AFTER the last ice age, 15,000 years ago, what we now know as our native mammals crossed the land bridge attaching us to the continent.

Of these creatures, many have become extinct, including the beaver, brown bear and wolf.

Others have been introduced by humans in some way, for pets, for meat and fur or by mistake by various means, via our transport systems.

There are at least 69 species of wild mammals living in and around Britain and another 29 bats and marine mammals that migrate.

Out of all our native mammals there are only three that truly hibernate during our colder months which are the bats, hedgehog and dormice.

With food no longer in abundance these animals sense that hunting would no longer be an energetically worth while activity, and in order to get through this veritable famine they hibernate to conserve energy.

Hibernation is not as easy as just going to sleep. As the weather gets colder these mammals will drop their body temperature to that of their surroundings and reduce their heartbeat to a few beats per minute.

By entering this state known as torpor, it enables them to make their food reserves last longer.

Bats have an amazing way of controlling their body temperature and during cold snaps, even in the summer, can go into a state of torpor.

Each mammal will select quite different places and ways with which to see through the winter months, these places of hibernation are called hibernacula and are chosen according to temperature and moisture.

With stories of 'bats in the belfry' largely untrue, they prefer somewhere warmer such as hollow trees, houses or caves, and Hedgehogs like to go under wooden buildings such as sheds, within a log pile or within your compost, somewhere nice and snug.

As the weather gets colder and the days get shorter the dormouse will create a nest at ground level and descend into a deep sleep.

Being one of the most elusive mammals and known as the seven sleeper' due to spending more time asleep than out and about, and also of nocturnal nature, this cute creature of the Wyre Forest is rarely seen.

But it' not all sleep, sleep, sleep, sometimes a hungry bat will emerge from torpor to feed.

In the past I have spotted bats on Christmas Day, which must have meant a bad autumn of foraging. Sensing a milder night, a spontaneous wake up begins, this wake up requires a substantial amount of energy, and can only be done so many times, which only highlights the problem that hibernation possess for bats and making the threat to hibernacula disturbance mainly from humans a serious problem.

All of our creatures out on the nature reserves have been well practised and all have evolved ways with which to deal with it. Some sleep, some hibernate and some find food sources that will keep them going through the cold nights. It will be interesting to see what seasonal change will influence this year.

11:51am Wednesday 9th January 2008

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