Relation of Lifespan to Brain and Body Weight 125 



It can be concluded that two independent factors are re- 

 quired to account for the observations. One of these may be 

 identified by the highly correlated pair of variables — metabolic 

 rate and body weight. The other factor may be identified by 

 the index of cephalization, which is orthogonal to body weight 

 and must also be nearly orthogonal to metabolic rate (see 

 above). Both of these factors are represented in the brain 

 weight, so that brain weight alone is almost as good a predictor 

 of lifespan as brain weight and body weight combined. 



Although the existence of two independent factors is very 

 reliably established, their resolution into the two orthogonal 

 terms specified above is not unique. The extension of these 

 procedures of multivariate analysis to larger sets of anatomical 

 and physiological variables may make it possible in future to 

 identify these factors more confidently, and perhaps also to 

 establish the existence of one or more additional factors 

 governing length of life. 



In spite of the coarseness of the measures used, this first 

 statistical investigation of the allometry of lifespan has been 

 rewarding. Much more remains to be learned from a more 

 detailed examination of these relations within individual 

 orders of mammals, and also in other vertebrate classes, birds 

 in particular. It is to be expected that the numerical values 

 of the coefficients will differ in these different groups, for brain 

 function can be specified by a single number such as total mass 

 only to the degree that brain structure is describable in all its 

 anatomical details by a single allometric coefficient. These 

 same considerations apply to the somatic dimensions. 



This completes the discussion of the statistical analysis and 

 the implications of these findings for the theory of ageing in 

 mammals will now be considered. 



First, let us consider the above findings in terms of a theory 

 of ageing put forth by Rubner almost exactly a half- century 

 ago (1908). He took note of the fact that several species of 

 domestic animals with markedly different body sizes and life- 

 spans all had lifetime basal energy expenditure of about 



