10 A. Comfort 



Peromyscus manicidatus gracilis 5 years 10 months (Dice, 

 1933); Perognathus longimembris > 1\ years (Orr, 1939)]. 

 These differences are not closely related to size — Micromys 

 minutus reaches nearly 4 years in similar circumstances 

 (Pitt, 1955). Leslie and Ranson (1940) and Leslie and co- 

 workers (1955) found a difference in specific longevity between 

 colonies of Microtus arvensis and Microtus orcadensis, though 

 this might reflect the results of domestication and better 

 culture. Such differences are clearly of great importance to 

 our understanding of age processes, but so far none of the 

 attempts to correlate them with quantities such as the 

 duration of pregnancy or the relative length of pre-adult 

 development is satisfactory — chiefly because the figures 

 assumed for the specific ages of the different species are 

 arbitrarily drawn from maximum age records, some of them 

 quite inaccurate. Most of the valid correlations which can 

 be made out have been reviewed by Bourliere (1946). 



In view of questions like these, one of the most important 

 requisites for the understanding of mammalian age processes 

 and the factors which time them is a full range of vertebrate 

 vital statistics, based on animals living under conditions of 

 captivity sufficiently good for a fair proportion of them to 

 reach old age. These figures are almost wholly lacking. So far 

 as can be ascertained, no life table has been published for a 

 captive population of any fish, reptile or amphibian. One 

 incomplete life table exists for domestic poultry, and it is 

 based on an assumed equation to cover losses from culling 

 (Gardner and Hurst, 1933); there is no other table for birds in 

 captivity. Apart from these, we have satisfactory actuarial 

 data only for man, laboratory rats and mice, and a few other 

 small rodents, with partial figures for culled populations of 

 agriculturally important animals [e.g. Merino ewes (Kelley, 

 1939)]. There are thus no data for any vertebrates other than 

 mammals; the figures which might throw light on the evolu- 

 tion of mammalian senescence, those for poikilotherms, birds 

 and marsupials, have never been sought. 



One consequence of this lack of information is that we have 



