Mutual Attraction 315 



stumble in a ditch, the remaining sheep will continue crowding for- 

 ward causing a "pile-up." Sheep men greatly fear the occurrence 

 of these pile-ups in which as many as 500 sheep may be killed within 

 a few minutes. 



Animals forming groups recognize others of the same species most 

 commonly by vision, but various other means are also used. Birds 

 migrating in darkness or fog, or moving through thick vegetation, 

 probably rely on call notes for keeping in touch with one another. 

 Sound is probably used in the aquatic environment also, to a much 

 greater extent than has been realized. Sound travels through the 

 water medium much more effectively than through the air medium 

 —quite the reverse of the relationship for other orienting stimuli, such 

 as light and odor. Various crustaceans, a great many fishes, and cer- 

 tain cetaceans, including blackfish and porpoises, produce grunts, 

 whistles, squeaks, and other noises that can be heard under water 

 for great distances and are undoubtedly useful for recognition among 

 aquatic animals (Kellogg, Kohler, and Morris, 1953). 



Some species of ants which are totally blind nevertheless manage 

 to keep close together by the use of their "contact odor" sense. In 

 tests with bullhead fishes each individual was found to move toward 

 any object of the same size and color as itself. The fish would then 

 touch the object with its barbels, and, if the sense organs encountered 

 a paraffin model, the fish would move away. In this instance the first 

 reaction causing the fishes to move together was a visual one; other 

 reactions came into play subsequently. Among land vertebrates 

 many mammals use odor for recognition. 



Sometimes the reaction to school or to form flocks is so strong that, 

 if no others of the same species are available, the animal will join a 

 group of another species. Thus we commonly find a few isolated 

 gulls flocking with a group of terns on the shore, or a single sander- 

 ling hurrying along the beach in company with a flock of least sand- 

 pipers. Examples among other types of animals will occur to the 

 reader. An amusing illustration of this tendency to associate with 

 a group, even of another species, was observed in the large basement 

 tank at the laboratory of the Woods Hole Oceanographic Institution. 

 A number of squid had been kept in this tank for some time, and 

 these animals always moved together in a dense school. One day a 

 small mackerel of about the same size was placed in the tank. Since 

 no other mackerel were present, this fish immediately joined the ranks 

 of the squid, swimming along with them at the same rate in close 

 formation. As the observer approached the side of the tank, the 

 squid were startled and shot backwards. Since the mackerel was 



