370 Relations between Species 



taining a sea anemone from a rock and placing it on the back of its 

 shell. When the growth of the crab causes it to move to a larger 

 shell, the crab loosens the base of the anemone with its claws and 

 transfers it to the new shell. In some species the base of the anemone 

 grows over the entire shell and extends beyond it; sometimes the 

 original shell is largely dissolved away so that the crab's house con- 

 sists almost entirely of the anemone's base. The stinging nematocysts 

 of the sea anemone are a powerful deterrent to predaceous fish and 

 protect the members of the partnership from being eaten. When 

 Nereis is also present as a junior partner, it is said to help keep the 

 inside of the shell clean and, as payment, to snatch fragments of food 

 from the pincers of the crab. 



Mutualism without Continuous Contact. A great many instances 

 exist of mutualism in which the partners are not attached to each other 

 or in which they are in contact intermittently or for only a short pe- 

 riod. In most mutual benefit associations of this sort the fulfillment 

 of a nutritional need plays a prominent role for at least one of the 

 species; but the other species, and sometimes both, may derive an 

 entirely difterent type of advantage. 



A commonly cited example of mutualism of two animal species in- 

 volves birds that alight on the backs of large grazing animals and 

 pick ofl: the ticks or other external parasites. The cowbird in North 

 America, the oxpecker, the little white heron in Africa, and certain 

 other birds obtain a ready supply of food in this way. The host ani- 

 mals are rid of their pests and are frequently warned of approaching 

 danger by the activity of the birds as watchmen. An amazing kind 

 of pest-control service is rendered by the crocodile bird in removing 

 leeches from around the teeth of the crocodile, which allows the bird 

 to enter its mouth for the search. Mixed groups of ostriches and 

 zebras are said to derive mutual benefit in guarding against attack 

 by the keener sense of sight of the ostrich and the greater powers of 

 scent possessed by the zebras. 



In the subterranean world we find various extremely complex re- 

 ciprocal relations between species. Various ants maintain a popula- 

 tion of aphids in their nests. The ants obtain a nutritive exudation 

 from the hind end of the alimentary tract by stroking the aphid's 

 abdomen with their antennae. This furnishes the basis for popular 

 accounts that ants "keep cows" and "milk" them. The aphids feed 

 on the roots of plants, or they are carried by the ants out of the nest 

 and allowed to feed on leaf stalks. The aphid eggs are laid on the 

 plants above ground, and, although of no immediate vise to the ants, 

 they are carried down into the nest where they are sheltered during 



