As snow lay thickly on the ground this month in south Wales, in the cold and the dark, the warm temperatures, luxuriant growth and abundant resources of the summer months seem far away indeed.
Even without the obvious impediment of snow, winter can be a challenging time for our wildlife. For some of our mammals, their adaptive strategy is to enter hibernation- an extended period of very deep sleep (or ‘torpor’), during which metabolic rate and body temperature drop to such an extent that the animal uses few resources and is thus able to survive harsh winter conditions. Most species that truly hibernate will stock up on resources and put on weight during the autumn in order to last the winter, or they may still wake up at intervals to ‘top up’. It is a complicated phenomenon that is not as well understood as one might imagine, and is much more complex (and perilous) than simply going to sleep.
Almost all mammal species conserve energy during the winter months by one or more specific strategies. Here are a few examples of British mammals that truly hibernate and a couple of myth-busters; species that people commonly think hibernate but which actually do not.
Hedgehog (Erinaceus europaeus)
Hedgehogs are true hibernators. Like most mammals their food (mostly terrestrial invertebrates) is much less abundant and accessible during the winter. The commencement of hibernation depends very much on prevailing conditions, but is normally between November and January.
Internal changes cause them to become immobile, and their bodies cool. Their heart rate decreases massively from around 190 to only 20 beats per minute, and their body temperature drops by 25ºC. In this condition they use hardly any resources.
Although they rarely leave their winter nests (typically in hedges and brash piles), they do actually wake up fairly often during the winter months.
Interestingly, hibernation is a response to cold conditions and not inherently necessary. For example, hedgehogs that are kept inside over the winter (e.g. young animals that have been rescued in the autumn because they have not gained enough weight to survive winter hibernation in the wild) will remain active all season. Our wild hedgehogs will certainly be hibernating at present, and probably won’t be seen till March or April.
All British bats also hibernate. Like hedgehogs, bats’ metabolic rate drops greatly, enabling them to survive the winter without foraging. This is essential since their food (invertebrates on the wing) is in particularly low abundance during the winter months and flying to hunt is extremely energy demanding.
To hibernate, bats need roosts that are relatively cool and remain at a constant temperature, in order to help them regulate their own body conditions and to avoid being woken in response to environmental fluctuations. For this reason they often chose underground sites, such as caves, or human equivalents such as large cellars, where temperatures are constant. This is essential as being woken from hibernation costs bats a lot of energy and over time can therefore lead to starvation.
Surprisingly little is known about British bats during the winter months. For example, pipistrelles may be our most abundant bat species, but it is not fully understood where they go in winter. Too few hibernation roosts have been identified to account for the numbers of animals recorded in Britain during the summer. Fascinatingly, bats mate prior to hibernating, during the autumn and early winter. The females are then able to store the sperm so that they do not actually become pregnant until the spring.
Hazel Dormouse (Muscardinus avellanarius)
The dormouse is the only one of our native small mammals that enters true hibernation during the winter. For dormice this is in a small woven nest at ground level, enabling them to regulate the humidity of their environment.
They normally enter hibernation around the time of the first autumn frosts (October or November) and are not normally recorded as active again until April or May, although this depends on conditions. Timing of hibernation is recorded on WTSWW nature reserves via the monitoring of dormouse activity in nest boxes that we install.
Hibernating dormice let their body temperature drop to that of their surroundings (the optimum being 1-4ºC, at which they consume very little energy). Their metabolism drops by around 90%.
Red Squirrel (Sciurus vulgaris)
Many people think that squirrels (both red and grey) hibernate. In fact, squirrels continue to forage throughout the winter, although they do cache food stores in preparation for the lean season. This behaviour is most obvious in urban grey squirrels but in fact is observable in our red squirrels also.
From January onwards when the breeding season starts, squirrels can already be observed to be active, often chasing each other round the canopy and up and down trees; this behaviour can be part of the mating process.
Badger (Meles meles)
Also contrary to popular belief, badgers do not hibernate. However, they too have developed strategies for conserving their energy during the winter when their staple diet (especially earthworms- consider the impact of frozen ground) is not available. They gain weight during the autumn to see them through the winter months. They will also spend many days underground conserving their energy during cold periods and will sleep more deeply and for longer periods.
Whilst hibernation sounds like the ultimate answer to winter hardship- how many of us, after all, have joked about wishing we could hibernate ourselves- it is actually a risky strategy in many ways. A hibernating mammal is very vulnerable to disturbance and predation. Dormice, for example, are very vulnerable to predation in their small nests at ground level and are unable to respond quickly to make their escape if found. For all hibernating mammals, remaining undisturbed is also critical to maintaining their essential low metabolism and eking out their scant resources till spring. Despite the risks however, hibernation has to remain one if nature’s most remarkable strategies.