Discussion 137 



Danielli: That would definitely mean that it was not just a 

 respiration effect. 



Comfort: With many of these animals you may increase their 

 metabolism and at the same time decrease their appetite, or some- 

 thing like that. One has to be careful. 



Sacher: If it did not have any effect on body temperature it would 

 not greatly increase the overall metabolism would it ? 



Comfort : I do not know whether there was that effect or not — the 

 paper may say. 



Danielli: Dinitrophenol would normally increase the metabolism 

 a lot, unless there were some compensatory mechanisms, and one 

 would expect appetite to increase rather than diminish. 



Bourliere: Long ago T. B. Robertson (1928. Aust. J. exp. Biol, 

 med. Sci., 5, 69) found that continuous treatment of the mouse with 

 desiccated thyroid, in quantities that stimulated growth, also 

 shortened the lifespan. 



Berg: We are studying the effect of thyroidectomy on the lifespan 

 of the rat. 



Maynard Smith : Another possible way of studying the effects of 

 metabolic rate on longevity is to use different genetic strains of the 

 same species. The most striking differences in longevity you can get 

 in flies are between inbred and outbred ; outbred flies will live about 

 twice as long as inbred ones. This certainly is not associated with a 

 lower rate of metabolism in the outbred flies. They are not animals 

 in which it is easy to measure the basal metabolic rate, but if one 

 judges it by rate of eg^g production, for example, the hybrids are 

 laying eggs at about twice the rate of the inbreds, as well as living 

 twice as long. Also the hybrids are much more active. Their greater 

 longevity is much more easily explained in terms of the other con- 

 cept that Sacher used, namely that the hybrids in many respects, 

 both physiologically and developmentally, seem to have far better 

 stabilizing mechanisms than do inbreds, and that what is wrong with 

 inbred animals is that they are just not good at regulating against 

 anything. 



Sacher: Yes, that is a view which is put forward systematically in 

 the concept of genetic homeostasis. 



Maynard Smith: I think it is a true one. 



Sacher: In general terms I think it is true also. We have the same 

 phenomenon in mice but not to the same degree. 



Verzdr: Thyroxine treatment of the tadpole and axolotl, which — 

 as you know — leads to transformation from larval to adult forms, 

 always leads to a shortening of life. The transformed animals never 

 survive long. 



