Small hibernators – winter survival
As the cold, dark weather draws in and the rain patters at the window many of us probably wish we could hibernate. Instead, all we can do is put on a big jumper and have a cup of tea. We like keeping ourselves warm because we are warm-blooded mammals who need to maintain a stable body temperature, allowing our metabolism to efficiently function. We are endotherms, generating our own heat rather than relying on a warm climate for survival.
It takes a great deal of energy to maintain the warm, constant body temperature of an endothermic mammal, especially if you are small. The smaller you are the larger the surface area you have in relation to your size and therefore the easier it is for you to lose energy in the form of heat. Smaller hearts also have to work harder to pump blood around the body. If you imagine a shrinking heart, the volume of blood it can pump reduces faster than the surface area of the heart. For these reasons small mammals have faster beating hearts than larger animals and a higher metabolic rate.
If you have a very demanding metabolism you will find it particularly difficult to survive when food is scarce and the weather is cold. This is why some small animals hibernate throughout winter. They are too small to migrate to warmer lands and fat reserves would not be enough to live off if fully active. So they take cover in a hibernaculum (a shelter in which an animal remains dormant for the winter). Their body temperature drops to within 1 to 2°C of ambient temperature and their metabolic rate falls to 1 to 2% of the rate when they’re active.
Incredibly, a hibernating hedgehog can have a body temperature of only 4°C and a heartbeat of only 5 beats per minute (bpm), whereas an active hedgehog has a body temperature of 35°C with a heartbeat of 200 to 280bpm. Hedgehogs are one of only three mammals that hibernate in Britain, the others being dormice and bats. These mammals can also enter a short-term version of hibernation called torpor to save energy when the weather is cold or food is scarce for only short period of time. When in this state their body temperature drops and metabolic rate slows, as when hibernating, but they stay in normal daytime refuges and generally return to normal activity after a few hours or days.
Bats are surprisingly long lived for their size, probably because they spend a substantial amount of time in a state of torpor and hibernation. They weigh less than 10g but can live over 25 years in the wild. In contrast other small mammals such as shrews and mice often live for less than a year. When the outside temperature is 15°C the metabolic rate of an active bat is 40 times higher than that of a torpid bat, illustrating the huge saving of energy when torpid.
Mammals do sometimes wake up during hibernation. Hedgehogs do this on average every 7 to 11 days and British bats about every 20 days. They may even leave their hibernaculum for a period of time, and in the case of the hedgehog, find an entirely different nest to resume hibernation. One reason why hibernating species are thought to do this is to boost their immune systems, as mammals are vulnerable to parasites during hibernation. They may also wake to defecate, urinate or munch on some food. It is still incredibly important not to wake an animal up during hibernation as natural bouts of arousal already consume 80% of energy reserves during hibernation.
Of course not all small mammals hibernate, as this is just one survival strategy. Field voles instead get by in winter by growing a thick layer of fur and surviving on roots, bulbs and bark rather than its more leafy summer diet. But wouldn’t you rather have a nice deep sleep? I think I would.
References
Photograph: by Jalte Peter Bregnehave Windum, 2007.
BBC Nature, (2014), Hibernation, available at: http://www.bbc.co.uk/nature/adaptations/Hibernation accessed 14 October 2014.
Discover Wildlife, (2010), How to tell torpor from hibernation, available at: http://www.discoverwildlife.com/british-wildlife/how-tell-torpor-hibernation, accessed 14 October 2014.
The Mammal Society, (2014), Field vole, available at: http://www.mammal.org.uk/fieldvole, accessed 14 October 2014.
Welsh Wildlife, (2014), To hibernate or not to hibernate, available at: http://www.welshwildlife.org/to-hibernate-or-not-to-hibernate/ accessed 14 October 2014.
Wildlife Trust, (2014), Hedgehogs, available at: http://www.wildlifetrust.org.uk/facts/hedge.htm accessed 14 October 2014.
