Tuesday, 12 December 2006

Bedding down for the winter

The mild November weather had its effect on our wildlife, with very late sightings of certain butterfly species and reports of hedgehogs still on the go. Most hedgehogs build their winter nests during October and are safely tucked-up by mid-November. However, winter hibernation remains a flexible option for our hedgehogs and many will continue to remain active if conditions (both in terms of temperature and food availability) are suitable. It is a common misconception that hibernation is an extended form of sleep, during which the body is rested. Instead, it is a complex behaviour used to conserve energy during periods when the animal may be faced with highly unfavourable conditions.

Since keeping warm is an energetically expensive strategy during winter, the hibernating hedgehog abandons this, allowing its body temperature to fall. This controlled reduction in body temperature, typically brings the animal’s temperature down to within a couple of degrees of the ambient temperature. If the ambient temperature falls to dangerously low levels, which would cause the hedgehog to literally freeze to death, then the hibernating hedgehog will burn off more of its body fat to generate the heat needed to keep the body temperature at a safe level.  This controlled reduction in body temperature results in a fall in oxygen consumption, heart rate and breathing rate, together with a restriction in the amount of blood flowing to the major organs. While an active hedgehog may have a heart rate of 250 beats per minute (bpm) this falls to just 5bpm during hibernation. Research has shown that hedgehog hibernation is most efficient at 4ºC and one of the features that helps the hedgehog operate at around this temperature is the winter nest itself. Made predominantly from leaves, pushed up against a larger object, the nest provides protection from the worst of the winter weather. Temperatures within the nest typically remain between 1 and 5ºC, ideal for hibernation.

Hibernation is not continuous and a hedgehog will “wake-up” every 7-10 days. This takes between three and four hours and the hedgehog will normally remain within the nest for a short period before re-entering hibernation. Very occasionally, individuals will venture outside, perhaps to establish a new winter nest. These periods of arousal are energetically demanding, collectively accounting for 80% of the energy expended during the entire hibernation period. As such, they must be important for the hedgehog. Fat reserves fuel hibernation and these are laid down prior to hibernation and need to be sufficient to last the winter. Any young or undersized hedgehog weighing less than about 1lb (450g) is unlikely to survive more than the briefest period of winter weather, so should be rescued to a hedgehog rehabilitation centre.

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