Mammals and Birds : Lifespans of Wild Populations 97 



cent of a population of little souslik {Citellus pygmaeus) 

 reached the age of one year in the Caucasus and Fitch (1947) 

 reported 9 to 38 per cent of cottontail rabbits reaching one 

 year in central California. 



Shrews appear even more short-lived than rodents. In 

 England, Crowcroft (1956) estimated that the common shrew 

 {Sorex araneus) lives for only 18 months at the longest and 

 that most individuals die before reaching one year of age. 



Bats, on the contrary, seem to have a higher survival rate 

 than other mammals of similar size. The expectation of 

 life in the colony of Myotis mystacinus ringed by Sluiter, Van 

 Heerdt and Bezem (1956) in Holland, was 4*4 years, with a 

 maximum lifespan exceeding 20 years! Such longevity is quite 

 unexpected in such small mammals and probably has some- 

 thing to do with the very peculiar metabolic pattern of these 

 animals. 



To sum up the available data on mammals, it seems certain 

 that, in natural conditions, small and fast-breeding species, 

 with a low ratio of mean to potential duration of life, show 

 very high and age-constant mortality rates. The only 

 exception, that of bats, is probably due to the rather peculiar 

 physiology of these animals. On the other hand, large and 

 slow-breeding species, with a high ratio of mean to potential 

 duration of life, tend to have age-specific mortality rates; 

 this pattern is nevertheless strongly influenced by ecological 

 conditions. 



In birds, we find the same difference as in mammals between 

 small and fast-breeding species on the one hand and relatively 

 large and slow-breeding ones on the other. Most of the 

 available data concerning this group have already been sum- 

 marized by Lack (1954), Farner (1955) and Hickey (1955) and 

 need not be mentioned again here. We will therefore limit 

 ourselves to a few examples. 



The best life-table we have for any bird of relatively large 

 size is that of the common terns {Sterna hirundo) studied by 



AGEING — ^V — 4 



