286 ANlsTJAL REPORT SMITHSONIAN INSTITUTION, 194 



the hole it is making. It is surprising how rapidly a mole can 

 progress by this method. After the runway is completed, the animal 

 traverses it without further digging other than enough to keep it 

 open. 



Gorillas {GoHlla) live a seminomadic life. They remain in a 

 given vicinity only a few days and then move on to another where 

 the food supply or other conditions may be better. In order that 

 they may add to their comfort they bend the limbs of trees and 

 shrubs so as to make crude platforms that are utilized by the females 

 and young as beds or hammocks. Generally an old male does not 

 use a platform; perhaps because he is too heavy for that type of 

 construction. Orangutans {Simia) make similar platforms in the 

 trees. 



The work of the beaver {Castor) is too well known to require dis- 

 cussion other than to point out that these animals make two types 

 of houses, one a mound out of the center of which they hollow a 

 living chamber, the other a living chamber excavated in a stream 

 bank. Both have turmels that lead into water that is ordinarily 

 so deep that it does not freeze to the bottom, so permitting the 

 beaver to go out of his house and return regardless of the weather. 

 In order that beavers may have uniform water levels adjacent to 

 their houses, they build dams of sticks, stones, and earth. Such 

 dams are sometimes as much as 12 feet high, and some have been 

 found that, although not so high, were as much as 550 feet in length. 

 The average dam is perhaps 75 feet long and 3 feet high. 



The tropical American fruit bats (Artiheus and Uroderma) have 

 adopted the practice of making shelters by biting the fronds of 

 certain kinds of palm in such a manner that the edges of the fronds 

 bend downward so as to form roofs beneath which small groups of the 

 animals can find shelter from daylight and from rain. Visitors to 

 the Panama Canal Zone can easily inspect some of these remarkable 

 shelters in the Botanical Garden at Summit. 



Birds' nests are the best known of animal homes. They are con- 

 spicuous and have been known by mankind since primitive man first 

 sought them for the eggs and the young he might obtain. The casual 

 observer may think that a bird's nest is merely a collection of sticks 

 and twigs in a tree but in reality each kind of bird builds a nest of 

 a type peculiarly its own. For example, the Baltimore oriole (Icterus 

 galbula) builds a deep sack-shaped nest pendent from the tips of the 

 limbs. It is really a woven fabric bag. The vireos {Vireo) on the 

 other hand, weave cup-shaped nests usually set in a horizontal fork so 

 that the nest is slightly pendent, though it is no deeper than wide. 

 In Africa the social weaver birds {Philetaerus socius) live together 



