Biological Approach in Study of Ageing 11 



no experimental mammal intermediate in size between man 

 and the small rodents whose rate of ageing is actuarially 

 known. There are no published actuarial data for rabbits: 

 their modal specific age for all strains is probably about 8 

 years, but large hybrids may reach ages as great as 15 years 

 (Comfort, 1956). Data for guinea pigs have been collected and 

 briefly reported (Rogers, 1950) but we have no comparison of 

 strains. Accordingly, many physiological and other differ- 

 ences described in the literature between young and "old" 

 animals are in fact differences between infant and young 

 adult animals, and even where this is not so it is impossible to 

 establish correlations between such changes and the rate of 

 ageing from maximum age records alone. 



Methods of obtaining vertebrate data 



It is difficult, but not impossible, to obtain actuarial 

 statistics for vertebrates, and as they are virtually essential 

 to any biologically directed attack on age problems, we should 

 devote our attention to getting them, as a matter of urgency. 

 They represent an expendable problem, moreover, since time 

 once spent will not require to be re-spent later. 



Age-mortality data can be obtained in three ways: (1) from 

 populations of animals specially kept under close observation 

 throughout life, (2) from analysis of existing records, (3) by 

 cross-sectional studies which indicate the simultaneous 

 mortality in each age group over one period of time, instead of 

 the successive mortalities of the survivors of a cohort followed 

 throughout life. The results of (3) will differ numerically from 

 those obtained from the same animals by the first two methods 

 if there is a secular change in mortality during the lives of the 

 longest-lived individuals. With this method we should include 

 cross-sectional studies of animals which can be aged by 

 inspection, particularly the analyses of fish populations by 

 means of catch curves (Ricker, 1948). The number of instances 

 in which wild populations can be used for ageing studies is, 

 however, so far small; although even in small birds whose 

 mortality is substantially constant, ringing studies show that 



