Dropping your body temperature and going into torpor is a sensible choice if you are an animal facing cooler temperatures and/or food scarcity. But when more favourable conditions arrive, you also need to be able to raise your body temperature again. Animals from arctic or temperate regions often stock up on their brown fat cells prior to winter, which, when stimulated with noradrenaline, burn off the fat as heat by a process called nonshivering thermogenesis (NST). However, do animals in warmer regions, such as Africa, also use NST or can they rely on passive heating from the environment? It's an area of much debate, explains Julia Nowack, from the University of Hamburg, Germany. With the help of her advisors, Kathrin Dausmann, also from the University of Hamburg, and Nomakwezi Mzilikazi, from Nelson Mandela Metropolitan University, South Africa, Nowack decided to investigate whether African lesser bushbabies, which sometimes have unusual difficulties rewarming after torpor, have the ability to use NST (p. 3811).
Nowack travelled to South Africa, where she trapped bushbabies by tempting them with banana, honey and peanut butter treats. She then kept them in outdoor enclosures, where the daily temperatures averaged 30°C in summer, but dipped as low as −5°C in winter. To test whether they could use NST, Nowack injected her bushbabies with noradrenaline and saw that they increased their oxygen consumption as well as raised their skin temperature by 1°C – good signs that they were burning off fat. Although there was no overall specific increase in NST during winter months, Nowack did find that during short colder snaps they did increase NST capacity to warm up. What's more, when she dissected two bushbabies that had died of natural causes, she clearly saw deposits of brown fat cells throughout their bodies.
It's clear that at least African lesser bushbabies can use NST. However, as there is no overall increase of NST during winter months it's possible that they rely more on passive heating or huddling together to warm up. Alternatively, bushbabies might use torpor throughout the year and as such don't go through a stocking up phase like their arctic friends, to increase NST.