The Biology of Senescence 



and third year of life. Its pulse rate of 340 beats per minute is 

 about 100 below normal' (McCay, 1952). The basal metabolic 

 rate of rats so retarded lay between that of normal young and 

 normal old animals (Horst et al., 1934). In rats retarded for 

 850 days, heat production per unit surface area was lower, but 

 heat production per unit weight higher, than in normal controls 

 (Will and McCay, 1943). The aorta and kidneys of retarded 

 rats showed in general a higher level of calcification than those 

 of controls (Hummel and Barnes, 1938) perhaps on account of 

 the relatively higher mineral concentrations in the restricted 

 diet (Barnes, 1942). A further series of experiments in the 

 dietary restriction of animals which had already reached matur- 

 ity was unfortunately complicated by the introduction of many 

 groups of variables (exercise, casein intake, liver supplements, 

 etc.) — in these experiments, underfeeding produced a significant 

 increase in life-span compared with fully-fed controls, but the 

 difference was far less conspicuous than in the retardation of 

 young growing rats, and the factors which were most important 

 in determining life-span were those which determined the degree 

 of body fatness (McCay, Maynard, Sperling and Osgood, 1941 : 

 Silberberg and Silberberg, 1 954) . This difference was largely 

 accounted for by the higher incidence of renal disease on a high 

 protein diet and in obese animals (Saxton and Kimball, 1941); 

 in contrast to the findings in animals retarded while young, the 

 incidence of chronic pneumonitis and of tumours was not 

 reduced by underfeeding in mature animals (McCay, Sperling 

 and Barnes, 1943; Saxon, 1945). A 33 per cent restriction of 

 calories, other elements in the diet being unaltered, produces 

 a significant prolongation of life in male C3H mice (King and 

 Visscher 1950). 



The results of these experiments indicate that at least some 

 mammals are capable while immature of undergoing prolonged 

 suspension of growth without any acceleration of senescence. 

 The suspension is not complete, since deaths occur unless some 

 increase in weight is allowed. The most important inference to 

 be drawn from the work would appear to be that senescence 

 itself is the direct consequence not so much of growth-cessation 

 as that of the attainment of a developmental stage, the timing 

 of which is partially, but not wholly, linked to the growth-rate 



150 



