MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 381 



wolves which hunted the buffalo show the advantage of mutual 

 help in aggression as well as in protection. In this banding 

 together of wolves there is active cooperation among individuals 

 to obtain a common food supply. What one wolf cannot do 

 that is, tear down a buffalo from the edge of the herd a dozen 

 can do, and all are gainers by the operation. 



On the other hand, the vast assembling of sea birds 

 on certain ocean islands and rocks is a condition probably 

 brought about rather by the special suitableness of a few places 

 for safe breeding than from any special mutual aid afforded; 

 still, these sea birds undoubtedly combine to drive off attack- 

 ing eagles and hawks. Eagles are usually considered to be 

 strictly solitary in habit (the unit of solitariness being a pair, 

 not an individual) ; but the description, by a Russian naturalist, 

 of the hunting habits of the great white-tailed eagle (Hali- 

 aetos albicilla) on the Russian steppes shows that this kind of 

 eagle at least has adopted a gregarious habit, in which mutual 

 help is plainly obvious. This naturalist once saw an eagle 

 high in the air, circling slowly and widely in perfect silence. 

 Suddenly the eagle screamed loudly. "Its cry was soon an- 

 swered by another eagle, which approached it, and was followed 

 by a third, a fourth, and so on, till nine or ten eagles came to- 

 gether and soon disappeared." The naturalist, following them, 

 soon discovered them gathered about the dead body of a horse. 

 The food found by the first was being shared by all. The 

 association of pelicans in fishing is a good example of the ad- 

 vantage of a gregarious and mutually helpful habit. The 

 pelicans sometimes go fishing in great bands, and, after having 

 chosen an appropriate place near the shore, they form a wide 

 half-circle facing the shore, and narrow it by paddling toward 

 the land, catching the fish which they inclose in the ever- 

 narrowing circle. 



The wary Rocky Mountain sheep (Fig. 234) live together 

 in small bands, posting sentinels whenever they are feeding or 

 resting, who watch for and give warning of the approach of 

 enemies. The beavers furnish a well-known and very interest- 

 ing example of mutual help, and they exhibit a truly com- 

 munal life, although a simple one. They live in "villages" 

 or communities, all helping to build the dam across the stream, 

 which is necessary to form the broad marsh or pool in which the 

 nests or houses are built. Prairie dogs live in great villages 



