Note that although hedgehogs are in the order Insectivora they do not just eat insects! The hedgehog ( Erinaceus europaeus) is an example of a hibernating insectivore in Britain, it hibernates from October/November to March/April. Hibernators are mainly found in the orders Rodentia, Chiroptera (bats) and Insectivora. Finally, they are better able to carry a thick coat (Section 1.3.2) and sufficient adipose tissue to last through the winter. Secondly, they have a smaller surface area to volume ratio and so can conserve body heat better than smaller species. Firstly, they would warm up too slowly and therefore use too much energy. There are several reasons why larger mammals do not hibernate. The largest mammal to hibernate is the marmot, which weighs about 5 kg. True hibernation only occurs in relatively small mammals, though not all small mammals living in temperate habitats hibernate in winter, as we have seen. The North American pocket mouse ( Perognathus californicus) is a facultative hibernator. There are some mammals that are categorised as facultative hibernators, entering hibernation in response to very cold weather and poor food supply. The arctic ground squirrel is described as an obligate hibernator because it hibernates every winter. A feature of hibernation that distinguishes it from other kinds of winter inactivity is that hibernators can arouse themselves spontaneously and are not dependent on external conditions, such as warm temperatures, to do so. Brown and black bears, for example, are lethargic during very cold periods but are otherwise active in the winter. This behaviour is in contrast to other winter states such as torpor or lethargy which are immediate responses to current conditions. For example, the arctic ground squirrel ( Spermophilus undulatus) enters hibernation between 5 and 12 October and emerges between 20 and 22 April, regardless of the weather on those dates. Some species are remarkably precise and predictable. Hibernation is an active process, that is, it is a state which animals enter into, not in response to immediate external conditions, but to internal stimuli. The heartbeat becomes slow and irregular and breathing rate also slows. At the same time, a hibernator's metabolic rate falls to as little as 1% of its normal value. Instead, body temperature falls, from around 38☌, to about 1☌ above ambient temperature, which is often close to 0☌. The phenomenon of hibernation is one reason why the term homeothermy is going out of fashion, to be replaced by endothermy, because maintaining a stable body temperature is the very opposite of what hibernators do. It occurs only in certain mammals and one bird species, the poorwill ( Phalaenoptilus nuttallii), a North American relative of the nightjar. Hibernation is defined as the condition of passing the winter in a resting state of deep sleep, during which metabolic rate and body temperature drop considerably. The word hibernation is often used loosely to refer to general inactivity but, in biology, it refers to a specific phenomenon, sometimes called 'true hibernation'. Many animals become inactive for periods of varying duration during the winter and there is a diversity of terms used to describe this state, including: sleep, torpor, dormancy, lethargy and hibernation.
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