A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 



487 



evaporation of water. Many mammals lose water from the body surface 

 in the form of sweat, secreted by sweat glands, but mammals such as 

 dogs, that have few sweat glands, pant vigorously and lose water from 

 the mouth and respiratory passages. Birds can cool themselves by the 

 evaporation of water from the respiratory tract, but none have evolved 

 sweat glands. 



The dentition of mammals is adapted for the purpose of obtaining 

 and handling a wide variety of foods. Their teeth are not all the same 

 shape, as is generally the case in reptiles, but are differentiated into 

 various types (Fig. 24.12). Chisel-shaped incisors are present at the front 

 of each jaw and are used for nipping and cropping. Next is a single 

 canine tooth, which is primitively a long, sharp tooth, useful in attack- 

 ing and stabbing the prey, or in defense. A series of premolars and 

 molars follow the canine. These teeth tear, crush and grind up the food. 

 In primitive mammals, the premolars are sharper than the molars and 

 have more of a tearing function. Most mammals do not swallow their 

 food whole, but break it up mechanically with their teeth and mix it 

 with saliva, which, in addition to lubricating the food, usually contains 

 an enzyme that begins the digestion of carbohydrates. Digestion is com- 

 pleted in the stomach and intestinal region. Numerous microscopic 

 villi line the small intestine, as they do in birds, and increase the surface 

 area available for absorption. 



A greater exchange of oxygen and carbon dioxide is made possible 

 by a many-fold increase in the respiratory surface of the lungs and by 



Figure 24.12. Teeth of mammals. A, The relatively unspecialized teeth of a prim- 

 itive insectivore; B and C, lateral and crown views of the left upper and lower molars 

 of an insectivore to show their occlusion; D, the stabbing and cutting teeth of a cat; 

 E and F, a crown view and a vertical section through the left upper molar of a horse 

 to show its adaptation for crushing and grinding. 



