ECOLOGICAL FACTORS 393 



animals, the avoidance of overheating is especially difficult, since their 

 own body temperature lies very close to the lethal condition, and 

 every active movement immediately gives rise to an increase in the 

 temperature, which becomes dangerous if cooling does not occur. 



Body cooling is frequently attained by retiring to water, as is illus- 

 trated by the habits of water buffalo, water buck, rhinoceros, elephant, 

 and especially by the hippopotamus. In Australia during the hot season, 

 rabbits may be seen at the edge of water holes with only their heads 

 above water. 55 The elephant showers itself with water. During great 

 heat many animals seek the shade of trees or caves where they fall 

 asleep and avoid continuous movement. Skin capillaries have reflex 

 dilation and so promote the radiation of heat at high temperatures. 

 The ears of rabbits and elephants, the hairless tail of the rat, and the 

 hanging tongue of the dog, all serve as heat regulators. 



A most effective means of reducing body temperature consists in 

 the evaporation of water, which possesses a high latent heat of evap- 

 oration ; 1 gm. of water that evaporates takes from the body more than 

 500 calories. The chief means of water evaporation is by breathing. 

 The accelerated breathing of many animals when greatly heated is 

 often accompanied by opening of the mouth and more rarely by exten- 

 sion of the tongue. Cold-blooded animals also utilize this means of 

 cooling; it has been observed for lizards that their temperature may 

 rise from 17° to 38° within 30 minutes on exposure to the sun; at a 

 body temperature of 39° the rate of respiration rises suddenly from 

 70-80 to 180-360 per minute, and the animal opens its mouth. A special 

 characteristic of many mammals is the possession of a large number 

 of sweat glands in the skin, as in ungulates, bats, and primates; others 

 have only limited numbers of such glands, especially on the ball of the 

 foot. With many species, as in the horse and man, the sweat glands 

 are so modified that they secrete large amounts of water and thus 

 serve to diminish body temperatures. 56 As the air approaches satura- 

 tion, water evaporates more slowly, and this means of cooling fails. 



Despite the inner stenothermal condition of the homoiothermal 

 animals, one ought not to infer that they are all eurythermal, and 

 therefore endure wide fluctuations of temperature. This is, however, 

 frequently true, witness the wheatear {Saxicola) that lives in Green- 

 land as well as on the hot Spanish plains, the two-humped camel that 

 must endure temperature ranges of —37° to -f38 057 in the Gobi Desert, 

 the tiger whose distribution extends from the Sunda Islands to the 

 Amur River, or the puma which occurs from Canada to Patagonia, are 

 examples of marked eurythermy. The homoiothermal animals are to 

 a certain degree independent of one of the chief factors of climate, 



