Friendship comes with complex pros and cons – possibly explaining why some individuals are less sociable, according to a new study of gorillas.
Scientists examined over 20 years of data on 164 wild mountain gorillas, to see how their social lives affected their health.
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.
Meanwhile, males with strong social bonds tended to get ill more – but were less likely to be injured in fights.
The study – by the Dian Fossey Gorilla Fund and the universities of Exeter and Zurich – may help to explain why such a wide range of traits have evolved in social animals, including humans.
"Having a lot of strong social relationships is often really good – but sometimes it isn't," said Dr Robin Morrison, lead author on the paper and Senior Researcher at the University of Zurich.
"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."
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 findings highlight the forces that affect the evolution of social behaviour.
"With these forces pushing in different directions, the 'optimal' social type will depend on an individual's sex, age, offspring and wider social group," said Dr Sam Ellis, from the University of Exeter.
"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 is 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.
"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," said Dr Tara Stoinski, CEO and Chief Scientific Officer of the Dian Fossey Gorilla Fund and one of the study co-authors.
The research was funded by the Swiss National Science Foundation and the Dian Fossey Gorilla Fund.
The paper, published in the journal Proceedings of the National Academy of Sciences, is entitled: "Group traits moderate the relationship between individual social traits and fitness in gorillas."
Case studies: the varied social lives of mountain gorillas
Gutangara – an adult female – lives in one of the largest gorilla groups. She has good relationships with many gorillas but spends most of her time with her offspring – some that are still young and others that are already adults. She is the most successful gorilla mother the researchers have seen, with eight surviving offspring.
Maggie was the highest-ranking female gorilla in the Bwenge group. She was one of the most frequently aggressive group members but was also quick to provide friendly support such as grooming. Maggie often led group interactions and acted as a protector, a role usually taken by males. 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 this 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 – had a difficult childhood. He 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. Titus had an unusual leadership style, being exceptionally gentle and calm. His closest relationships were with the females in his group, who often stayed in physical contact with him, something rarely observed to this extent in other groups. 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. He was known for his authoritative but peaceful nature, rarely initiating or entering fights, but was quick to protect others and resolve conflicts in the group. He had a particularly close relationship with his son Gicurasi, whose mother left when he was young, and who eventually took over leadership of the group in Cantsbee's final years. When Cantsbee later became ill, he chose to leave the group, spending his final months alone, except for one brief visit to the group shortly before his death.
