48 SECRETS OF ANIMAL LIFE 



edible frog or water-frog, Rana esculenta, habitually 

 spends its winter in the mud of the pond, while 

 in some other species the females hide during 

 winter under moss, among leaves, and the like, 

 while the males take to the moist mud. The 

 internal economy of the winter torpor (perhaps the 

 word hibernation is best kept for mammalian 

 winter-sleepers) is of great interest. The fire of 

 life burns low; no food is taken; a minimum of 

 energy is expended; the reserves stored in the liver 

 and in the " fatty bodies " are slowly used up ; 

 the respiration sinks back to a primitive mode by 

 means of the blood-vessels spread out in the skin. 

 None the worse for their long fast, the frogs 

 bestir themselves as the winter disappears, and 

 pair in the pools, often in very unsuitable places 

 where the spawn is soon left stranded. The males 

 call to their mates, and their croaking capacities 

 (due to vocal cords in the larynx) are enhanced by 

 the presence of two internal resonating sacs which 

 lie at the posterior corners of the mouth and bulge 

 outwards when inflated. These sacs are not de- 

 veloped in the females, who give voice much 

 more rarely, and certainly do not respond vocally 

 to the males* serenading. In the calls of different 

 species of frog there is a striking individuality, and 

 we cannot hear even the dull "grook, grook" of 

 the grass-frog in the early spring without a thrill 

 deeper than the cuckoo's wandering voice gives us 

 later on. For, apart from the instrumental music of 

 insects, the first voice in the evolution of animals to 



