The Long Winter Sleep: The Biology of Hibernation

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At this time of year, as the nights begin to draw in, and a chill appears in the air, the idea of simply curling up in a warm spot and sleeping through the winter months is very tempting. Unfortunately for us, Humans are not among the species which undergo hibernation. However, many other mammals reduce their activity levels during the winter, and a few undergo full hibernation, biochemically altering their metabolism to wait for more favourable conditions.

There are two alternative ways to deal with the long, cold winter months in temperate regions; increase your metabolism to maintain your body temperature despite the poor conditions, or conversely, to reduce your metabolism and essentially just ‘sit it out’. The former necessitates a massive increase in energy consumption, which may be difficult as food availability tends to decrease at this time of year. Perhaps it is no coincidence that many human societies have winter festivals that involve feasting! The latter option, commonly known as hibernation, involves a reduction in metabolic rate and an associated drop in core body temperature, and this requires a number of specific adaptations. True hibernation is specifically defined a seasonal, or environmentally driven reduction in metabolic rate, often, but not necessarily associated with a reduced body temperature. It involves a reduction in metabolic rate below the basal metabolic rate (BMR). A lower limit on metabolic reductions is known as the universal minimum metabolic rate (UMMR), which appears to represent a bottom line for all mammalian species, big and small. True hibernation relates directly to the metabolic state of the animal, and can range in duration from 1 day to 1 year. Some animals such as black bears hibernate whilst pregnant, and young are either born while the mother remains dormant, or shortly after hibernation ends. Hibernation usually involves periods of several weeks (mean 14.8 days) of reduced metabolic rate, punctuated by brief arousal periods of 12 – 24 hours, during which body temperature and metabolic rate rises back to basal levels.

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