While most mammals can adapt to the cold, many small mammals give up the struggle. Stephen Harris explains the strategies they adopt to cope with chilly weather.
As winter approaches, large mammals can often migrate to warmer areas or grow a long, dense winter coat, but these options are not open to small ones.
Small mammals face other disadvantages, too – they have a larger surface area to volume ratio (and so lose heat more quickly), and short, thin fur.
And since small mammals already need a high metabolic rate to maintain body temperature, they cannot increase this further when food is scarce.
So the solution is simple: give up the struggle and let your body temperature fall. Small mammals do this in two ways: torpor and hibernation. Both are strategies to reduce energetic costs.
WHAT ARE TORPOR AND HIBERNATION?
- Torpor is a short-term reduction of body temperature on cool days. Hibernation is an extended form of torpor.
- Torpor is driven by ambient temperature and food availability; hibernation is associated with day length and hormone changes.
- There are features associated with both processes: a controlled reduction of body temperature, typically to within 1-2˚C of ambient temperature; a fall in oxygen consumption and breathing, heart and metabolic rates; a restriction of blood flow to the main organs and the ability to awake spontaneously whatever the outside temperature.
- In Britain, torpor is recorded in dormice, bats, hedgehogs and wood mice. It lasts a few hours.
- Animals go into torpor in their normal daytime refuges.
- Torpid animals can’t perform co-ordinated movements and barely respond to stimuli.
- To go into torpor, a bat simply reduces its metabolic rate – the decline in body temperature typically takes about two hours.
- Once torpid, the metabolic rate is very low. When the outside temperature is 15˚C, the metabolic rate of an active bat is 40 times higher than that of torpid bat. Hence there are considerable energy gains to be made, which increase as the ambient temperature falls.
- To come out of torpor before an evening’s foraging, a bat displays violent shivering and muscle contractions. Arousal takes just under an hour.
- Hibernators generally select special hibernacula, but hedgehogs may use summer nests. Bats often have very specific temperature and airflow requirements.
- Common dormice hibernate at ground level in woven nests partly covered by leaf litter or moss or in hollow trees. Edible dormice have been recorded under tree roots and beneath floorboards in outbuildings.
- Body temperatures drop a lot lower than in torpor, with metabolic rates falling to between 1 and 2 per cent of the active animal. The heart rate of an active hedgehog is 200 to 280 beats per minute, but only 5bpm in hibernation.
- The body temperature of a hibernating hedgehog is around 4˚C. If the ambient temperature drops below zero, the metabolic rate is increased to prevent the animal from freezing. This is still less expensive than the energy costs of complete arousal.
Do mammals wake up during hibernation?
- Yes – though it’s more extended than torpor, hibernation is not continuous.
- Waking up from hibernation takes a few hours. Hedgehogs do this on average every 7 to 11 days and British bats about every 20 days. Hedgehogs may be seen in winter moving to another nest before resuming hibernation.
- When bats emerge from hibernation, they fly around in or emerge from their caves. They may drink as they lose a lot of water during hibernation.
- It must be important for mammals to wake up periodically, since bouts of arousal consume 80 per cent of energy reserves expended during hibernation.
- Recent research suggests that mammals are vulnerable to parasites during hibernation and so wake periodically to boost their immune systems.
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