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DISCUSSION 



Parkes: Dr. Comfort's paper brings up the old problem of why in some 

 species one sex lives longer than the other. You showed that the female 

 wolfhound lives longer than the male, so to that extent the wolfhound 

 is similar to man. Are there any other species known to you where that 

 happens? 



Comfort: It seems to be very nearly general, not only in such mammals 

 as have been investigated, but in a great number of other animals as well. 

 The suggestion was made at one time that the longevity of the female 

 depended on her being the homogametic sex, and that it ought to be the 

 other way round in birds and butterflies, but as far as I know the results 

 so far do not bear that out: in some butterflies the female is longer-lived, 

 in others the male. In birds, at any rate in poultry, greater vigour of the 

 female seems to be the rule. I don't know whether that is true, but it 

 has been claimed. 



Parkes: So that is a fairly general phenomenon for which there is no 

 general explanation? 



Comfort: Darwin gave the best "explanation" for it, when he said that 

 it was a natural peculiarity due to sex alone! 



Parkes: That is a description, not an explanation! 



Verzdr: The "biological age" in individuals as they get older in time 

 is extraordinarily different. I think that your great variations, Dr. 

 Comfort, may be explained by the fact that there are some individuals 

 who are biologically still young, while others are already very old. The 

 same appears from your survival curves ; at least in many of your curves 

 there seems to be some similarity to the Z-shaped survival curve in 

 humans. There is, of course, an initial quick decrease in the first years 

 of life, but if you cut that out, the curve is first more or less flat, and then 

 it starts sinking in a Z-shape. This we see in man, in rats and, as you 

 have shown us, in many other species. Can you explain why in rats, 

 inbred for 25 years, we get this life survival curve ? 



Comfort: I cannot offer an explanation of it; I am still trying to 

 arrive at a description of it. So far as the Zoo populations were concerned, 

 we were not getting so much a Z-shaped curve as an arithmetical straight 

 line, but it is clear from the behaviour of the sheep that this is due to the 

 life not being so good in Zoos. If we were to keep them under really good 

 conditions and if we were to choose strains that are hardy, we should 

 expect to get a Z-shaped curve with a plateau of varying length. We 

 get the effect in guppies where there may be virtually no deaths before 



