Study reveals pros and cons of gorillas being social

By Stephen Beech

A study of gorillas in the wild has revealed the "complex" pros and cons of friendship.

The findings may explain why some people are less sociable than others, say scientists.

An international team of researchers examined more than 20 years of data on 164 wild mountain gorillas to see how their social lives affected their health.

Their findings showed that costs and benefits changed depending on the size of gorilla groups, and differed for males and females.

For example, friendly females in small groups didn’t get ill very often but had fewer offspring – while those in large groups got ill more, but had higher birth rates.

And males with strong social bonds tended to get ill more – but were less likely to be injured in fights, according to the findings published in the journal Proceedings of the National Academy of Sciences (PNAS).

The study – by the Dian Fossey Gorilla Fund and the universities of Exeter and Zurich, Switzerland – may help to explain why such a wide range of traits have evolved in social animals, including humans.

Lead author Dr. Robin Morrison, Senior Researcher at the University of Zurich, said: “Having a lot of strong social relationships is often really good – but sometimes it isn’t.

“For example, our study found that strong and stable social bonds are generally linked to less illness in female gorillas – but more illness in males.

“We can’t be certain why this happens, but it appears it’s not a simple matter of social contact leading to more disease risk.

“It’s possible that males expend more energy by having close social ties, as they have to defend females and offspring, and the stress of this may reduce their immune function.”

She explained that the study focussed on the strength of each gorilla’s key social bonds, and its integration in the group – along with wider context such as group size, stability and conflict with other groups.

The researchers say their findings highlight the forces that affect the evolution of social behavior.

Dr. Sam Ellis, from the University of Exeter, said: “With these forces pushing in different directions, the ‘optimal’ social type will depend on an individual’s sex, age, offspring and wider social group.

“In humans and other social mammals, the social environment is one of the strongest predictors of health and lifespan.

“But our study shows this isn’t a straightforward case of more and stronger social ties always being better.

"In some situations social traits that we’ve previously thought of as maladaptive can have important benefits.”

The study was based on long-term observations of mountain gorillas in Volcanoes National Park, Rwanda, which usually live in groups of about 12 with a single dominant male.

Gorillas followed included Gutangara, an adult female, living in one of the largest groups.

She is the most successful gorilla mother the researchers have seen, with eight surviving offspring.

Maggie, the highest-ranking female gorilla in the Bwenge group, was one of the most frequently aggressive group members but was also quick to provide friendly support such as grooming.

When the dominant male died unexpectedly, Maggie took charge, guiding the group until they merged with a neighbouring group.

But Maggie struggled to fit into the new group and eventually chose to leave. She travelled alone for a month until she crossed into Congo, where her trail was lost.

Titus – a “silverback” adult male mountain gorilla – lost his father and many other group members to poachers, and his mother died when he was only four.

He became the dominant male of his group at the early age of 15.

His gentle nature made him very attractive to females and contributed to his 20-year tenure as dominant male until his death in 2009.

Cantsbee, also a silverback, led his group for 22 years – the longest dominance tenure ever recorded – and fathered at least 28 offspring.

Known for his authoritative but peaceful nature, Cantsbee had a particularly close relationship with his son Gicurasi, who eventually took over leadership of the group in Cantsbee’s final years.

Study co-author Dr. Tara Stoinski, chief scientific officer of the Dian Fossey Gorilla Fund, added: “This paper highlights the incredible value of long-term studies to furthering our understanding of the evolution of sociality and how the benefits or costs of sociality can vary considerably across different environments.”