Most of us tend to slow down in winter, as the cold weather and icy frost creeps into our bones, but some clever species avoid it all together by spending the darker months in a process known as hibernation. For some, hibernation is the only way to get through until spring.
How do animals hibernate?
Various animals living in or near wetlands hibernate to conserve energy through the winter, when their natural food sources become scarce or impossible to find. To do this, they first need to ensure that they fatten up during summer to provide a source of essential energy.
Once they’re ready to hibernate, they instinctively start slowing their metabolism down to use as little energy as possible; their heart rate drops, as does their breathing, and they enter a state of deep hibernation or ‘torpor’ - where the metabolism slows down to less than 5% of their normal rate. The difference between hibernation and torpor is that hibernation appears to be a voluntary state, whereas torpor is usually for a shorter time period and appears involuntary.
We describe this state as 'hibernation' rather than 'sleep' because the brain cycles that the animal goes through are so drastically different from sleep patterns - it’s more of an extreme slowing down.
What kind of animals hibernate?
Reptiles and amphibians are cold blooded, and so if they remained active would not be able to perform their bodily functions. None of our wetland bird species hibernate, and have instead developed adaptations to deal with the cold – such as migration. Only three British mammal groups truly hibernate. All of our bat species, hedgehogs and the dormouse retreat to their cosy boltholes from around October/November until March/April the following year.
Do hibernating animals stay ‘asleep’ all winter?
You may notice from season to season that the animals you see in nature seemingly vanish in winter, but they may not all be hibernating. You might also notice that some animals you thought hibernated are up and about! It’s a myth that hibernating animals can’t go back to hibernation after waking up - although ideally, they want to stay in hibernation to conserve energy. However, they sometimes get up for a bathroom break, because of a warmer spell of weather or because it’s become dangerously cold (below freezing, they may wake up to top ice crystals forming in their blood).
As the only flying mammals, bats need to use a lot of energy, and to cope with this bats have developed several energy saving tactics. They hibernate during winter when food is scarce and have a very low birth rate. Bats will often pick hibernation roosts in old buildings or even the eaves of your house, so watch out for them if you’re moving things around or renovating during this period.
Many reptiles can be found in wetland habitat - but they’re hidden in winter. This group go into a version of hibernation called brumation, a specific term that applies to reptiles, but it is similar to hibernation. This is because reptiles rely on the sun to regulate their body temperature, so hibernation is essential for their survival in colder climates.
You’ll probably have noticed a lack of frog, toad and newt activity in autumn and early winter, and they do in fact become dormant during these months for survival. Frogs and newts will hibernate on the bottom of ponds, but if these ponds are full of leaf matter this can prove deadly to a hibernating amphibian if the pond freezes over.
Underneath that thick layer of ice, the leaf litter and plant matter starts to decompose, and that can release toxic gases that unfortunately can be fatal to the amphibians living below the surface.
Some wetland insects hibernate in larvae form, stashed safely in reedbeds and tussocks of grass, or crevices in walls or bug hotels. Some insects like honey bees rely on their family to survive, huddling together and using their joint heat to combat the cold.
There’s also a process called diapause which can occur at any stage of an insect’s development, which could be as an egg, larvae or adult form. Simply put, this is a state of stasis where the insect’s appetite and development is slowed right down, until temperatures warm up and they appear again.
Who doesn’t hibernate?
Although there is at least one bird species in the world that does hibernate, birds in the UK at least do not use hibernation as a survival strategy. The two main threats to birds in winter are a lack of food and extreme weather conditions. But instead of hibernation, wetland birds have certain adaptations that allow them to survive the winter. They use their power of flight to migrate to more suitable climes. They also perform social flocking behaviours to keep warm and escape predation. They rely on their hardy physiology to survive extreme cold.
You might assume that water voles are hibernators as they seemingly vanish from view, but actually they’re active throughout the winter. Like us, they tend to spend a lot more time in their burrows. They store food such as tubers and rhizomes that they forage here as winter snacks.
It’s not easy being a water vole, and numbers in the UK have dwindled in recent years, due to predation and loss of habitat. Luckily our reserves are a lifeline for them, and you stand a good chance of seeing one scuttle around the banks if you’re patient and look closely.
Otters are active throughout the year. European otter coats are waterproof and 200 times denser than human hair, so they are well insulated from the cold. This enables them to breed all year round, as is shown with the surprising set of wild otter cubs arriving at Slimbridge in December 2018. Thanks to conservation efforts to reduce the amount of agricultural chemicals polluting the water, otter numbers are now on the rise from near extinction in England.
Scientific investigations have indicated that rising temperatures in the UK and beyond are proving disruptive for hibernating species, as the warmer temperature spells are causing them to stay for longer periods in their ‘active’ breaks from hibernating, causing them to use up vital energy. If the cold snap returns and no food is around, their chances of survival are drastically reduced. This means that our gardens are becoming more and more important as refuges for wildlife.
Things you can do to help wildlife in winter
- Don’t set fire to piles of leaves without first checking for small mammals burrowed under.
- In autumn, why not set up a designated haven in your garden to help hibernating animals? This could be a special wildlife pond, bat box or insect hotel that you know to leave there all winter. This helps lessen the risk of animals looking for hibernation spots in awkward or dangerous places for them. Our gardening for wetlands hub is full of things you can do to help.
- Supply a source of un-frozen water for non-hibernating birds to drink and bathe in.
- Clean out your pond before winter comes to make sure there’s no plant matter that will decay and produce noxious gases that can poison amphibians. You could consider floating a ball in the water to create movement and offset freezing in all but the coldest of temperatures.
- Don’t smash your pond if it freezes over, as this can shock the fish and amphibians that live there. Instead, make a small hole with the bottom of a hot pan.
- Why not rewild a small section of your garden to provide a shelter for hibernating insects? Logs, stones and old bricks can be piled up to make a makeshift insect hotel. Cover it up with leaves and branches, and you’re good to go.