AGGREGATIONS OF ANIMALS 109 



then there is some advantage in being so consti- 

 tuted that the available space can be shared by 

 others, perhaps by many others. Here we come 

 upon one of the first steps towards the develop- 

 ment of a closer social structure than the very 

 general one that pervades all ecological commu- 

 nities; and we may call it the social level of tolera- 

 tion, mere toleration for the presence of other 

 animals within the same limited space. 



Another widely spread type of collection of 

 annuals in nature occurs with many species 

 during the breeding season. Then animals which 

 are ordinarily solitary, such as frogs, move into 

 favorably located bodies of water where they 

 collect in large numbers which are well advertised, 

 to human ears at least, by the spring chorus of the 

 males. The formation of such breeding aggre- 

 gations is widespread in nature. Worms, insects, 

 fishes, snakes, birds and mammals as well as 

 less familiar animals are known to collect so. 



In dry regions, even near Chicago in dry sum- 

 mers, many moisture-requiring animals aggre- 

 gate together either in some damp spot or in a 

 niche when the group may conserve the bodily 

 moisture of its constituents. Such collections 

 frequently show what is called "summer sleep" 

 or aestivation, as opposed to the "winter sleep" 

 or hibernation with which we are more familiar. 



