Animal Societies, From Slime Molds to Man 



By R. E. Snodgrass 



Honorary Research Associate 

 Smithsonian Institution 



Though the great majority of animals as adults lead solitary lives, 

 except at the mating seasons, there are some that live in groups of 

 interdependent individuals. Among the lower invertebrates the indi- 

 viduals of such a group are usually anatomically connected, and these 

 species are termed "colonial" by zoologists. On the other hand, there 

 are true social organizations of free individuals, as with some of the 

 insects and man, which are known as "societies." The two terms, 

 however, are more commonly used interchangeably. Then there are 

 many animals that associate in herds, flocks, schools, and swarms. 

 Such species may be said to be sociable, but they are merely gregarious. 

 With the truly social animals there is always something that acts as 

 a bond for unifying the members by making them mutually dependent 

 on one another. The nature of the bond, however, may be quite 

 different in different societies. 



THE SLIME MOLDS 



A slime mold, or slime fungus, in its usual form would never be 

 suspected of being a colonial or social animal. It is a flat mass of 

 protoplasm known as a plasmodium, generally a few centimeters or 

 inches in diameter, containing numerous nuclei but no cell walls. It 

 lives in damp places on decaying leaves or logs, over which it slowly 

 moves by a flowing motion or by sending out pseudopods, picking up 

 food particles as it goes. In the plasmodium stage the creature is 

 an animal that might be likened to a giant ameba. In its reproductive 

 stage it becomes plantlike. Zoologists therefore call its kind Myce- 

 tozoa, and the botanists call it Myxomycetes. In the reproductive 

 stage, spore-bearing stalks like the sporangia of a fungus grow out 

 from the back of the plasmodium. The spores when ripe are scattered 

 by the wind ; those falling on water become minute flagellate protozoa, 

 each of which multiplies rapidly by division. Then these simple one- 



425 



