Monday, October 29, 2007
I LOVE the autumn and in the part of the countryside I live this is a particularly fine one. The Indian Summer is lingering on even into November, which will help to shorten the winter.
Dividing the year into four seasons can be useful, but it’s an artificial concept that doesn’t always work in the natural world. Snails, for example, seal up their shells and go into a sort of hibernation in mid-winter. But if we get a period of hot and dry weather in the summer they do exactly the same thing. This is aestivation.
True hibernation only occurs in warm-blooded creatures, and snails, which are molluscs, are not warm-blooded so they don’t really hibernate. Because there are no bird species that are known to hibernate, the habit is confined to mammals, and not many mammals at that.
In Ireland the only animals that hibernate are our various bat species and hedgehogs. They drop their temperature and heart rate and go into a deep coma for several months. The normal body temperature of a hedgehog is 34C, but in hibernation it drops to between 4C and 6C.
Other Irish mammals such as badgers and squirrels can manage a sort of semi-hibernation in which they sleep for a few days in bad weather and there is a much smaller reduction in heart rate and body temperature. But there are quite a lot of cold-blooded creatures that survive the northern winter by shutting down all their systems and going into a form of suspended animation. Apart from the molluscs, many insects, reptiles and amphibians do it.
At least two of our butterfly species "hibernate". They are the small tortoiseshell and the peacock, right. Personally I believe that as a result of global warming small numbers of red admirals have also started over-wintering.
The textbooks say that all red admirals in this country are migratory, arriving from the south in late summer. But recently I’ve seen them flying around my garden in the spring and I believe these are individuals that have over-wintered as adults. But there is another possibility. Some butterfly species over-winter not as winged adults but as eggs, pupae or caterpillars.
Other insects that "hibernate" include queen bees and queen wasps of various species and green lacewing flies, which actually turn red when they’re in suspended animation. Two-spot ladybirds "hibernate" communally with anything of up to 50 individual sclustering together.
Many of these insects have adopted the warmth and shelter of houses or sheds as places to spend the winter. We’ve all had the experience of finding small tortoiseshell butterflies in the curtains of a seldom-used room. The common lizard "hibernates" usually under a rotten log or in a rock crevice or a hole in a wall. They reappear surprisingly early in the spring and can be active in late February in milder coastal areas. Newts have rather similar habits. When the weather starts to get cold they crawl out of the water that they’ve been living in since spring and go in search of a log or a rock to sleep the winter away. While they’re searching for the right spot these slow-moving creatures are vulnerable to predators.
Although no bird species hibernate, wrens in Ireland come the closest to it. In cold weather they practice "communal roosting" in which a number of birds will huddle together for warmth in a disused nest or sometimes in the rood space of a house.
* dick.warner@examiner.ie
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