CLASS MAMMALIA. MAMMALS 



479 



OJI 



B 



Figure 351. A, young American opossums attached to the mammary glands in the brood 

 pouch of their mother. When born the young opossum is strikingly undeveloped and smaller 

 than a honey bee. The opossum is considered the most primitive of America's mammals and is 

 often referred to as a "living fossil." Except for its larger size, it is almost the same as its 

 ancestor of millions of years ago. B, four fetal nine-banded armadillos that have developed from 

 a single egg. (A copyright by General Biological Supply House, Inc., Chicago.) 



it. The placenta presents great variations in 

 the individual orders, in its special develop- 

 ment and in the mode of its connection with 

 the uterine walls. 



Hibernation and migration 



During the winter, mammals either re- 

 main active like the rabbit, or hibernate to 

 tide them over the winter scarcity of food 

 and cold rigorous weather. During hiberna- 

 tion the metabolism drops to a low level, 

 the temperature of the body decreases, and 

 the animal falls into a profound torpor. 

 Respiration almost ceases, the heart beat is 

 slowed, and no food is taken into the body; 

 but the fat masses stored up in the autumn 

 are consumed, and the animal awakens in 

 the spring in an emaciated condition. Chip- 

 munks and ground squirrels hibernate in 

 underground nests, and some bats in caves 

 or buildings. 



Comparatively few mammals migrate. 

 Among those that do are the fur seal, rein- 

 deer, bison, red bat, and some whales. The 



fur seals (Fig. 352F) that breed on the 

 Pribilof Islands in Bering Sea spend the win- 

 ter months making a circuit of about 6000 

 miles. The reindeer of Spitzbergen migrate 

 regularly to the central portion of the island 

 in summer and back to the seacoast in the 

 autumn. Some bats and certain birds leave 

 their northern summer habitats when food 

 becomes scarce and migrate southward for 

 the winter. 



The lemmings of Scandinavia are cele- 

 brated for their curious mass movements. 

 They multiply on high plateaus until the 

 food supply is exhausted; then they swarm 

 over the valleys in vast hordes, swimming 

 lakes and streams, and turning aside for 

 nothing, eating crops in their paths. Rem- 

 nants of the lemming army finally reach the 

 sea, plunge in, and perish. There are many 

 theories to explain why the lemmings jump 

 into the ocean and drown, but probably 

 none is better than the one, that they at- 

 tempt to swim the ocean as they have other 

 bodies of water and fail in the too great 

 undertaking. 



