140 ANIMAL ECOLOGY 



their food, or upon other factors in their surroundings. Such 

 variations have a regular periodicity in the case of the shrews ^Sa 

 at any rate. 



1 6. Rodent fluctuations have always been recognised, 

 although the universal existence of the phenomenon has not 

 always been realised. It is less well known that the larger 

 herbivorous mammals (ungulates) are also subject to periodic 

 epidemics in nature, which are separated by much greater 

 intervals of years than those of rodents, owing to the slower 

 rate of increase of the former consequent upon their larger 

 size. A rough idea of the rates of increase of the mammals 

 of various sizes may be gained by the following figures. If 

 one pair were allowed to increase unchecked, or at any rate 

 were not subject to very severe checks (as by epidemics), a 

 dense population would be produced by mice or lemmings 

 in three or four years, by squirrels in five years, by hares or 

 rabbits in about ten years, by sheep in about twenty years, 

 by buffaloes in about thirty years, and by elephants in about 

 fifty or more years. These figures are of course rough approxi- 

 mations, but they enable one to see why mammals of different 

 sizes have different intervals between their epidemics and 

 between their maxima. 



17. Good evidence on the subject of fluctuations in wild 

 ungulates is rather scattered and hard to obtain, since during 

 the last few thousand years there have been many epidemics 

 among man or his domestic animals which have become 

 communicated to wild species and have thus interfered arti- 

 ficially with the natural balance of numbers of the latter. For 

 instance, cholera and liver-rot seem to have been serious 

 factors affecting the numbers of wild boars and deer on the 

 continent of Europe during early times. ^^^^ But there are 

 several bits of information which give us a glimpse of the way 

 in which large ungulates used to regulate their numbers when 

 living in an undisturbed state of nature. Brooks ^ says that 

 on Vancouver Island the wild deer are subject to cycles of 

 abundance and scarcity. These are followed closely by the 

 cougar or mountain lion, which preys exclusively on deer, 

 just as the lynx does upon rabbits. We do not know in this 



