6o LIFE : OUTLINES OK GENERAL BIOLOGY 



White's tortoise, that he wrote about so often, used to dig out a 

 hibcmaculum in the garden, and actually survived fifty-four 

 British winters. 



TRUE HIBERNATION.— Turning to mammals, amonj^ which we 

 fmd the true hibernators, we at once find a clue put into our hand in 

 the fact that the capacity for winter sleep has a curiously irregular 

 distribution. The mole is not very far removed zoologically from 

 the hedgehog, and both are largely insectivorous. Why does it not 

 hibernate as the hedgehog does? The probable answer is that the 

 mole, being a burrower, can get below the frost's grip, and thus 

 does not need to hibernate. W^ater-shrews do not hibernate, while 

 bats do; and the probable reason is that while fl3'ing insects are 

 far to seek in winter, there are many aquatic insects and pupae to 

 be found by looking for them. Similarly, it is not difficult to find 

 plausible reasons why true squirrels, rabbits, hares, lemmings, 

 water-voles, porcupines, beavers, ermine, foxes do not hibernate, 

 while ground squirrels, true marmots, dormice, Canadian jumping 

 mice, racoons, skunks and spiny ant-eaters do. It seems that the 

 capacity for winter sleep has arisen as an adaptation in the course 

 of natural selection, and is not a necessity impo.sed by cold and 

 scarcity on certain kinds of constitution. We are not aware of any 

 a priori reason why birds should not hibernate; they evade the 

 winter by migration, and in all probability have never given winter 

 sleep a trial. It should also be noted that polar mammals do not 

 hibernate, for the hibernation of a warm-blooded animal ends 

 fatally if there is very prolonged exposure to very low temperature. 

 The polar mammals meet the winter by thick fur, by stores of fat, 

 by migrating, and so on, but not by hibernation. It is true that the 

 mother polar bear lets herself be snowed up (all but a ventilating 

 hole), and gives birth to her cubs (in January) in this retreat, but 

 there is no true hibernation of the type which the hedgehog and 

 hamster illustrate. 



There is, as one would expect, considerable diversity in the dura- 

 tion and the depth of the winter sleep. Marmots and bats often 

 hibernate for six months, dormice and hedgehogs for three. Eats 

 are occasionally seen flying in winter in Britain, but they tend to 

 sleep uninterruptedly; marmots may waken up five to ten times in 

 the winter, and go to .sleep again. The hedgehog is probably more 

 penetratingly affected by its hibernation than any other mammal. 

 On the whole, it may be said that the true hibernation of .some of 

 the warm-blooded mammals is a much deeper change than the 

 lethargy of the cold-blooded reptiles and the like. Lizards are often 

 reawakened by a very sunny day in mid-winter, and everyone is 

 familiar with insects which awaken unseasonably and to their own 

 destruction. 



