Physiological Approach in Study of Ageing 27 



climate permits this lizard's population to be actually active 

 every month of the year and individuals grow throughout 

 their short lifespan. On the other hand, the northern race of 

 the same species (Sceloporus undulatus hyacinthinus) usually 

 lives for more than 4 years in Maryland, probably reaching a 

 maximum age of 8 years. In this colder climate the fence 

 lizard is active only seven months of the year and its growth 

 appears to stop after about the fifth or sixth year. In this 

 case growth thus appears to be determinate in one part of the 

 animal's range and indeterminate in another. 



All these observations concern poikilothermal vertebrates, 

 and one may wonder if something comparable may occur in 

 homeothermal animals. Recent ecological studies suggest 

 that such may be the case. The efficiency of thermoregulating 

 mechanisms varies greatly in the various groups of birds and 

 mammals. Some species regularly enter periods of hibernation 

 and of estivation during which their internal temperature may 

 fall to a point close to that of the environment. In such a 

 state the metabolic rate is greatly reduced, often during many 

 months a year. Others, like bats, normally have a very low 

 metabolic rate, almost at the hibernating level, during about 

 20 hours a day, their oxygen consumption increasing only 

 during flight. Such a periodic reduction of the energy metabol- 

 ism, so well studied by Pearson (1947), is not at all restricted 

 to the growth period, but continues throughout life. It seems 

 to me highly significant that these mammals which have a 

 poor temperature regulation and a normally low rate of 

 metabolism are precisely those whose lifespan is far longer than 

 that of other mammals of similar size. In 1947, I reported the 

 case of a wild Rhinolophus hipposideros banded as adult in 

 October 1938 and still living in fine condition in May 1946. 

 More recently, Dorst (1954) reported the cases of marked 

 Rhinolophus ferrum-equinum and Miniopterus schreibersi 

 having reached 15 and 14 years respectively in wild conditions, 

 potential longevities of 8 to 12 years being very frequent in 

 these bats. Cockrum (1956) has even more recently published 

 additional observations for North American species which 



