Mammals and Birds : Lifespans of Wild Populations 91 



these ungulates breed once a year, have a herbivorous diet and 

 a long maximum lifespan. For such species sources of eco- 

 logical data for the construction of life-tables are of three 

 kinds: (1) knowledge of age at death for an adequate and 

 reasonably random sample of the population; (2) knowledge 



1000 



500 



100' 



MALES 



OALL SHEEP 



Fig. 1. Survival curves for males of five ungulate 

 populations. 



of the fate of individuals of a single cohort, at frequent 

 intervals; and (3) knowledge of the age structure among the 

 living. As Deevey (1947) pointed out, the first and third types 

 of information can be used only if one is prepared to assume 

 that the population is stable in time. 



The survival curves of males and females of four species 

 (roe deer, red deer, black-tailed deer and dall sheep), based on 



