CLASS MAMMALIA 417 



Proboscidea (pro bo sid' e a; G., pro, in front, and hoskein, to feed) 

 contains the elephants, which are the largest terrestrial mammals and 

 which live to a great age, sometimes as much as two hundred years. 

 The order Sirenia (si re' ni a; G., seiren, a sea nymph) includes the man- 

 atees and dugongs, which are aquatic and herbivorous animals with 

 several characteristics betraying a relationship to the elephants. The 

 manatees are found in the fresh waters along the coasts of southern 

 North America, northern South America, and Africa; the dugongs inhabit 

 Oriental and Australian waters. 



445. Cetacea. — In this section are the whalebone whales, sperm 

 whales, porpoises, and dolphins. They have a single or double nostril 

 in the median line at the end of the snout and when they rise to the 

 surface expire forcibly, throwing a great column of water into the air, 

 an act called "blowing." They have to come frequently to the surface 

 for the purpose of obtaining air and each time they come they "blow." 

 Among the whales are the largest animals that have ever lived, reaching 

 a maximum length of 85 feet. The teeth of the whalebone whales are 

 rudimentary and functionless and are replaced by whalebone, or baleen. 

 Whalebone is a horny material developed from the epidermal lining 

 of the mouth, which is arranged as a series of curtain-like plates to form 

 a sieve. These whales feed on relatively small animals which occur in 

 large numbers and which they strain from the water with this sieve. 



446. Hibernation. — Hibernation has previously been noted as an 

 adaptation to terrestrial life and was briefly discussed in connection 

 with Amphibia (Sec. 403). Among the reptiles, turtles bury themselves 

 during winter in the mud of the shores and bottoms of the bodies of water 

 in which they live, and lizards and snakes hide in crevices in rock ledges 

 or crawl into holes in the ground. No birds hibernate. While many 

 mammals remain active in the winter, protected by their heat-conserving 

 covering of fur and subcutaneous fat and find a sufficient supply of food to 

 meet their needs, others hibernate during all or a part of that season. 



In preparation for hibernation a mammal becomes very fat, storing 

 up a supply of heat-producing food. During true hibernation the 

 temperature of the animal falls, frequently to within a few degrees of 

 freezing; respiration becomes very slow and shallow; the heart beats 

 slowly, and the circulation is sluggish; in fact all metabolism is carried 

 on at a very slow rate, and the temperature-regulating mechanism is 

 temporarily suspended. In other words the organism becomes for 

 the time cold-blooded. The muscles are rigid and the animal is uncon- 

 scious. The mammals of this country that undergo true hibernation 

 include jumping mice and pocket mice, the woodchuck, or ground hog, 

 ground squirrels, and some of the bats. 



Some mammals, such as the skunks, badgers, and raccoons, do not 

 undergo a true hibernation but only spend the time in their winter 



