346 



THE RISE OF ANIMAL LIFE 



munks, beavers, porcupines, guinea pigs 

 (Fig. 13-65) and even the pestiferous rats 

 (Fig. 13-65) and mice. They must have 

 been highly successful as a group because 

 all the families alive today have been in 

 existence a very long time, and none has 

 become extinct as is the case v/ith most 

 other groups. They are characterized by 

 their large chisel-like incisor teeth which 

 are self-sharpening and which are kept al- 

 most incessantly active. The incisor teeth 

 grow continuously and if not worn down 

 would soon become so long that the animal 

 could not close its mouth. If, as often hap- 

 pens, a member of this group loses one of 

 the incisors, the tooth opposing it will then 

 have nothing against which to grind, and 

 continues to grow, passing through the an- 

 terior portion of the skull. Since the tooth 

 is curved, as it grows longer it tends to form 

 a circle. As the movement of the jaws be- 

 comes restricted such an animal is in danger 

 of starving to death. In some cases squirrels 

 with one or more teeth grown into com- 

 plete circles have been reduced to feeding 

 on the soft food found in garbage. 



Most rodents are small today, altliough 

 at one time there were giant beavers which 

 grew to the size of a small bear ( Fig. 25-2 ) . 

 Rodents have a high degree of intelligence 

 and are usually secretive animals, many liv- 

 ing in burrows. One, the beaver, is almost 

 human in its ability to build dams. It selects 

 a place in a small stream, which is just 

 what any engineer would do if he were 

 building a dam. Beavers cut trees in such 

 a way that they fall in the exact spot that 

 will do the most good; they entwine the 

 branches so that the debris coming down 

 the river will catch there and make the dam 

 water-tight. When they have finished the 

 job, a first-rate dirt dam is the result. In 

 some regions where conservation laws have 

 made it possible for them to come back, 

 they have become almost pests because of 

 their habit of damming every small stream 

 in large areas, thus flooding all the fields 

 and roads in the surrounding country. 



The bats (Chiroptera) have conquered 

 the air. To be sure, other mammals such as 

 the flying squirrel are able to soar from tree 

 to tree but they have not approached the 

 bat in any sense as a flying creature. The 

 bat competes very well with the birds; in 

 fact, it can perform some feats that birds 

 cannot. The wing is merely a modified hand 

 with the fingers greatly extended and cov- 

 ered with skin. Many species of bats live in 

 deep caves, such as the Carlsbad Caverns 

 in this country, where they are forced to fly 

 in complete darkness. Just how these ani- 

 mals could avoid objects as small as tiny 

 wires strung across a room that is totally 

 dark has puzzled biologists for a long time. 

 Recently, however, with the aid of delicate 

 electronic equipment, it has been discov- 

 ered that the bat, instead of being a silent 

 animal as all had thought, does emit bursts 

 of high frequency sound waves during 

 flight. These have a frequency of about 

 50,000 vibrations per second and are there- 

 fore beyond the range of the human ear, 

 the upper limit of which rarely exceeds 16,- 

 000 vibrations per second. When these high 

 frequency sounds made by the bat strike 

 an object, no matter how small, they 

 bounce back to its ears; this happens at 

 such speed that the animal can respond 

 soon enough to avoid obstacles without the 

 use of its eyes. Man now uses this same 

 principle in detecting distant objects but 

 instead of sound waves he uses radio waves. 

 Thus the bat has had in operation for ages 

 the first radar system. 



Bats usually feed on insects, although 

 there are the so-called "flying foxes" which 

 are herbivorous. A small number are blood 

 suckers, though not as gruesome as current 

 stories and moving pictures would imply. 



The whales (Fig. 13-66) and porpoises 

 (Cetacea) are probably the most special- 

 ized of all mammals. They are descendants 

 of land carnivores which have returned to 

 the sea, and become so successful in this 

 environment that they are found in all of 

 the oceans of the world. The story of how 



