THE NUMBERS OF ANIMALS 125 



with courtship and nesting habits. These and other matters of 

 great ecological interest may be found described and discussed 

 in the works of Howard. It should be pointed out that 

 territorial systems among birds are not always for the purpose 

 of dividing up the food-supply in a suitable way. They may 

 also be equally important in limiting numbers by being con- 

 cerned with nesting-sites. 



36. Less is known about territory conditions in carnivorous 

 mammals, but it is pretty clear that they do in many cases have 

 territories, and for the same reasons as vv^ith hawks and warblers. 

 It is well known that animals like the African lion or Indian 

 tiger are few in numbers, and that each district will have one 

 pair or one family living in it. We know practically nothing 

 about the way in which such animals settle the size of their 

 territory, and little enough about the extent to which the terri- 

 torial system is found among mammals at all. There are 

 indications that some herbivorous mammals limit their numbers 

 to some extent in this way. For instance, Collett ^^^ says 

 that the lemming does so in normal years. It is probable that 

 insects like ants, which live in great towns, have some system 

 of spacing out their colonies so as to avoid overcrowding. The 

 whole subject requires more investigation before we can say 

 exactly how important a part it plays in the lives of animals ; 

 but Howard's work on birds is sufficient to show that it is 

 certainly a very effective method of limiting numbers in some 

 species, and it will probably be found to occur very widely 

 among animals which do not possess any other convenient 

 means of regulating numbers. 



37. We may conclude this chapter by referring to the 

 animals which live by exploiting the work of other species, but 

 do not actually eat them or destroy them in the process. The 

 majority of parasites belong to this class. Such animals as 

 tapeworms do not always harm their hosts very greatly, except 

 that they divert a certain amount of food from its proper 

 destination in the tissues of the host. In the same way fleas, 

 if not too numerous, do not necessarily do much direct harm 

 by withdrawing blood from their host, although they may 

 accidentally spread the germs of disease in that way. Parasites 



