8o ANIMAL AGGREGATIONS 



To the human senses these bird and bat roosts are easily detected 

 by their odor, and perhaps that is a factor in guiding the bats to the 

 common sleeping place. 



Allen (192 1 ) has banded clusters of these bats. He records re- 

 covering three of a group of four from the same place where they 

 were banded, after an interval of three years. 



These sleeping aggregations appear to be without mating signifi- 

 cance. The Raus did not see copulation among the insects they ob- 

 served; and, in fact, in many cases the sleeping groups were com- 

 posed of males only. The robin roosts may contain both sexes and 

 all ages of birds above the nestlings. With crows the common roost 

 ends with the beginning of the breeding season, except for the bache- 

 lors; and in general these roosts are not occupied by the breeding 

 birds. After the breeding season the birds may return in family 

 groups, a situation to be discussed later at some length. Among bats 

 the sexes are segregated (Howell, 1920) during the time of gestation 

 and of the care of the young, at a time when contact sleeping aggre- 

 gations were observed. 



At the low level of integration of aggregations, with which we are 

 especially concerned, the appearance of social appetite is an inter- 

 mittent phenomenon. It may be awakened by gonadal activities 

 that precede the breeding season or by the conditions which induce 

 hibernation or aestivation. These varying exhibitions of a stronger 

 social appetite are ordinarily part of an annual rhythm, but in many 

 marine forms the rhythm may be a lunar one during the warmer 

 season of the year. In the slumber aggregations, the periodic 

 strengthening of the social appetite has a diurnal rhythm. Aggrega- 

 tions may be induced or controlled by conditions of moisture or by 

 the lack of normally favorable conditions ; this phenomenon may or 

 may not be rhythmical in nature. At this low level of social integration 

 the social appetite is not constant in appearance and in this regard 

 becomes more like the sex and hunger appetites, in which rhyth- 

 mical or spasmodic appearance is one of the usual characteristics. 

 In more highly integrated social groups the action of the social 

 appetite is steadier, and therefore less spectacular and less easily 

 recognized. 



