Large groups limit social diversity among mountain gorillas 

A new study reveals that living in large groups limits social diversity among mountain gorillas. Scientists from the Dian Fossey Gorilla Fund and the University of Exeter report that mountain gorillas usually form groups of 12 to 20, which they found to be the optimal size for cultivating a variety of relationships.

The researchers identified seven categories of relationships among the gorillas, ranging from close maternal ties to less significant acquaintances. Some groups had as many as 65 individuals, and groups of this size were characterized by much lower social diversity. 

“It is often assumed that animals living in larger groups will have more diverse and complex social lives,” said study co-author Dr. Robin Morrison.

“However, our study suggests that social diversity is lower in very large groups where gorillas must maintain a larger number of relationships – with most relationships falling into the weakest category.

Dr. Morrison said that strong social relationships still exist in the big groups, but they seem to make up a smaller proportion of the total relationships.

“We cannot say for certain why this is, but it may be that gorillas only have enough time and mental energy to maintain a certain number of relationships at a given strength.”

“So they keep their key relationships and simply maintain weak ties with all the other gorillas in the group.”

According to Dr. Morrison, living in a social group requires mental effort. “Indeed, one of the big ideas in social evolution is that humans developed large brains and language to deal with social complexity.”

The research was focused on more than a decade of observations of 13 gorilla groups, which were monitored by the Fossey Fund in Volcanoes National Park, Rwanda. The numbers of mountain gorillas have grown in recent years, and this could explain why unusually large communities have formed.

The gorillas live in stable social groups that forage together in the day and nest together at night. To measure social relationships, the study authors studied proximity data which showed how much time certain individuals spent close together.

“In many primates, social interaction can be measured by how much time individuals spend grooming each other,” said Dr. Morrison. “However, gorillas spend less time grooming than most other primates. Instead, a lot of gorilla society is about who individuals choose to sit next to, and who they move away from.”

The relationship categories identified by the scientists relate to different patterns of interaction, representing a scale from close to weak associations. However, the categories cannot be expressed in human terms such as best friends or close friends.

“Not only were groups above a certain size not more socially diverse, but individuals living in the same group had variable levels of social complexity – some gorillas had a greater diversity of social relationships than others,” said study co-author Professor Lauren Brent.

“This adds to a rich body of evidence that shows that, whether you are a human, gorilla or another type of social animal, not everyone experiences their social world in the same way.”

The social relationships experienced by individual gorillas were found to vary by age and sex. The females maintain a relatively consistent diversity of relationships during adolescence and adulthood, but for males social diversity declines rapidly as they enter adolescence.

At around 14, males are still several years from full sexual maturity yet exhibit some signs of young adulthood. Adolescence is also the time when males are faced with the decision of whether to leave the group they were born into, so it is possible that social distancing may lead up to their potential departure.

About half of the males choose to stay with their childhood community. In this case, they gradually build a diverse set of relationships through adulthood as they take on fatherhood and protective roles.

The scientists explained that the findings could be useful for gorilla conservation, such as efforts to limit the spread of disease.

“By better understanding these social relationships, we can better understand how diseases would spread through these social groups,” said study co-author Dr. Tara Stoinski, who is the president and CEO of the Fossey Fund.

“This is really important right now for mountain gorillas because disease is one of the major threats to their conservation. They catch many of the same diseases as humans, including Ebola, and it’s extremely likely they would also catch COVID-19 if they were exposed to it.”

“Long-term monitoring and protection of endangered mountain gorillas is crucial, not only for their conservation but also for what we can learn from this intelligent and highly social species about how complex social behaviour, such as our own, has evolved.”

The study is published in the journal Proceedings of the Royal Society B.

—

By Chrissy Sexton, Earth.com Staff Writer