Mammals and Birds : Lifespans of Wild Populations 95 



In such a case, it would be expected that the population 

 dynamics would be different from those found by Andersen 

 (1953) in Kalo; the older animals would be competing with 

 younger, physiologically more efficient animals. Similarly, if a 

 dall sheep herd were not culled by predators, the population 

 would presumably be limited by food supplies, and the 

 mortality among prime adults would increase accordingly. 

 Taber and Dasmann are thus certainly right in pointing out 

 the danger of considering the population dynamics of a given 

 animal under given circumstances as typical of that species in 

 general. 



All the species of long-lived and slow-breeding ungulates 

 considered above, belonging either to hunted or unhunted 

 populations, show in most cases age-specific mortality rates. 

 The situation seems very different in small mammals, which 

 are both short-lived and fast-breeding. All the species investi- 

 gated so far appear to have age-constant mortality rates 

 (after very early life). 



In his study of the survival of wild brown rats on a Mary- 

 land farm, Davis (1948) shows, for instance, that no more than 

 about 5 per cent of the rats live for a year. In the tropical 

 environment of Malayan jungles, the situation looks very 

 much the same for the 12 species or subspecies of Murids 

 studied by Harrison (1956). Table I indicates the estimate of 

 mean survival rates per month for marked animals, together 

 with the mean and maximum length of life (in months) and 

 the age at which only 5 per cent of the population can be 

 expected to survive (effective maximum, 95 per cent). 



The white-footed mice (Peromyscus leucopus) of the George 

 Reserve in south-eastern ^lichigan have likewise a very low 

 survival rate (Snyder, 1956). With the high rate of mortality 

 of the winter 1950-1951, only one mouse in a thousand could 

 be expected to reach an age of 93 weeks; with the lower 

 rate of the previous winter, 34 mice could be expected to reach 

 93 weeks, and at least one would probably reach 197 weeks. 

 The mean length of life from birth for such individuals would 



