486 VERTEBRATE LIFE AND ORGANIZATION 



phenomena of migration, particularly such things as the ability of young 

 birds to reach their destination on their first migration even though 

 unaccompanied by adults. More observations and experiments are neces- 

 sary before the riddle of bird migration will be solved. 



211. Characteristics of Mammals 



Mammals are the familiar haired creatures, such as cats, mice, pigs 

 and men. They are the group of organisms to which the term "animal" 

 is often restricted by laymen, though zoologists object to such a usage. 

 A jaw joint between the dentary and squamosal bones, and the presence 

 of three auditory ossicles within the middle ear, are convenient osteo- 

 logic features for distinguishing between mammals and the extinct mam- 

 mal-like reptiles from which they evolved. Osteologic criteria are 

 necessary in dealing with fossil material, but contemporary reptiles and 

 mammals can be distinguished in many other ways. The presence of hair 

 and mammary glands is the most obvious diagnostic feature of mam- 

 mals, but these are only two reflections of more fundamental changes- 

 increased activity and greater care of the young. 



Increased Activity. Birds are the most active of all vertebrates, but 

 mammals are a close second, and they are certainly the most active of 

 the primarily terrestrial vertebrates. Their appendages extend directly 

 down to the ground in the vertical plane, instead of out from the body 

 in the horizontal plane as the proximal segment of the limb does in 

 amphibians and most reptiles. This improves the effectiveness of the 

 limbs in support, and permits them to move rapidly. A firmer support 

 is also provided for the pelvic girdle and hind limbs, because most 

 mammals have three sacral vertebrae in contrast to the two of most 

 reptiles. Arboreal species use the tail for balancing, and it plays a major 

 role in the propulsion of aquatic mammals such as the whales, but in 

 most mammals it has lost its primitive role in locomotion and is fre- 

 quently reduced in size. Further details of the mammalian skeleton, and 

 of other organ systems, will be emphasized in succeeding chapters, but, 

 in short, the whole skeleton reflects the increased activity and agxlity. 



The increased speed of locomotion also entailed changes in the 

 neuromuscular apparatus. Shifts in many of the muscles concerned with 

 support and locomotion are correlated with the new limb posture. 

 Moreover, the muscular system of mammals is considerably more elab- 

 orate than that of reptiles, for many primitive muscles have been sub- 

 divided. This, together with a more highly developed nervous system, 

 permits more varied responses and adjustments to environmental con- 

 ditions. 



A consistently active life naturally requires a high and constant 

 rate of metabolism, and mammals have had the same problems to solve 

 in this respect as their avian relatives. Mammals are also homoiothermic, 

 but there are differences in the way temperature regulation is achieved. 

 Hair, rather than feathers, entraps air and forms an insulating layer 

 over the body surface that reduces heat loss. Heat is dissipated, when 

 necessary, by an increased blood flow through the skin and by the 



