74 ANIMAL ECOLOGY 



are either undigested or only partly digested. The next stage 

 in this series is the crocodile bird, which sits inside the open 

 mouth of the crocodile and picks bits of food from amongst 

 the teeth of its " host." The arctic skua employs similar but 

 more drastic methods, when it chases kittiwake gulls and 

 terrorises them into yielding up their last meal, which is skil- 

 fully caught by the skua before reaching the water below. 

 Another method employed by some animals is to take small 

 bits out of their food-animal, without actually destroying it, just 

 after the manner of the hookworm or the filaria. For instance, 

 Lortet ^^ says : " The fishes of the lake of Tiberius [Tiberias], 

 very good to eat, serve as a pasturage for the myriads of crested 

 grebes {Podiceps cristatus) and of pelicans. Frequently the 

 grebes snatch at the eyes of the chromids, and with one stroke 

 of their long sharp beaks lift out as cleverly as would a skilful 

 surgeon the two eyeballs and the intro-orbital partition. These 

 unhappy fish, now blind, of which we have taken numerous 

 examples, have thus the entire face perforated by a bloody 

 canal which cicatrises rapidly. It is only the larger individuals 

 who are thus operated on by the grebes, for, not being able to 

 avail themselves of the entire fish, these voracious birds take 

 the precaution to snatch only the morsel of their choice." 

 Put less poetically, this means that the bird is able to carry 

 on the ordinary carnivorous method of destroying the whole 

 of its prey as long as the latter is below a certain size, but when 

 it grows above this limit new methods are adopted, which 

 closely resemble those of a true parasite. A further stage in 

 the series which we have been tracing is the arctic fox, which, 

 although an ordinary carnivorous animal in summer, when it 

 eats birds and lemmings, often travels out on to the frozen 

 sea-ice in winter and there accompanies the polar bear and 

 subsists on the remains of seals killed by the bear, and upon 

 the dung of the latter. The bear does all the work, and the fox 

 gets a share of the proceeds. From this point it is only a short 

 distance to a true carnivore, like the polar bear itself, which 

 is, after all, only living by exploiting the energies of the seal. 

 5. To imagine that parasites are unique in exploiting the 

 activities and food-products of their hosts is to take a very 



