MAMMALS IN THE WATER 241 



sponge-fishers and the Arab divers must have sight 

 almost as keen below water as that of the sea-otter. 

 They have even learnt by practice to control the con- 

 sumption of the air-supply in their lungs. The usual 

 time for a hippopotamus to remain below water is five 

 minutes. The pearl-fisher can remain below for two 

 and a half minutes. In a tank a diver has remained 

 under water four minutes. But temperature marks the 

 limits of man's amphibious habits. Its effects seem less 

 potent on other mammals in the water. The hairless 

 amphibious beasts of the tropics hippos, tapirs, 

 elephants, and manatees need warm waters to swim in ; 

 but in temperate Europe, or even in the Arctic seas, 

 certain animals seem indifferent to constant wet, and the 

 intense discomfort of * wet clothes ' when out of the 

 water. A polar bear is wet, literally, to the skin. The 

 otters, though they have an inner coat, look thoroughly 

 drabbled when out of the water. The land -rat's 

 coat also becomes wet through. The latter avoids 

 water in cold weather ; but the otters sit cheerfully 

 on the bank in winter frosts or even in wind. So 

 do the Zoo beavers, but their lower fur is probably 

 impervious to wet. A piece of beaver fur, with the 

 long coat taken off, was dry at the roots after soaking 

 for two and a half hours in a basin. If the temperature 

 of aquatic animals were naturally low, like that of a fish, 

 their indifference might be explained. A hibernating 



16 



