MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 383 



or communities which spread over many acres. They tell 

 each other by shrill cries of the approach of enemies, and they 

 seem to visit each other and to enjoy each other's society a 

 great deal, although that they afford each other much actual 

 active help is not apparent. Birds in migration are grega- 

 rious, although at other times they may live comparatively 

 alone. In their long nights they keep together, often with 

 definite leaders who seem to discover and decide on the course 



FIG. 235. Prairie dogs. (Adapted from photo- 

 graph by Merriam.) 



of flight for the whole great flock. 

 The wedge-shaped flocks of wild 

 geese flying high and uttering their 

 sharp, metallic call in their south- 

 ward migrations are well known in many parts of the United 

 States. Indeed, the more one studies the habits of animals 

 the more examples of social life and mutual help will be found. 

 Probably most animals are in some degree gregarious in habit, 

 and in all cases of gregariousness there is probably some de- 

 gree of mutual aid. 



An interesting series of gradations from a strictly solitary 

 through a gregarious to an elaborately specialized communal 

 life is shown by the bees. Although the bumblebee and the 

 honeybee are so much more familiar to us than other bee kinds 

 that the communal life exemplified by them may have come 

 to seem the usual kind of bee life, yet, as a matter of fact, there 

 are many more solitary bees than social ones. The general 

 character of the domestic economy of the solitary bees is well 

 shown by the interesting little green carpenter bee, Ceratina 

 dupla. Each female of this species bores out the pith from 

 five or six inches of an elder branch or raspberry cane, and 



