Physiological Approach in Study of Ageing 25 



well fed, while their metabolic rate, as indicated by the heart 

 beat frequency, is significantly lower than in normally fed 

 individuals. Kopec's (1924) experiments with caterpillars of 

 Lymantria dispar gave similar results. Among rotifers the 

 lifespan of the adults seems likewise to be inversely propor- 

 tional to metabolic rate. Encysted adults may survive as 

 long as 59 years in diapause (Rahm, 1923), while most active 

 individuals live only a few weeks. 



Besides these experiments, numerous field observations 

 appear to confirm both the influence of temperature and the 

 effect of food restriction and reduced activity on the duration 

 of life of invertebrates. To take but a few examples, a tropical 

 butterfly studied by Fountaine (1938) in Cambodia reached 

 its imaginal stage in only 7 days, while some Arctic Satyrids 

 need more than 2 years to reach the adult stage. Among 

 some molluscs living in the same area, growth rate, ultimate 

 size and duration of life seem to depend on the amount of 

 food available in the various environments. Such appears to 

 be the case, at least, for the limpets observed by Fischer- 

 Piette (1939) in Brittany. 



Thus, it seems quite certain that the rate of energy metabol- 

 ism has something to do with the duration of life in inverte- 

 brates, but one may wonder if such a correlation exists in 

 vertebrates, especially in those groups whose internal milieu 

 is kept at a constant temperature by efficient thermoregula- 

 ting mechanisms. McCay's experiments (McCay, Crowell and 

 Maynard, 1935) have shown that it is possible to lower the 

 energy-turnover of rodents by supplying them with the least 

 possible amount of food, and consequently to almost double 

 their lifespan — a fundamental discovery which has been con- 

 firmed many times. But many other facts seem to indicate the 

 major influence of the metabolic rate on the ageing processes 

 of vertebrates. 



Fishes offer remarkable opportunities for study from that 

 point of view. Large samples of populations living in definite 

 environments can be easily collected and an accurate deter- 

 mination of their age-composition can be made in most cases. 



