https://phys.org/news/2018-05-huddling-survivalmonkeys-social-partners-winter.html

This method of keeping warm, called social thermoregulation, means that the macaques with more grooming partners would stay warmer, spend less energy on maintaining body temperature and be less exposed to environmental stress, increasing their probability of surviving winter.

The study is the first to show that such social huddling may be a mechanism that connects social bonding to higher "fitness—the term used by scientists to measure of how well animals can cope with their local ecological conditions, usually measured by reproductive success and survival.