ANIMAL AGGREGATIONS 



411 



already been called to the decreased activ- 

 ity and lengthened life of crowded sper- 

 matozoa (p. 395); many other similar ex- 

 amples could be cited. 



Positive social facilitation is also com- 

 mon, even when one uses social in a more 

 restricted sense. Carefully proved in- 

 stances include the faster improvement in 

 maze learning, in grouped than in isolated 

 goldfishes (Welty, 1934), and in green 

 sunfishes, Lepomis cyanellus (Greenberg, 

 1947); an increased food consumption in 

 grouped guppies. A Camponotiis ant digs 

 more rapidly in the presence of others 

 (Chen, 1937); and many animals, includ- 

 ing monkeys, chimpanzees, and men, are 

 stimulated to eat more and even to con- 

 sume marginal foods in the company of 

 others. Much of human education is facili- 

 tated in groups. 



Some social facilitations in nonhuman 

 animals approach what is called imitation 

 in human behavior and may be spoken of 

 objectively as being "contagious." Song 

 sparrows reared in the semifreedom of an 

 ornithologist's study showed a variety of 

 contagious activities. When one sparrow 

 ate, bathed, or preened, the others often 

 did likewise; when one flew to the desk, 

 for example, another usually followed 

 (Nice, 1943). Grouped chimpanzees that 

 have considerable freedom of movement 

 show waves of specialized activities appar- 

 ently passed along by contagion (Kohler, 

 1925). 



The food-procuring behavior of many 

 different kinds of animals changes, depend- 

 ing on the number present. The group 

 fishing of the double-crested cormorants 

 near San Francisco gives an example of 

 elaborate and flexible group cooperation. 

 These cormorants may fish singly, in small 

 coordinated flocks of from ten to twelve, 

 or in larger flocks that may contain as 

 many as 2000 birds. Fishing usually begins 

 before the larger flocks are fully formed. 

 The basic pattern in small flocks consists 

 of a circle with all birds facing the same 

 direction. This pattern changes with the 

 large flocks; then, a long, narrow, well- 

 packed line moves forward, fishing as it 

 goes. Some cormorants swim at the sur- 

 face, others dive and swim at the same 

 rate; those left behind by the rapid ad- 

 vance take to the air and fly forward again 

 to become members of the line of fishers. 



The large flocks swim decidedly faster than 

 do small fishing groups— an example of an- 

 other kind of social facilitation; they also 

 pursue a given school of fish until the hun- 

 ger of the cormorants is satiated, or until 

 the school escapes (Bartholomew, 1942, 

 1943). Thus the persistence of a large 

 flock is greater than that of a small one. 



ORGANIZATION OF ANIMAL GROUPS 



The simplest animal aggregations show 

 little, if any, discernible social organiza- 

 tion. Primitive, partial integrations arise, in 

 some instances from the constant pushing 

 toward the center of those on the periphery, 

 as in schools of very young bullheads 

 {Ameiunis) . Highly integrated insect 

 groups— colonies of bees or ants, for exam- 

 ple—have types of organization that we 

 can only partially perceive and which we 

 do not yet fully understand. Our present 

 knowledge of integrating and regulating 

 mechanisms in social insects will be sum- 

 marized later (p. 426); for present pur- 

 poses we can state that in many insect 

 colonies the individual seems to be strongly 

 group-dominated. 



The contemporary organization of verte- 

 brate groups, often more or less crudely 

 approaching some aspects of human or- 

 ganization, is based on the application of 

 three general principles: the holding of ter- 

 ritory; domination-subordination; and lead- 

 ership-followership. These different types 

 may occur in fairly pure form, or they may 

 grade into each other, even in schools of 

 fishes, to give complicated organizational 

 patterns. 



Each of these principles— territoriality, 

 social hierarchy, and leadership— operates 

 to some extent among invertebrates. Cer- 

 tain crabs, spiders and insects defend 

 small territories (Pearse, 1939). The small 

 hermit crab (Eupa^iirtis lon^icarpiis) dis- 

 plays dominance based on individual con- 

 tacts (Allee and Douglis, 1945); an order 

 of social ascendancv exists among associat- 

 ed fertile females of the wasp Tohstes p,aJ- 

 lictis fPardi, 1948; and p. 430); and some 

 ants show leadership relations (Schneirla. 

 1933; Chen, 1937a). The greater part of 

 our information concerning these principles 

 of group organization deals with verte- 

 brates, especially among the bonv fishes, 

 lizards, birds, and mammals. 



