onsen is world-famous as the preferred simian hot spring.
But life hasn't always been so simple for the snow monkeys of Jigokudani ("The Valley of Hell," named for its natural hot springs). They were initially forced from their habitat in the 1950s by the development of nearby ski resorts.
As a result, they moved into nearby towns and quickly found trouble, raiding fruit from local farmers' orchards. The farmers petitioned the government and were granted the right to hunt the macaques.
A Nagano Railway Company worker and local nature enthusiast, Sogo Hara was determined to protect the monkeys and worked for months to persuade troops to leave the creatures alone. Hara's strategy was to lure the monkeys into the mountains with apples.
His group soon discovered a nearby ryokan (a traditional Japanese inn). As is typical for ryokans, Korakukan's property included outdoor onsen for guests' enjoyment. One day in 1963, so the story goes, an errant apple fell into the human's onsen. A young monkey tested the steamy water and emerged with the apple. He didn't exit the waters immediately, though, and many of the monkeys looked on as their buddy quietly enjoyed the water.
It wasn't long before other young monkeys began mimicking the water-loving monkey's behavior.
Within months, all of the female monkeys had taken to bathing in the hot springs. While this behavior delighted human visitors, it created a clear hygiene issue for the inn.
This led to the establishment of the Jigokudani Monkey Park in 1964, thanks to the efforts of Hara. Today, this is where the snow monkeys live, freely soaking in their dedicated hot springs peacefully.
Damage resulting from Typhoon No. 19 had impacted the spring's water levels and temperature but restoration work is now underway and the park is open to tourists.
The Jigokudani Facebook page offers daily updates on the restoration work and presence of the monkeys.
A relaxing soakThe snow monkeys' onsen practice isn't solely about warming their bodies or bobbing for apples -- like the Japanese and visitors to Japan, they seem to appreciate onsen for its stress-reducing properties.
Since the macaque's thick coats keep them warm, and they take a dip even when there are no apples in sight, researchers' studying the monkeys at Jigokudani believe hot springs function primarily to ease their tension.
Staff encourage the macaques to keep the park's onsen in their daily routine, offering a steady diet of soybeans, barley and, of course, apples.
Researchers also discovered the monkeys' indifference to the nearly 500 daily visitors to the park. In other words, their stress hormones did not increase as the number of park visitors increased.
Even during the busy tourist season, the monkeys meander nonchalantly about the park, climbing in and out of the hot spring as humans snap photos and ooh and ahh in delight.
Unlike some of Japan's other open animal experiences, such as Nara Deer Park, touching or feeding the monkeys is strictly prohibited. But this rule doesn't detract from the joy of sitting poolside as the macaque play leapfrog.
Meet the monkeysThe baby monkeys love a good snow romp.
Courtesy Joshua MellinSince the monkeys don't swim every day (in spite of their warm fur coats, they seem to prefer the hot springs on colder days), visitors will want to plan accordingly before heading north to the park.
A flexible itinerary is best, and January and February are prime visiting months. Fresh snow is ideal -- the monkeys are at home and perhaps at their playful best in the snowy climate. If you're lucky, you might even catch baby monkeys wrestling and making snowballs.
Time of day is a critical piece of planning, too: Jigokudani Monkey Park requires an overnight stay to catch the monkey's early morning soak. While it is possible to do it as a day trip from Tokyo, even catching the earliest train will make for a later arrival, when crowds are at their greatest, and that's if everything goes off without a hitch. If you miss seeing the monkeys, you'll wish you had allowed for more time.
The park opens at 9 a.M. And closes at 4 p.M. Daily, with extended hours from 8.30 a.M. To 5 p.M. Seasonally.
You're better off arriving the night before you plan to visit. Stay at a ryokan, delight in its onsen and take off for the park first thing in the morning. If you can, leave time for onsen exploration beyond your ryokan, for at the base of the mountain leading to the park is a main street packed with nine unique onsens for weary travelers.
Each meant to aid a particular ailment, the onsen encourage visitors to walk the charming street in a yukata (light kimono) and wooden sandals to collect stamps on a towel passport from all the onsens. Doing so is believed to ward off evil spirits and bring divine favor. The thin strip contains no shortage of mom-and-pop restaurants eager to share their house-made versions of Japanese staples.
Getting thereMost of these inns offer complimentary morning shuttles to the Jigokudani Monkey Park's information center. To best even these crowds, early risers can trek up the scenic path themselves for a chance at some private time with the monkeys.
From the information center, the monkey's onsen is only accessible via a one-mile path through the forest, about 30 minutes each way. Unfortunately, Jigokudani Monkey Park is not accessible to wheelchairs as the final ascent consists of tight winding paths and steep steps.
Jigokudani Monkey Park's pool-side views offers visitors a delightful insight into this unique behavior. Relaxing with the chilled-out macaques is a quintessential excursion from Japan's mega-cities.
Getting to Jigokudani Monkey Park from Tokyo requires an hour and a half ride on the shinkansen (high speed train) to Nagano, then a 50-minute trip on a local train to Yudanaka station in Yamanouchi, the town the park is located in. The Jigokudani Monkey Park information center is two miles from the train station.
For winter enthusiasts exploring nearby Shiga Kogen, the largest ski field in Asia, it's less than a hour's bus ride away.
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Older Macaques With Smaller Social Circles
Do you find yourself prioritizing close friends and family over a wide circle of friends as you grow older? It's a pattern commonly observed in humans. Now, scientists have found the same behavior in female rhesus macaques.
To explore the phenomenon, scientists examined eight years of data that tracked over 200 macaques living on Puerto Rico's Cayo Santiago. They looked at grooming interactions and spatial proximity among the macaques, which many studies use as a measure of social connectedness. They found that female macaques reduced their grooming partners by 44% on average over the 8-year span, and also decreased the number of other macaques they approached. The individuals that they did spend time with tended to be relatives or old friends.
Because the macaques spent similar amounts of time socializing when they were older as they did when they were younger, the team ruled out other explanations for the reduced social circles, like death of partners, or reduced ability or interest in socializing.
The scientists noted that the benefits of wider versus smaller social circles may change as the macaques age. While wider social circles could benefit younger macaques by offering a wider pool of potential social partners, older macaques may have more to risk and less to gain. For example, wider social circles also increase the potential for disease transmission or conflict. New relationships also take more mental effort, so smaller social circles may help older macaques may conserve mental energy.
All explanations aside, maybe there's just something special about spending time with old friends and family.
Reviewer: Clare Kimock, Nottingham Trent University
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