370 MUSCARDINID.E— MUSCARDINUS 



quite likely that in many mammals the accumulation of fat,' 

 which in temperate and arctic climates is usual in autumn, 

 causes a progressive decrease of metabolic activity, and that 

 the advent of a certain degree of cold, which is no doubt fairly 

 definite for each species, consummates the matter by bringing 

 the vital processes almost to a standstill. Add to this the 

 effect of long custom regulating by inheritance the exact time 

 or conditions under which the machinery shall slow down, and 

 we seem nearer to a satisfactory explanation. But the actual 

 process is in life influenced by questions of food and of 

 individual differences. Thus, while dormice seem to have 

 acquired such similarity of constitution that all the individuals 

 may be said to become "ripe" for hibernation at about the same 

 time, the same process in hedgehogs is spread over a much 

 longer period, and the " ripening " is reached in a much more 

 uncertain manner. The difference is probably connected with 

 the more abundant food and the greater hardiness of hedge- 

 hogs ; it also indicates a higher degree of specialisation in 

 dormice. Both animals agree in that hibernation, having been 

 once properly started, runs a more or less normal course until 

 the body, having absorbed the fat by which its vital processes 

 have been clogged, is stimulated to fresh activity by the warmth 

 of spring, a warmth coincident with a renewed food-supply. 



Bats exhibit two forms of hibernation. Some species, such 

 as Leisler's Bat, are highly specialised and experience a regular 

 and normally continuous torpidity, and probably in these the 

 process has been so ingrained by inherited habit that the 

 stimulus of temperature and fat accumulation are hardly 

 needed. In other species, such as the Pipistrelle, hibernation 

 is intermittent, and here temperature would seem to be the sole 

 stimulus— the temperature at which the food-supply disappears. 

 In this respect the Pipistrelle is less specialised than bats which 

 hibernate more regularly. 



Hibernation may, therefore, be of two kinds— continuous, as 

 in the Dormouse; intermittent, as in the Pipistrelle. It does not 

 preserve the animal from cold, since it will die if not sheltered 



• As suggested for Glis by Aug. Forel, who likened hibernation to catalepsy and 

 hypnotic sleep {R^vue de F hypnotism, translated in Zoologist, 1887, 281) ; and by 

 W. L. Hahn (see above, Vol. I., 29) for bats. Thin animals certainly cannot 

 hibernate successfully. 



