By Dr Jean Holley, UNE Discovery

I have to confess, I’m not the biggest fan of Winter. Give me warm summer evenings and days at the beach any time! Nonetheless, I am grateful that I have a warm home to go to when it gets cold. But what about other animals? How do they cope with cold or adverse conditions? Most of us have heard of the term hibernation, but what exactly does this mean. What strategies can animals use to survive the cold?

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Mammals (and birds) are endotherms, meaning that they can maintain a high, and constant body temperature that is independent of the environmental temperature. This is possible because they have a high metabolic rate. High and constant body temperatures allow endotherms to be active at a wide range of environmental temperatures. This comes at a cost however, because maintaining a high body temperature requires lots of fuel in the form of food or stored fats. This is particularly true for smaller mammals because their high surface area/volume ratio means they lose heat quickly, and must produce a lot of heat to compensate for heat loss, especially during cold periods.

A Nifty Strategy

What can a mammal do during very cold winters when they need enormous amounts of fuel to maintain a high body temperature? Instead of trying to find enough fuel, they reduce their body temperature and metabolism through entering a state known as torpor. By reducing their body temperature (sometimes by as much as 30°C, depending on the species and other factors), animals can reduce energy expenditure, and therefore the amount of fuel needed, allowing them to survive cold conditions where food is sparse. But don’t be fooled, torpor is not just taking a ‘cool snooze’; it is a very complex physiological process involving changes to metabolism, body temperature and heart rate.

An eastern pygmy possum in a state of torpor. By curling up, heat loss is minimised. Photo credit: Chris Wacker.

There are two main types of torpor: daily torpor and hibernation (also known as prolonged torpor). Daily torpor is where an animal enters a state of torpor for several hours during the day, and is usually interrupted by daily foraging and feeding. Daily torpor can occur throughout the year, although its use often increases in winter. It isn’t just the cold that torpor is used to cope with. Some small marsupials like the brown antechinus, for example, will use torpor to reduce their energetic needs when food is scarce after a bush fire. Similarly, the marsupial Mulgara can use daily torpor during pregnancy to store fat in anticipation for the energetically demanding task of lactation. Daily torpor is common in birds, and is known from a very large number of small marsupial and placental mammals (Geiser 2002; 2004).

Hibernation on the other hand is often seasonal, usually lasting from autumn to spring. During hibernation, animals have longer, and deeper torpor bouts than those used in daily torpor. During these bouts, body temperatures are low and bodily functions are reduced to a minimum. Torpor bouts during hibernation last for several days or weeks, and are interrupted by periodic rewarming and brief (usually less than one day) resting periods when the body temperature is high (Geiser 2002). That is, hibernators do not remain in a state of torpor for the whole winter, but rather go through bouts of torpor, broken up by brief periods of rewarming. Many animals fatten extensively before entering hibernation and rely to a large extent on stored fat for an energy source during winter. Australian natives that use hibernation include echidnas, pygmy-possums and feathertail gliders.

What a nifty strategy!

Trading off activity to find food for cooling down helps animals survive with a lot less food or fat on board. If only I could enter hibernation and ‘sleep’ my way through winter…I suppose I will have to content myself with evenings snuggling on the couch, with Netflix and a hot chocolate- I guess it isn’t too bad after all.

To read more about Torpor and Hibernation, see this piece in The Conversation written by UNE Postdoctoral Research Fellow, Chris Wacker.

References and further reading:

Geiser, F. 2002. Hibernation: Endotherms. DOI: 10.1002/9780470015902.a0003215.pub2

Geiser, F. 2004. Metabolic rate and body temperature reduction during hibernation and daily torpor. Annu. Rev. Physiol. 2004. 66:239–74.