Hibernation - How some wildlife withstands winter

(c) Danny Green

As winter approaches, sleeping through sounds ideal. While some animals grow a thicker coat or migrate south to avoid the freezing temperatures and food shortages winter brings, others hibernate. It is more than a big sleep. The body lowers its temperature, heart rate, breathing and metabolism to the minimum levels required to sustain life. Usually warm-blooded mammals will feel cold to touch, their breathing barely noticeable. Some species can also go into partial hibernation, or torpor, which requires less energy to wake-up from than hibernation, enabling some to emerge during milder temperatures to seek food and water.

Hedgehogs, Dormice and Bats

Hedgehogs, dormice and bats are the only three native mammals in the UK which fully hibernate. From around October, all 17 species of British bat will begin seeking winter roost sites such as hollow trees, caves, cellars and purpose-built bat boxes. Suitable sites need to keep a stable temperature and humidity to minimise water loss. A bats heart rate while hibernating drops to around four beats per minute (compared to over 1,000 beats a minute while flying). On warmer days they may rouse to forage for insects and water. But, when the temperature drops again, they will return to hibernation.

Hedgehogs make sure they have a well-constructed nest to hibernate in consisting of leaves, twigs and grass. This is usually sited beneath hedges or tree roots, inside compost heaps or underneath garden sheds and can be up to 50 centimetres thick. By March a hedgehog may have lost a third of its body weight, prompting hedgehog-lovers to put out fresh water and meaty dog or cat food for the awakening hogs.

As for dormice, their name - from the French term dormir meaning ‘to sleep’ - says it all. Even during the summer, dormice drop into torpor during cold spells or periods of food  shortage to conserve energy supplies. In autumn, a dormouse will fatten-up on nuts and berries increasing its normal body size by up to 40% before a long hibernation lasting sometimes until May.

Snakes, frogs, ladybirds and bumble bees

Snakes, like all reptiles, need warmth to function hence hours spent basking in the sun. The native British snakes (adder, smooth and grass snake) are not actually asleep during the cold months, but they are deeply inactive and usually underground.

Amphibians such as frogs, toads and newts also lie low during winter. Like reptiles, they are cold-blooded creatures. Toads and newts tend to seek frost-free places such as log piles or old walls as the weather gets colder and some frogs may move to the bottom of ponds burying themselves in the silt. Here they stay for much of the winter occasionally taking advantage of mild weather to forage.

The hibernating gang includes some insects. Around November ladybirds start looking for suitable hibernation sites such as under tree bark, piles of leaves or undisturbed shed corners. While most butterflies overwinter as caterpillars, a few including brimstone, peacock, comma, small tortoiseshell and red admiral hibernate as adults in sheds or hidden in dense ivy.

Honey bees do not hibernate whereas bumble bees do. Or rather only the queen hibernates – the rest of the bumble bee colony dies. The queen will choose south-facing banks to burrow into soft soil. When spring comes, she will emerge and gorge on pollen and nectar before finding a suitable place to lay her eggs, creating the next colony.

On our nature reserves, with help from volunteers, we create opportunities for wildlife’s winter survival. We build log and grass piles and make sure there is plenty of long grass, dried vegetation and old trees for shelter.

You can do your bit too. Leave compost heaps undisturbed and grass uncut in the garden. Gather dead leaves and add to piles of old wood under hedges and shrubs. Gardens are a wildlife haven in winter - and the less tidy the better.