16th November 2009
Hedgehogs
This is a difficult time of the year for hedgehogs in the Highlands as the variation in temperatures, sometimes ten degrees in twenty four hours, can affect their hibernation. The last few weeks has included such changes and more than one hedgehog has been tempted out of hibernation. Such was the case last week just west of Inverness when a hedgehog came from a hedge and walked out. Fortunately someone gave it some food and it went back to the hedge and presumably its hibernating place. We find hibernation difficult to understand and this is partly because we think of hibernation as a deep and long sleep. It is far more complex than that and one of the reasons is the question of keeping the body warm. This is very energy expensive and as hedgehogs are not very well insulated they opt out to shut down some of their body functions. As a result their body temperatures falls from about 35 degrees C. to 10 degrees C or even less to match the local conditions.
In very cold conditions the body functions rise a little so that it does not fall so far that it freezes. Incredibly the heart rate slows to less than 20 beats per minute and there are long periods, sometimes up to an hour, when there are only occasional short periods of breathing. The result is that the hedgehog gives up normal body functions to the barest minimum needed to stay alive and that is hibernation. In this state the animal is so drowsy it cannot feed. What little supply it needs comes from the huge fat reserves it has managed to store inside itself. These reserves are called white fat and may make up to one third of its body weight. There are also stores of brown fat that it uses to suddenly boost the body activities if it briefly come out of hibernation for some reason such as disturbance or a rise in temperatures.
In the past we have assumed that hedgehogs just hibernate through from November to March but recent studies have shown that this is not the case. Whilst some hedgehogs will hibernate for three or four months others can wake up about once a week. Waking up has its problems as to be active the body temperature need to rise to 30 degrees C. and this usually takes three to four hours. The hedgehog then may spend a day or two as fairly normal but then it goes back into hibernation. It is now known that very few of them use the same nest for the hibernation and new nests are built on mild nights.
Hedgehogs are one of the most difficult mammals to study partly because they are nocturnal. Whilst most conservationists believe their numbers are plummeting there is little factual evidence to support this. Discussions over the number of them killed on the road is fraught with problems. On the one hand the apparent increase in such road kills could mean more hedgehogs in the countryside but could be because there are more cars on the road and we are driving faster. There is no doubt that there have been fundamental changes in the countryside to which the hedgehogs have had trouble adapting. The population of badgers has risen markedly and these are the only mammals that can easily deal with the hedgehogs spines. Records suggest there are 100,000 more badgers than there were fifty years ago and both these mammals compete for the same food, earthworms. As to the future of hedgehogs there must be serious cause for concern.