January - Deep Winter Sleep

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January seems like a good time to think about the process of hibernation as a survival strategy. Anyone who experiences the seasonal cold and dark days of winter that are part of our British climate will understand the concept of hibernation. However, for some wildlife hibernation is a crucial life cycle mechanism. Hibernation is an extended period of deep sleep, sometimes referred to as torpor, which reduces metabolic rate and body temperature. During the cold months when food is scarce and what is available has low energy value shutting down in this way allows some animals to subsist on the fat reserves built up during the seasons of plenty.

In Britain several wildlife species survive the winter by hibernating, although the process does take different forms. Hedgehogs, bats and dormice are UK species that hibernate, although, bats and hedgehogs do sometimes break their torpor at intervals to defecate or top up on food or water. Whilst these three species are considered to truly hibernate, frogs, toads, slowworms and some insects also have periods of torpor in extremely cold periods. The latter group, however, do reawaken on warmer days and continue to feed.

The physiological changes to a species when it hibernates are quite amazing. Flying bats can have incredible heart rates of approximately 1000 beats per minute. During hibernation this reduces to around 20 beats per minute in order to conserve energy! The heart rate of an hedgehog during hibernation also reduces from 190 beats per minute to 20 beats per minute. Breathing also slows down as far less oxygen is required to supply the system and body temperature of the hedgehog can drop by approximately 25°C.

Timing of hibernation varies between species and also changes from year to year depending on climatic conditions. Hedgehogs generally hibernate between November and March, waking several times during the period. October to March is the typical time span for hibernating bats with gradual reducing and increasing activity at the beginning and end of the torpor. Dormice, however, curl up for hibernation when temperatures drop below 15°C and generally remain in torpor until spring.

If you would like to help hibernating wildlife then there are plenty of things that you can do. Bats will rest in cool, humid places like tree hollows, caves and garden sheds but you could invest in or make a bat box for a suitable area. Please remember that bats are protected by conservation policies and should not be disturbed even if they have chosen an inconvenient site such as your attic!

Hedgehogs may find a use for an undisturbed log pile but do not forget to check the area if you light a bonfire! Placing a tennis ball on a pond will prevent the water from freezing over and allow oxygen to reach sleeping frogs at the bottom. A garden compost heap can also be of benefit to toad and slow worms moving in and out of torpor. The best advice is to be aware of hibernating wildlife in and around your area and take special care not to disturb their rest until the warmth of spring returns.

References and Further Reading

www.britishhedgehog.org.uk

ww.bbc.co.uk/nature/species/Hazel_Dormouse
www.bats.org.uk

Geiser.F. 2004. Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor. Annual Review Physiology. 66:239-74.