For years, only primates living on the island of Madagascar have been known to hibernate. But now, according to new findings published in Scientific Reports this week, the pygmy slow loris – a little primate that lives in the tropical forests of Vietnam, Cambodia, Laos, and China – hibernates too.  

During the energy conservation state known as torpor, body temperatures and metabolic activities are reduced. These bouts can last for less than a day (that’s called daily torpor) or for more than 24 hours (multi-day torpor, or hibernation). Both daily torpor and hibernation are common and occur in at least 11 orders of mammals: from rodents and bats to marsupials and monotremes (like echidnas) spanning arctic, temperature, and tropical habitats. But primates are a peculiar case: While the African lesser bushbaby is capable of daily torpor as an emergency response, Malagasy lemurs are the only true hibernators. No primate outside of that island has been known to hibernate, until now. 

A team led by Thomas Ruf from the University of Veterinary Medicine in Vienna hypothesized that the pygmy slow loris, Nycticebus pygmaeus, is a good candidate species for the use of hibernation outside of Madagascar. It lives in a habitat with seasonal changes in temperature and precipitation, and thus changing food availability; and at 400 grams (14 ounces), it’s small like most hibernators (bears are an obvious exception to this size rule). Also, high rates of heat loss and low winter temperatures of a cool, dry 5°C (41°F) create demanding energetic costs for thermoregulation.

Using implanted data loggers, the researchers monitored the core body temperatures of six pygmy slow lorises exposed to natural climate conditions in outdoor enclosures in northern Vietnam’s Ninh Bình during the autumn, winter, and spring. These animals had been confiscated from illegal wildlife traders and were kept at the Endangered Primate Rescue Center in Cúc Phương. 

Lorises of both sexes entered a state of multi-day torpor from mid-December to mid-February. The first bouts started during a cold spell in late October, and the last one occurred in early April. These bouts lasted 43 hours on average, though some stretched to 63 hours. 

Even though torpor may be induced by shortage of food, these lorises were continually offered (and consumed) a broad spectrum of food items throughout the study period. That means the hibernating lorises, surprisingly, weren’t food restricted or nutritionally stressed. The benefits of torpor in terms of energy savings, on the other hand, are evident. As body temperature decreases during hibernation, energy expenditure is reduced to approximately five percent of the animal’s basal metabolic rate.

^{Images in the text: Tilo Nadler}