The Biology of Senescence 



decline can be reduced by mild hypothyroidism (Turner and 

 Kempster, 1948) The pair of crowned pigeons which lived, 

 according to Fitzinger (1853) for over 40 years, mated and laid 

 throughout life, but hatched no offspring after the age of 18 or 

 20 years (Flower, 1938). Spermatogenesis likewise appears to 

 decline (Payne, 1952). The life-span of birds is longer in pro- 

 portion to size and metabolic rate than that of mammals, and 

 the scatter of age in senescence as shown by aviary records 

 appears, superficially at least, to be rather greater within a 

 species. 



We are familiar with the ageing of warm-blooded animals 

 because we keep them. It is among the 'cold-blooded' verte- 

 brates that the real uncertainty begins. We keep fish, but only 

 the smaller forms — we do not, apart from zoological gardens 

 and occasional pet tortoises, keep reptiles; as for amphibia, 

 Hilaire Belloc wrote uncontrovertibly concerning lonely people 

 who keep frogs that 



by the way 

 They are extremely rare. 



The general assumption that all vertebrates must necessarily 

 undergo a senescence at least superficially similar to that of 

 mammals has prejudiced even those biologists who have kept 

 frogs for long periods, with the result that very little real in- 

 formation unbiassed by this assumption has been published. 

 Very possibly the assumption may prove to be correct, but it 

 cannot be lightly made. It is evident that some senile change, 

 in the form of an accumulation of injuries, must occur in all 

 vertebrates with the passage of time, and be reflected in the 

 force of mortality. But this effect is certainly small and incon- 

 stant compared with the 'morphogenetic' senescence which 

 determines the life-span of mammals. It is this morphogenetic 

 component which we are concerned to detect and estimate in 

 lower vertebrates. Unfortunately for such a study, the life of 

 many of these creatures, whether it ends in senescence or not, 

 is, as we have seen, long enough to make ordinary short-term 

 laboratory observation useless. 



Bidder's opinions on the relation between perpetual youth 

 and continuing growth have already been quoted (p. 12). 



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