Biological Approach in Study of Ageing 15 



progressively better environmental conditions, it may be 

 possible to use them as a source of information about the 

 nature of the processes which operate in domestication. This 

 is an important question in age studies, since in studying the 

 gross loss of vigour which occurs late in senescence we are, in 

 fact, studying a phenomenon which only rarely occurs in the 

 wild state, and when we study it in circumstances which allow 

 a high proportion of animals to reach old age we are, in fact, 

 producing by domestication an organism different from that 

 which arose in the first instance by natural selection. Life 

 tables obtained from zoo populations represent a special 

 biological situation, in which the animals receive sufficient 

 protection from predators to enable them to reach old age, 

 but are subject to higher mortalities from epidemics, parasites, 

 and behavioural causes such as fighting due to the inter- 

 ruption of territory behaviour. 



We can compare the performance of the sheep with that of 

 Merino ewes, and also with that of the wild sheep studied by 

 Murie (1944) who prepared a life table by ageing the skulls of 

 animals dying in the Mount McKinley National Park. This is 

 one of the few cases where senescence has been demonstrated 

 in a wild mammalian population. The comparison is interest- 

 ing for the much better performance of the wild sheep, even 

 when extensively preyed on by wolves, compared with that of 

 the zoo animals, but its main interest is in showing the re- 

 lationship in time between the mean, median and limit of the 

 two survival curves, since the percentages reaching extreme 

 ages were similar in all three cases. Such information, and the 

 comparison of longitudinal and cross-sectional studies under 

 different environmental conditions, could be used to study 

 directly the distribution of individual vitalities, or potential 

 lifespans, in a population. 



In other cases it is necessary to work with specially main- 

 tained populations. We have set out to obtain detailed vital 

 statistics for one poikilothermal vertebrate, the guppy 

 (Lebistes reticulatus), by observing the lifespans of several 

 thousand specially kept individuals. We have found that this 



