ADAPTATION 339 



animal resting on its belly on the ground. In this position the 

 animal remains completely quiet, the deathlike immobility 

 being heightened by the closing of the eyes and a slowing down 

 of respiratory movements. The newly hatched young of the 

 common tern, Sterna hirundo hirundo, can be handled without 

 exciting fear, but later when about a third grown and not yet 

 able to fly, the same birds will feign death by lying perfectly quiet 

 in the grass when one approaches them. If a bird is picked up, 

 it maintains its lifeless attitude even when feathers are pulled out. 

 Its patient indifference to such treatment does not last long, 

 however, and comes to an end abruptly, with the bird struggling 

 frantically to escape. This death feint is not shown by terns 

 after they have learned to fly. Feigning a broken wing or 

 lameness is a device employed by a number of ground-nesting 

 birds during the breeding season to direct the attention of 

 intruders away from the young or the nest. The tactics of the 

 opossum, Didelphis virginiana, in shamming death when uncov- 

 ered in its nest is an example of the same general sort of behavior- 

 ism that is displayed by a number of other smaller mammals. 

 Feigning death or injury is widespread in different groups of 

 animals. The tonic immobility produced in many animals may 

 merely be the result of fear and have no especial adaptive signifi- 

 cance. They may be "scared stiff." If the temporary immo- 

 bility aids in concealing the animal, as in the case of young terns, 

 there results a certain benefit which may be considered adaptive. 

 The simulation of bodily injury by adult birds would certainly 

 seem to be directed toward protecting the young. 



Animal Associations. — Certain kinds of animals are ordinarily 

 found in groups of individuals. Such animals are gregarious— 

 they live in flocks or herds. The horse, antelope, elk, buffalo, 

 elephant, etc., illustrate this mode of existence. The herd is 

 made up of individuals of the same species. The leader of the 

 herd, in the case of horses, is a male and the same is true of other 

 groups. In the buffalo (bison) herd the leader is a female. 

 Sometimes two or more species are associated as in the presence of 

 flocks of white herons with elephants, or of the ostrich with the 

 zebra and gnu. All domestic animals except the cat are gregari- 

 ous in habit. Herding in mammals is confined almost entirely 

 to herbivorous forms. Such animals subsist on plant food and 

 require a relatively large amount which must be gathered from 



