The Biology of Senescence 



In all the groups whose susceptibility to senescence is doubt- 

 ful, there is wide variation in life-cycle and growth-pattern, 

 which is very probably reflected in differences of their capacity 

 for age changes. Some aquarium species offish certainly 'age' 

 as judged by their declining reproductive powers: in the larger 

 sea fish this has not been clearly demonstrated. The contra- 

 dictory views offish senescence given to Flower (1935) by two 

 acknowledged authorities, one on aquarium and the other on 

 marine ichthyology, and based on small and large teleosts 

 respectively, are possibly both correct. In other forms there is 

 an obvious sex difference in growth-maintenance, in longevity, 

 or in both. The specific age might also be indefinite in animals 

 which nevertheless became more liable to die, as individuals, 

 with increasing age. It was implied by Ricker (1945) that fish 

 might senesce individually, i.e. undergo a waning of vitality and 

 resistance with age, but that there is no sharp specific age — the 

 life-span of each individual would be limited by senescence, but 

 the senile process would reach its critical point at a much more 

 variable age than in mammals: as if the menopause in human 

 beings were to occur with approximately equal probability in 

 any year after the menarche. Such senescence would be real, 

 but could not readily be detected actuarially. 



Very nearly all these problems require abundant new data 

 to settle them finally. The general evidence of the distribution 

 of vertebrate senescence which will be given here is both frag- 

 mentary and equivocal. It does, however, contain some facts 

 which suggest that Bidder's hypothesis is too simple and that 

 the manner of growth-cessation, rather than the fact of it, is the 

 main determinant of the mammalian pattern of senescence. 



2-4-1 FISH 



The 'indeterminate' growth of fish, on which Bidder based 

 his hypothesis, has often been discussed (Hecht, 1916; Keys, 

 1928; Huxley, 1932; Vaznetzov, 1934; Thompson, 1942; Wel- 

 lensieck, 1953). Many large species of teleosts can continue to 

 grow throughout life, and the rate of decline of their growth- 

 rate is considerably slower than e.g. in most reptiles. The locus 

 classicus of continued growth without evidence of senescence, 

 actuarial or reproductive, is the female plaice. Here the evi- 



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