I’VE been seeing quite a few hedgehogs recently and on several occasions when I’ve let my dog out at night there has been an uproar of barking as he discovers one and dares it to unroll and fight like a man.
I like hedgehogs and I know they help to control slugs and other pests in the garden so I’m pleased to see so many around.
Pleased and a little surprised. Hedgehog populations in Ireland fluctuate on a cyclical basis. Typically a hard winter causes a population crash and it takes three to five years for numbers to build up again. During the very cold weather last December and January I imagined that many hibernating hedgehogs were freezing to death and that this would be one of the lean years.
But a mild summer with plenty of invertebrates around must have meant a good breeding season for those that survived last winter and they seem to be quite plentiful.
Food supplies are diminishing, though they can augment their usual diet of insects, slugs and worms with some fruit and berries, but they must build up their fat reserves if they’re to survive hibernation.
In fact they have to increase their body weight by around 30% to about 500 grams to give them a chance.
It used to be thought Irish hedgehogs went into hibernation in October or early November and most of the reference sources still say this. But recent research in Britain has shown over there it’s considerably later – December or even January – and it seems quite likely this is also true in Ireland.
They are the only Irish mammals, apart from bats, that hibernate.
Contrary to what a lot of people think, badgers, squirrels and other such creatures become less active during the winter, often dozing for days at a time when the weather’s bad, but they don’t go into true hibernation.
But some time over the next few months a hedgehog will build a special nest, a much more elaborate structure than the ones it uses to sleep in during the day in the warmer months. It will usually be in a thick bush or a pile of brushwood and will be about 50 centimetres in diameter. It will bring in dead leaves in its mouth and use its body to pack them into a dense, insulating layer.
At the same time it will be busy accumulating two kinds of fat in its body, white and brown. The white fat will feed it throughout hibernation. Its food needs will drop to about one fiftieth of what it requires when it’s active so it should be able to eke this fat out to survive until hibernation ends in the spring.
The brown fat is an emergency heat supply. In hibernation the temperature of the outer parts of the animal’s body is the same as the ambient air temperature – like that of a fish or a reptile. But if the ambient temperature drops below about 4 degrees there’s a danger of frostbite so the brown fat is burned to supply extra heat.
Hedgehogs usually wake up every week to 10 days when they’re hibernating. We’re not quite sure why they do this, though they usually take the opportunity to urinate. Sometimes they even build themselves a new nest, and then go back into hibernation. It’s a risky business for the hedgehog and rather mysterious to us.
But the fact that the species survived last winter’s long freeze-up and is still with us in good numbers shows it works.
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