The Biology of Senescence 



Hopkins (1930) found that in Venus mercenaria growth was con- 

 tinuing actively at 20 years. The oldest specimens aged by 

 growth rings were in general not the largest shells. Some small 

 examples had reached an estimated age of 40 years, and ap- 

 peared to have grown abnormally slowly. This observation, like 

 Fischer- Piette's (1939) and Weymouth's (1931) findings, should 

 lead to a great deal of caution in regarding the life-span of 

 any mollusc as indeterminate in the same sense as that of 

 actinians. 



2 6 Senescence in Wild Populations 



Senescence as a potential part of the individual life-cycle is, 

 as we have seen, widespread: in discussing the evolution of 

 senile processes, however, it is important to know how far it 

 really occurs in wild animals. The weight of evidence suggests 

 that senescence in the wild is rare but not unknown. Its com- 

 monest form is undoubtedly the pseudo-senescence which follows 

 reproduction, but genuine senescence analogous to that of man 

 is occasionally reached, at least by individuals, while there are 

 probably some forms in which it is normally reached. If our 

 observation of animal life-cycles were confined to small birds 

 and mammals in the wild, however, we should probably not 

 recognize senescence as an entity except in ourselves. 



2-6-1 VERTEBRATES 



Although data from bird and small mammal populations 

 have perhaps led to an overstatement of the case against 

 'natural' senescence, old age is undoubtedly a relatively rare or 

 very rare termination to the life-cycle of vertebrates studied in 

 the field — as it is for man in societies where medical and 

 economic conditions are bad. For large numbers of animal 

 species, the typical curve is one in which a high or very high 

 infant mortality rate is succeeded by a high adult mortality 

 rate which does not increase with age. These species, even when 

 they are capable of senescence, never reach it. This type of 

 curve has been repeatedly demonstrated in population studies, 

 (see Lack, 1954). Whereas in voles kept in the laboratory the 

 survival curve approximates to that of man (Leslie and Ranson, 



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