62 ANIMAL ECOLOGY 



certain point ; this is explained in the section on the Pyramid 

 of Numbers. Leaving aside the question of parasites at 

 present, it may be taken as a fairly general rule that the enemy 

 is larger than the animal upon which it preys. (This idea is 

 contained in the usual meaning of the word " carnivore.") 

 But such is not invariably the case. Fierceness, skill, or some 

 other special adaptation can make up for small size. The 

 arctic skua pursues and terrorises kittiwake gulls and compels 

 them to disgorge their last meal. It does this mainly by naked 

 bluff, since it is, as a matter of fact, rather less in weight than 

 the gull, but is more determined and looks larger owing to a 

 great mass of fluffy feathers. In fact, when we are dealing 

 v^dth the higher animals such as birds, mammals, and the 

 social ants and bees, the psychology of the animals very often 

 plays a large part in determining the relative sizes of enemies 

 and their prey. Two types of behaviour may be noticed. 

 The strength of the prey and therefore its virtual size may be 

 reduced ; this is done by several devices, of which the com- 

 monest are poison and fear. Some snakes are able to paralyse 

 and kill by both these methods, and so can cope with larger 

 animals than would otherwise be possible. Stoats are able 

 to paralyse rabbits with fear, and so reduce the speed and 

 strength of the latter. It is owing to this that the stoat can 

 be smaller than its prey. The fox, which does not possess this 

 power of paralysing animals with fear, is considerably larger 

 than the rabbit. The second point is that animals are able 

 to increase their own effective size by flock tactics. Killer 

 whales in the Antarctic seas have been seen to unite in parties 

 of three or four in order to break up the thick ice upon which 

 seals, their prey, are sleeping.^^^ Wolves are another example. 

 Most wolves are about half the linear size of the deer which 

 they hunt, but by uniting in packs they become as formidable 

 as one very large animal. The Tibetan wolf, which eats small 

 gazelles, etc., hunts singly or in twos and threes.^^^ On the 

 other hand, herbivores often band together in flocks in order 

 to increase their own powers of defence. This usually means 

 increased strength, but other factors come in too. Ants have 

 achieved what is perhaps the most successful solution of the 



