GROUP DISCUSSION 



Comfort: I would like to emphasize the variety of the material to 

 which we have been obliged to apply the term lifespan. I am 

 inclined to say that if a single parameter must be used to designate 

 a curve — it is better it should not be, but if it must be — for most of 

 the purposes we have been talking about I would favour the modal 

 age of adult death which Dr. Benjamin showed us. 



Unfortunately there are many curves for which you cannot use the 

 modal age of death. In zoo animals there is effectively no mode, 

 since the survival curve is almost an arithmetic straight line (Com- 

 fort, A. (1957). Proc. zool. Soc. Lond., 128, 349; Ciba Found. Coll. 

 Ageing, 3, 14.) 



Another possible parameter that has been mentioned is the median. 

 It has the advantage for experimental purposes that you need not 

 wait till the animals studied are all dead — you can rush into print 

 when half of them are dead. But I think its standard error is a little 

 difficult to handle. It also has the drawback that it is very sensitive 

 to the effects of environment on the survival curve. The last decile 

 is far more stable in this respect. 



You could also use the limit. The limit has the advantage that 

 even in small populations of animals one or two commonly survive 

 much longer than their fellows — their performance is a better index 

 of "physiological" performance than the crude mean or median. Its 

 drawback is the existence of a large number of doubtful records of 

 very old age in man and animals. 



In Bourliere's curves for birds, and also many of Beverton's 

 curves for fish, standing mortality at low ages is so high that it is 

 effectively independent of age; the most obvious parameter is the 

 half-life, but since these populations contain some long-lived 

 individuals the limit is also possible. But they cannot be made to 

 indicate what happens to the mortality at ages which are so rarely 

 reached. 



To compare lifespans we might fit a set of curves and compare 

 their time scales. Without using any equations I superimposed 

 those for the K/B Drosophila, for a human population (1941 United 

 States males), for Murie's wild sheep, and for my thoroughbred 

 mares, on different time scales, by fitting the last three quartiles of 

 each unsmoothed curve. That is another way of defining and com- 

 paring lifespans ; but if you do that you must allow for the fact that 

 man has a uniquely long developmental period, whereas sheep have 



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