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 
