352 GENERAL ZOOLOGY 



the body is only a little above 0°C. The integument of such 

 animals is without any insulating mechanism for preventing loss 

 of heat. 



Birds and mammals are warm-blooded or homoiothermous. 

 In such animals the range of body temperature compatible with 

 living is much more limited and must be maintained in the 

 neighborhood of 39°C. for birds and 37°C. for mammals, except 

 in a hibernating animal. Most homoiothermous animals are 

 provided with feathers or hair which, in addition to serving as a 

 protection from mechanical injury, also aid in conserving body 

 heat. Secretion of sweat and evaporation of water from the 

 lungs are important means for preventing overheating. Feathers, 

 heavy fur, or hide constitute good protections against cold. 

 Aquatic mammals, such as whales, are provided with a layer of 

 fat or blubber beneath the skin. A thick hide is also effective 

 as a protection from the heat of the sun, as in the case of the 

 hide of the elephant and rhinoceros. The human body in its 

 natural state is poorly adapted for living in cold climates. The 

 problem has been met by the use of artificial covering in the form 

 of clothing. 



Hibernation. — Many of the invertebrate animals such as 

 snails, crustaceans, insects, spiders, and myriapods undergo 

 hibernation. Among vertebrates hibernation also occurs during 

 the winter in most amphibians, and in terrestrial reptiles such as 

 lizards, tortoises, and snakes, and also in some mammals, par- 

 ticularly many rodents and some carnivores. Hibernation in 

 mammals occurs in holes, hollow trees, caves, dens, etc., where 

 the animal is protected from cold and enemies. During hiberna- 

 tion the animal is in a deep torpor resulting from a greatly lowered 

 rate of metabolism. In some cases there are records to show 

 that the hibernating animal awakes from time to time to take 

 food from a supply previously stored in the nest. The time 

 spent in hibernation varies roughly from three to six months, 

 depending upon the kind of animal and also upon the latitude 

 of its habitat. 



The decline in metabolic rate of a hibernating animal is indi- 

 cated by the rates of respiration and heart beat, body tempera- 

 ture, and loss in weight. The respiratory rate of a species of 

 ground squirrel, Citellus tridecemlineatus, during hibernation 

 ranges from 3^ to 4 per minute as compared with 100 to 200 per 



