The best known of these strategies is hibernation but it’s actually rather rare among Irish wildlife, at least in the strict definition of the word.
Many insects, including ladybirds, tortoiseshell butterflies and queen bumble bees, seek out a sheltered spot at this time of year in which they can survive the winter in a torpid state. But, to be biologically pedantic, this is not hibernation, it’s called diapause. Only our hedgehogs and bats really hibernate. There are a couple of bird species around the world that become inactive in winter but it’s generally accepted that no birds hibernate.
Hedgehogs here normally go into hibernation in October or November but this does depend on the weather. If it’s particularly mild they may still be out and about in December. They seek out a warm, dry place, usually a pile of twigs and leaf litter in the bottom of a hedge or under a furze bush, and make a well-insulated nest.
Then they curl up into a ball in the nest and all their metabolic processes start to slow down. Their body temperature drops from its normal value of around 34C down to the ambient air temperature, their heart beat slows from around 150 to 190 beats a minute to about 20 and they go into a state of torpor that is similar, though not identical, to sleep.
But research has shown they don’t simply nod off in the autumn and wake up in the spring. They regain consciousness quite frequently, normally to urinate, and sometimes, for reasons that aren’t fully understood, they leave their nest and move to a spare one that they prepared during the previous autumn.
Their survival depends on stores of fat laid down in the late summer and autumn. They accumulate two kinds of fat — white fat and brown fat. The brown fat is to keep them warm and the white fat is to keep them fed.
Hedgehogs are probably not native Irish animals. One theory is that they were introduced by the Vikings, who used them for food. The earliest definite record we have is from the mid 1,400s and comes from the Viking city of Waterford. Ireland is probably at the northern margin of their viable range and, as a result, many of them fail to survive hibernation. Their accumulated reserves of fat run out before the weather gets warm enough, in March or April, for them to start foraging again.
This is a particular problem for young hedgehogs, born late in the year, that go into hibernation weighing about 500 grams. If young hedgehogs visit your garden at this time of year you can significantly improve their chances of survival by feeding them. Use tinned dog or cat food (not fish-based) with some added roughage like dog biscuit.