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13 Oct 2021

The real hibernators

Written by Roddy Hamilton, Ranger in our North-East region
A red squirrel sits on a log, framed by two branches. It holds its front paws up in front of its white tummy.
Hibernators? Red squirrels can be spotted on snowy days
It’s often thought squirrels hibernate, but in the depths of winter squirrels are often seen bounding through snow and scrabbling for caches of nuts. They may spend more time cosied up in their drey, but they don’t hibernate.

Come those dark nights, many of us just want to pull the duvet up over heads and hibernate. Some (and I’m thinking about teenagers in particular here) can become experts at it.

The clocks go back at 2am on Sunday 31 October and from then on the days shorten incrementally towards 21 December, the shortest day of the year. The sleepless nights of high summer, when daylight permeated the curtains in the small hours, are a memory. Lack of light instructs our brain to produce melatonin, the sleep-inducing hormone. We instinctively want to curl up and drift off.

In the animal world, many animals sleep more, conserving energy. But in some, their adaptation to winter goes well beyond sleep, with bodies reducing heart-rates, breathing and brain activity to the bare minimum, and entering the state we know as hibernation. But who are the real hibernators?

An aerial colour photograph of Killiecrankie in autumn, showing the trees in their autumn colours. The rivers runs through the centre of the image, with the footbridge in the distance.
The leaves turn and the nights grow shorter as autumn sets in

Popular beliefs about which animals hibernate varies. For instance, it’s often thought squirrels and mice hibernate, but in the depths of winter at Crathes squirrels are often seen bounding through snow, scrabbling for caches of nuts. And as any country cottage owner will tell you, mice often move indoors in winter, seeking warmth, but they still have to remain active to forage.

In fact, our only mammalian hibernators are hedgehogs and bats. Hedgehogs are perhaps the most famous, and expert, of hibernators. Instead of keeping going in winter and finding food that is in short supply, their heart rate comes down from 190 beats per minute to only 20, enough to keep their vital organs going. This way, the hibernating hedgehog stores energy, and won’t waken until spring’s sunshine hits.

It is the cold, more than the length of day, which dictates the change in species habits. Bats move from their attic maternity roosts when the temperature drops. They will look for somewhere reliably cool and damp to spend winter, like the cellar at the Tower of Drum. And they will rouse themselves from hibernation once in a while. This allows them to excrete, for their bodies to deal with any toxins which might have built up, and for them to sleep for a while – their bodies still need sleep, even in hibernation.

As a beneficial strategy, hibernation is not without its physiological costs. Waking from hibernation is slower than waking from sleep, so there can be an increased risk of predation. This is why the cellar in the Tower of Drum, being enclosed, is a safe house for bats. But there are studies of hibernators which suggest there are other negatives. Hibernation can affect memory on waking, and adversely affect the auto-immune system.

And extreme cold can be dangerous. Frogs depend upon sugars in their bloodstream to protect them against frost. While some frogs choose to hibernate out of the way in leaf litter, some choose the mud in a pond. There is a trade-off here between danger of predation and danger of the hiding place. Frogs breathe through their skin but if the pond freezes, the frog’s submarine hidey-hole can quickly be without oxygen.

For butterfly species there is an adaptive choice which can be made. Some butterflies migrate to warmer climes, and some hibernate – for example, in the dark corners of sheds. Some red admirals move south, and some stay.

And when we switch on that jarring light in the winter morning and coax ourselves and our teenagers out for a walk, it is remarkable to think as the snow whirls and ice forms, that ladybirds in trees like those at Castle Fraser are clustered deep in the nooks of gnarly bark, waiting for spring to waken them.

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