Among gorillas, as among human beings, friendship is a question of compromise. — AFP
It's said to be precious, comforting and essential to our wellbeing, but behind the hugs and shared experiences sometimes lurks a darker side. What if making friends also means taking a risk? This surprising hypothesis is supported by observations made among mountain gorillas.
A long-term study carried out in Rwanda reveals that, for these primates, friendships can have complex pros and cons.
Published in the journal Proceedings Of The National Academy Of Sciences, this study is the fruit of two decades of observation by the Dian Fossey Gorilla Fund, in collaboration with the Universities of Exeter and Zurich.
Over a period of 20 years, the researchers tracked 164 mountain gorillas living in the wild in Volcanoes National Park. Their aim was to understand how social relationships influence health, conflict and reproduction in these great apes.
Although the scientists expected to identify positive effects of strong social bonds, the results turned out to be more nuanced, sometimes even surprising. The impact of relationships varies according to the sex of the individuals and the size of their group.
In females, small, close-knit groups are associated with better health and less exposure to disease. However, this stability seems to hinder reproduction.
Conversely, females living in larger groups have higher birth rates, but are more exposed to disease.
A source of stress?
In males, too, the effects of social relationships are mixed. Those with close relationships fall ill more frequently, but are less likely to be injured in fights.
As such, for mountain gorillas, “having a lot of strong social relationships is often really good – but sometimes it isn’t”, explains Dr Robin Morrison, lead author on the paper and senior researcher at the University of Zurich, quoted in a news release.
So how can this be explained? The researchers suggest that the stress of protecting the group, particularly in males, may weaken their immune systems.
“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,” says Dr Morrison.
The study was not limited to quantifying social interactions or observing isolated behaviours. It analysed the strength of each individual’s primary bonds and level of integration within the group, taking into account contextual factors such as group size and stability, and intergroup conflict.
These different variables offer a better understanding of the forces shaping 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,” says study coauthor, Dr Sam Ellis, of the University of Exeter.
And this idea extends beyond gorillas.
“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,” adds Dr Ellis.
Differing temperaments
The gorillas observed in this study generally live in groups of a dozen or so individuals, organised around a dominant male. But this structure does not eradicate the great diversity of characters and backgrounds.
This is one of the major lessons of this research: there is no single model of social success, but several, sometimes very different ones, which coexist depending on the context.
To illustrate this diversity, the researchers drew up portraits of a number of outstanding individuals as case studies. Their trajectories show how singular temperaments – gentle, dominant or more daring – can influence group dynamics, sometimes overturning established rules.
Among them, Gutangara, a sociable female living in one of the largest groups, is also the most prolific mother on record, with eight surviving offspring.
Maggie, another dominant female, combined authority and tenderness. When the male in her group died, she took over as leader, before venturing off alone to the Democratic Republic of Congo.
Among the males, Titus, a precocious orphan, led his group for 20 years with a rare gentleness, benefitting from special attention from the females.
Cantsbee, on the other hand, remained in charge of his group for a record 22 years. The father of at least 28 cubs, he maintained a strong bond with his son Gicurasi, who succeeded him.
Ill in his last months, he gradually isolated himself, joining his group only once before his death.
In short, in gorillas as in human beings, friendship is based on compromise. It can protect or weaken, promote or hinder.
For while social bonds are a driving force in evolution, they are never free of cost or consequence: they are shaped, transformed and can sometimes become a burden. – AFP Relaxnews
