The Distribution of Senescence 



1940, Fig. 30), in wild voles (Hinton, 1925, 1926; Elton, 1942) 

 and in Peromyscus (Burt, 1940) senescence is never observed, 

 judging from the state of the teeth and bones of recent and 

 fossil animals. In some populations the vole must be regarded 

 as an annual (Elton, 1942). Tooth wear is a reliable index of age 

 in short- tailed shrews, those over 2 years of age being edentulous, 



8 16 24 32 40 48 56 64 72 80 88 96 

 Fig. 30. — Smoothed survival curve for the vole, Microtus agrestis in captivity 

 (from Leslie and Ranson, 1940). 



but age limitation by this mechanical form of senescence is more 

 potential than actual since few survive to exhibit it. They may 

 survive in captivity up to 33 months (Pearson, 1945). 



The log-linear pattern of decline in survivorship is highly 

 characteristic of birds. It has been demonstrated in the black- 

 bird, song- thrush, robin, starling and lapwing (Lack, 1 943a, b, c) , 

 redstart (Buxton, 1950), American robin (Farner, 1945) and 

 herring gull (Marshall, 1947). In a series of robins ringed by 

 Lack (1943a), 1 1 1 out of 144 leaving the nest (77 per cent) died 



109 



