412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 5 



must either migrate, starve, or hibernate, take the last alternative, 

 whereas they might have been expected to choose the first, seeing that 

 they are the only mammals capable of powered flight. A few species 

 of bat, particularly in America, do migrate southward in the winter, 

 but the majority of those inhabiting the temperate regions hibernate; 

 nevertheless, their hibernation is by no means so complete as was once 

 thought, for many species wake up at intervals and not only move 

 about in the caves in which they pass the winter but go outside and fly 

 from one shelter to another. 



DORMICE AND HEDGEHOGS 



A hibernating mammal like the dormouse rolls up into a ball when 

 it becomes torpid, the head being bent down so that the chin rests on 

 the abdomen. The hind feet are curled forward about level with the 

 nose, and the hands, clinched into fists, are held either under the chin 

 or alongside the cheeks. The eyes and mouth are tightly closed, the 

 ears folded back downward close to the surface of the head, and the 

 tail is tucked forward between the legs, its tip wrapping over the face 

 and back. The body temperature is so low that the animal feels cold 

 to the touch, and the muscles are held rigid so that the creature can be 

 rolled along a flat surface without disturbing its pose. Dormice 

 hibernate from September to April, but their sleep is not necessarily 

 uninterrupted ; their record for continuous sleep is 6 months and 23 

 days. Pledgehogs are much less regular hibernators, and often do 

 not lie up for the winter until the end of the year, so that they may not 

 be torpid for more than 3 months. Bats of the temperate regions 

 vary; some species may hibernate for over 5 months, but here again 

 many of them awake at intervals during the winter. 



Hibernating mammals are aroused by warmth; extreme cold may 

 also produce this effect. None of them, however, can normally with- 

 stand being frozen, hence the necessity for them to lie up in the pro- 

 tected nest of the hibernaculum where frost is excluded. Although 

 bats make no nests, they hibernate in places such as caves where the 

 temperature is fairly constant and, low as it may be, never drops to 

 freezing. The alpine marmot and many allied species enter upon a 

 true hibernation, but bears, contrary to popular belief, do not. Bears 

 "den up" for lengthy periods during the winter, but they do not become 

 truly torpid, and their normal basic rate of metabolism is maintained 

 throughout their sleep ; it is in the winter den that the cubs are born, 

 and there that the mother nurses them for some weeks "licking them 

 into shape." Similarly the badger may lie up for several successive 

 days in extremely cold weather but it does not hibernate in Great 

 Britain — what it may do in northern Europe has not yet been definitely 

 ascertained. 



