Western Morning News (Saturday)
SECRETS OF OUR WINTER SLEEPERS
Why endure the hardships of winter when you can sleep through it? Charlie Elder looks at the strategy adopted by our mammals that hibernate
This is a time of year when some of our most characterful mammals take time out, save energy and sleep off the worst of the weather. Sounds like a nice idea! Hibernation is a great survival strategy, but is not without its risks, and species that spend the winter in a prolonged torpor have to prepare well and build up fat reserves to sustain them through such a lengthy period of inactivity.
It is well known that hedgehogs hibernate, and they are joined by the famously dozy dormouse and by insect-eating bats that would struggle to find sufficient airborne invertebrates to eat during the cold months.
So while we enjoy this festive season in the warm indoors, spare a thought for those creatures battling the elements, putting their faith in fine-tuned biology to see them into the new year and beyond. This winter has been far from frosty so far, though mild temperatures and variable weather present challenges if hibernating creatures stir from their slumber and find it hard to find food.
Here I explore the strategies true hibernators use to beat the odds and wake up fit and well in early spring.
HEDGEHOG
The hedgehog is probably the best known of our hibernating mammals, adopting the strategy in order to make it through lean times and hostile winter weather.
Curled up in a prickly ball they are well protected from potential predators, but at the mercy of the elements and so choose sheltered sites such as under a heap of leaves, beneath a garden shed, log pile or even in a rabbit hole.
They eat invertebrates foraged from the ground, such as earthworms and beetles, and in the depths of winter, when the ground is frozen hard, food is hard to come by, so hibernating during prolonged cold conditions makes sense.
However, they can become active on mild nights and may even relocate to different hibernation nest sites.
While they hibernate from roughly November to March, hedgehogs will remain active well into December if it is mild enough – as it has been this year. And the more they eat, the more they can attain and maintain a sufficient weight to survive hibernation.
Research shows that younger hedgehogs often remain active until later as they need to pile on fat reserves rapidly to hit an optimum 600 grams or so in weight. Females that have recently raised young also need to get back up to prime condition and feed for longer before hibernating.
Given the climate is less harsh in the Westcountry compared with further north, one would expect our hedgehogs to emerge from hibernation earlier than those in Scotland. However, a citizen science survey run by the People’s Trust for Endangered Species and British Hedgehog Preservation Society has found that emergence dates across the UK do not necessarily follow clear regional differences.
During hibernation hedgehogs enter a torpor, their heart rate and body temperature falling and breathing and metabolism slowing.
One should avoid disturbing hibernating hedgehogs, as rousing them from their deep slumber burns up precious reserves of energy.
DORMOUSE
The Westcountry is fortunate in being a stronghold for the hazel dormouse – although these secretive mammals keep out of sight, being active after dark when they forage above ground among the branches of trees and hedges.
They are famously dozy zy – the name thought ht to derive from the French nch ‘dormir’ to sleep – and may spend up to half their lives asleep, either hibernating or in a torpor during colder weather between nocturnal foraging trips.
When hibernating – which can last as long as from October until May – they curl up in nests of tightly woven vegetation beneath the leaf litter on the woodland
floor, among roots or under hedgerows. They hold their body in a ball to avoid heat loss, with their furry tail curled up and over to cover the face. When active in the war warmer months dormice construct nests off off the ground in which to rest up during the day or raise young. These are constructed of plant material, including honeysuckle bark, and may be located in tree holes, brambles, nest boxes or old birds’ nests.
They eat a variety of food, including pollen, berries and insects, but it is hazelnuts that they really feast on in the autumn in order to gain weight in preparation for their long winter hibernation. The discarded empty shells scattered beneath hazel trees can help identify their presence – they have a neat hole in one side so that they resemble tiny wooden clogs, and the gnawed opening has a smooth inner rim.
BATS
Flying burns up a huge amount of energy, so bats need plenty of airborne food and the warmer months offer up a smorgasbord of moths and other flying insects after dark.
In winter the picture is very different, with too little invertebrate life on the wing, and our bat species get through the testing months by hibernating.
They seek out quiet places to roost with a cool and constant temperature, such as caves or tunnels, cavities in trees or buildings. Secreted in these sheltered sites, either alone or in groups, their metabolic processes slow and their temperature drops as they fall into a deep and extended Th torpor. They look t all but dead, and yet like a slow-burning candle, life flickers within.
It can take a while for a bat to rouse from this vulnerable state of inactivity, much like a car warming up. Anything that causes them to wake risks bats burning up precious stores of energy – though on warmer nights they may leave the roost site to find food and water.
Survival depends on building up sufficient fat reserves in autumn, and there may be short periods of torpor that increase in length as they enter hibernation, while emergence from this state after winter can also be a gradual process.
Just like the hedgehog and dormouse, when the going is good they need to make the most of it. Summertime and the living is easy, and spring and autumn too, but winter for these creatures is a season they cross out of the diary.
We can look forward to lengthening days and early spring bringing with it the possibility of spotting bats back in flight at dusk, the chance of coming across hungry hedgehogs rootling around in our garden beds and the knowledge that dormice are roaming our woods once again. Until then, to all our hibernators, sleep tight…