It’s three in the morning in South Africa, in the middle of winter. Temperatures have dropped to just below freezing and a vervet monkey—silver-furred and black-faced—is very, very cold. Soon, the rising sun will heat the land to a much nicer 25 Celsius, but for now, the vervet faces five hours of bitter chill. So, it seeks out some friends for warmth. And as they huddle together in a shivering heap, tiny thermometers in their bodies record their temperatures.
Richard McFarland from the University of the Witwatersrand implanted the devices in a dozen vervets in January 2011. His goal: to see if their social ties could help these wild animals to cope with wildly varying temperatures.
McFarland is no stranger to such research. In 2008, he studied two groups of Barbary macaques—stocky, stump-tailed monkeys, living wild in Morocco’s Atlas Mountains. He found that those with more social ties were more likely to survive the winter of 2008—an exceptionally cold and snowy season that killed 30 of them. It was the first study to show that an animal’s social connections can affect its odds of surviving a bout with the elements. The question was: why?
One possibility was that the well-connected macaques were better at keeping warm, because they could more easily find partners to huddle with or could huddle in bigger groups.
To test that idea, McFarland’s team travelled to the opposite end of Africa. They tranquilised a dozen female vervets and surgically implanted wax-sealed temperature recorders into their abdomens. When the monkeys came to, the team released them and counted how many social partners they had—that is, how many other vervets they groomed and were groomed by. Nineteen months later, the team recovered the recorders and the precious data they contained.
The data revealed that during winter, the monkeys’ core temperatures were often three degrees lower at night than in the day. As the months wore on, these fluctuations became increasingly severe. But the more partners the monkeys had, the steadier their body temperatures were. Sociability meant stability.
These results put a new twist on the value of group-living among primates. In some now-classic studies, Joan Silk from the University of California, Los Angeles, showed that young baboons are more likely to survive if their mothers have strong social bonds. There could be many reasons for this, but one popular idea is that such bonds help animals to deal with stress. Certainly, well-connected monkeys have lower stress hormones than isolated ones, and that alone might give their immune systems a break, and improve their survival. The same applies to humans—people with bigger social networks tend to have better physical and mental health.
But the vervets show that social ties have another important benefit. “Better-networked monkeys stay warmer in winter,” says Katie Hinde from Harvard University, who was not involved in the study. “That has the potential to protect animals from illness and leave them with more calories for building babies.”
And unlike Silk’s baboons, “it’s not just strong bonds that matter,” says Louise Barrett from the University of Lethbridge, who led the research. “Weak bonds matter too. Having a lot of partners that you can call on means you can have a bigger huddle, or find a partner if yours is already busy. There’s a utilitarian function to maintaining a lot of bonds.” In other words, quantity matters as much as quality. This effect might even be important in keeping a vervet group together, and stopping its members from splintering into smaller factions of tightly knit—but very cold—individuals.
Huddling isn’t the only way in which groups stay warm, either. Vervets cement their friendships by grooming each other, and that improves the insulating properties of their fur. “It’s like fluffing a duvet,” says Barrett.
“I think this study has important implications for the behavioural ecology of personality,” adds Hinde. She wants to know the vervets’ personality traits affect their social networks and how that, in turn, affects their survival and reproductive success. Do gregarious monkeys stay warmer than live longer than loners? And how does that vary between parts of Africa with different climates? In regions where temperatures swing more manically throughout the day, do monkeys have better social integration?
Reference: McFarland, Fuller, Hetem, Mitchell, Maloney, Henzi & Barrett. 2015. Social integration confers thermal benefits in a gregarious primate. Journal of Animal Ecology http://dx.doi.org/10.1111/1365-2656.12329