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.

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