XXV 



RODENTS AND RABBITS 



1 . Characteristics of rodent life 



The animals loosely known as rodents are the most successful of 

 modern mammals other than man. They live in all parts of the world, 

 from the tropics nearly to the poles. Three thousand species are known, 

 as many as are found in all other mammalian orders put together. They 

 inhabit a considerable variety of ecological niches, mostly on the land, 

 often in burrows, but many in the trees and some in the water. This 

 most successful type of mammalian life is, however, in several ways un- 

 typical of the rest and indeed has been isolated since the early Tertiary. 

 One striking point is that the animals have never become large in size, 

 although such increase is a tendency found in almost all other mam- 

 malian groups. The South American capybara, the largest living 

 rodent, is the size of a small pig, and few fossil forms were much larger. 

 Rodent life has specialized in rapid breeding and this system of pro- 

 duction of large numbers of small animals has been very successful. 

 The total rodent biomass today may well be greater than that of the 

 whales, which are at the other extreme, and have all the advantages of 

 aquatic life. The rapid reproduction presumably brings considerable 

 evolutionary advantages, enabling the population to make the adjust- 

 ments necessary to meet changing circumstances. One of the charac- 

 teristics of rodent populations today is their great fluctuations (p. 663), 

 notorious in the case of the voles, mice, and lemmings, but marked also 

 in rats and other forms. The pressure of rodent life is such that no 

 stable equilibrium is reached with the environment and extreme oscil- 

 lations occur, often with results of great importance to man and to his 

 crops. 



In spite of the similarities of all these animals with gnawing teeth, 

 zoologists consider that the rabbits and hares are not closely related to 

 the others and are therefore to be placed in a distinct order Lagomorpha, 

 the order Rodentia being retained for all other 'rodents'. It is not even 

 certain that the two orders are in any way related. Fossil rodents (in 

 the strict sense) certainly occurred in the late Palaeocene period and a 

 probable lagomorph is reported from the same time. Both groups 

 retain many primitive mammalian characters, for instance, a long, low 

 skull, with small brain and small cerebral hemispheres, temporal fossa 



