Biological Approach in Study of Ageing 9 



Actuarial senescence almost certainly occurs in all birds and 

 mammals. It is by no means certain that it occurs in all 

 poikilotherms, beyond the accumulation of evident injury 

 with age. The ageing of birds and mammals has been termed 

 "endogenous", and it has been suggested that this type of 

 ageing is in some way connected either with the evolution of 

 homoiothermy or of a fixed adult size. Bidder (1925, 1932) 

 attributed it to the direct action of a growth-inhibitory 

 mechanism evolved during the transition to life on land. 

 From maximum age records it is evident that the lifespan of 

 smaller vertebrates declines fairly steadily from phylum to 

 phylum in ascending phylogenetic order. It is longest in fish, 

 amphibia and reptiles. We have records of 35 years in Triton 

 (Smith, 1951) and 40 years in goldfish (Harvey and Hems, 

 1948). The lifespan of birds is in many cases substantially 

 greater than that of mammals of comparable size and activity. 

 The lifespan of the laboratory mouse is normally less than 

 three years, while that of a chaffinch in captivity may exceed 

 20 (29 — Moltoni, 1947). This suggests that in phylogeny other 

 causes than the increase in metabolic rates have operated to 

 shorten the maximum lifespan — the metabolism of small 

 birds measured by their oxygen consumption is higher than 

 that of rodents, and apparently it does not decline with age 

 like that of many mammals (Benedict and Talbot, 1921). The 

 growth of birds, as Bacon (1645) pointed out, ceases relatively 

 earlier than that of mammals, and much more definitively; 

 the epiphyses of rats never join, and they may continue in 

 growth, or be made to grow in response to somatotrophin, at 

 any age, while no further growth occurs in birds after the 

 attainment of adult size. 



There are also important discrepancies between the 

 maximum ages reported in closely related mammals. The 

 most extensive figures are for rodents: thus Mus bactrianus 

 has been reported to live and remain fertile longer than any 

 strain of Mus musculus (Green, 1932), while species of Pero- 

 myscus and Perognathus live almost twice as long [Peromyscus 

 maniculatus gambelli 5 years 8 months (Sumner, 1922); 



