6o ANIMAL ECOLOGY 



to live entirely on mice in the years when the latter are very 

 abundant, but prey on larger animals like rabbits at other 

 times. 



11. It is thus plain that the size of the prey of carnivorous 

 animals is limited in the upward direction by its strength and 

 ability to catch the prey, and in the downward direction by the 

 feasibility of getting enough of the smaller food to satisfy its 

 needs, the latter factor being also strongly influenced by the 

 numbers as well as by the size of its food. The food of every 

 carnivorous animal lies therefore between certain size limits, 

 which depend partly on its own size and partly on other factors. 

 There is an optimum size of food which is the one usually eaten, 

 and the limits actually possible are not usually realised in prac- 

 tice. (It is as well to point out that herbivorous animals are 

 not strictly limited by the size of their plant-food, except in 

 special cases such as seed-eating birds, honey-collecting 

 insects etc., owing to the fact that the plants cannot usually 

 run away, or make much resistance to being eaten.) We have 

 very little information as to the exact relative sizes of enemies 

 and their prey, but future work will no doubt show that the 

 relation is fairly regular throughout all animal communities. 



12. Three examples will serve to illustrate the part played 

 by size. There lives in the forests round Lake Victoria a kind 

 of toad which is able to adjust its size to the needs of the 

 moment. When attacked by a certain snake the toad swells 

 itself out and becomes puffed up to such an extent that the 

 snake is quite unable to cope with it, and the toad thus achieves 

 its object, unlike the frog in ^sop's fable.^'^'^ Carpenter ^^ has 

 pointed out another curious case of the importance of size in 

 food. The tsetse fly {Glossina palpalis)^ whose ecology was 

 studied by him in the region of Lake Victoria, can suck the 

 blood of many mammals and birds, in which the size of the 

 blood corpuscles varies from 7 to iS^ut, but is unable to suck 

 that of the lungfish, since the corpuscles of the latter (41^ in 

 diameter) are too large to pass up the proboscis of the fly. 

 A third case is that noticed by Vallentin ^^^ in the Falkland 

 Islands. He found that the black curlew {Hcematopus quoyi) 

 ate limpets {Patella cenea) on the rocks at low tide, but was only 



