294 



SUBPHYLUM VERTEBRATA: 



rhinarium' 



OPOSSUM 



Figure 16.26 A marsupial and a placental mammal. (From Malcolm Jollie, C/iorc/afe Morpfio/ogy^ 

 Reinhold Publishing Corp., New York, 1962.) 



opment, young may use pouch as a shelter; mostly 

 Australian, also South American and the possum in 

 North America (Figure 16.26). 



Infraclass Eutheria (Placentals) 



Diagnosis: without a pouch; embryos become well 

 developed in association with a placenta; newborn 

 often helpless, but are not embryonic; include all 

 aerial, aquatic, and terrestrial mammals other than 

 monotremes and marsupials. 



We now turn to the final class of animal kingdom. 

 The mammals are familiar to us because both man 

 and many of his domestic animals — cats, dogs, cattle, 

 sheep, goats, horses, and others — are members of this 

 class. Again, discussion is limited to certain aspects 

 of biology. 



Mammals are a very successful group of animals. 

 Moreover, they seem to be more independent of their 

 environment than are most other animals. Mammals 

 have an essentially constant body temperature that is 

 maintained within a few degrees by internal body 

 mechanisms (endothermism) which are found in no 

 other animals except the birds. Few mammals are 

 strictly diurnal or nocturnal; however, the brightest 

 and darkest periods of the day tend to be times of 

 inactivity. Among animals, mammal activities seem 

 to be most controlled by their inner workings, or 

 physiology, rather than by their environment. 



MOVEMENTS 



Mammals display nomadism, emigration, and mi- 

 gration; and most mammals have well-developed 

 homing abilities. The homing sense may or may not 

 play a part in the three kinds of movements. Even 

 among the nomads, e.g., wolves, the seemingly aim- 

 less wandering is not strictly so, but is in relation to a 

 defined route. 



Emigrants on the other hand do not seem to use 

 any "homing" sense. The desertion of its home by 

 part or all of a species is at least partly due to over- 

 population. Although United States mammals such 

 as the eastern gray squirrel, snowshoe hare, beaver, 

 pika, and woodrat, are known to emigrate, the most 

 striking example is found in the mouse-like lemmings 

 of the birch-willow habitat near the timberline of the 

 high Norwegian mountains. These animals live on 

 succulent grasses and roots and normally bear one or 

 two litters of four or five young each year. Periodi- 

 cally, there is an enormous increase of reproductive 

 potential; three or four litters are produced in the 

 summer, the first litter is bearing young by August, 

 and there are up to eleven young per litter. In addi- 

 tion to this high reproductive rate, the young show 

 exceptional vitality and can ward off disease in a re- 

 markable manner. Needless to say, these conditions 

 lead to overpopulation and lack of food for the ex- 

 panded population. The vast majority of the popula- 

 tion then emigrates downhill, seemingly letting 



