56 Discussion 



Comfort: I do not think one can get much along those Hnes out of 

 my data. The differences for the two sexes scored separately was 

 very small. The subjects were all fillies, but you have to score both 

 parents to get a significant difference. 



Hartwig: Have you also studied the influence of parental age on 

 the fertility of the offspring ? 



Comfort: No. I could probably do so now, by going through the 

 data again. But it would mean following each life from start to finjsh 

 and counting the number of foals. In many instances there is some 

 doubt whether the animal missed or whether it miscarried. The 

 Stud Book usually distinguishes cases where the foal was born dead, 

 or where there was a miscarriage, from those where the mare failed 

 to conceive ; but one would have to be sure of differentiating between 

 unsuccessful pregnancies, and pregnancies which did not take place 

 at all. The other trouble is that as the animals are not crossed 

 twice with the same stallion in succession, one would have to allow for 

 the fertility of the stallion, which varies a great deal. In these 

 thoroughbreds there is a surprisingly high rate of infertility. 



Kershaw: You showed a death curve [not printed] which starts 

 with a slow rise on the left. The figures which we had on industrial 

 horses, that is draught horses and police horses, show that while the 

 general survival curve is the same, the death curve may have its 

 slow fall on the right (Chalmers, T. A., Kershaw, W. E., and King, 

 J. O. L. (1956). Nature {Lond.),178y4<8). I assume that the arbitrary 

 end-point in the racehorses is in part economic; in those figures of 

 ours for the draught horses and the police horses it was certainly 

 economic. This suggests that the curve of death is arbitrarily 

 determined by the index that one uses. We had assumed that the 

 working life had some relation to natural longevity. It does seem 

 now, in animals for which one can get the same data for different 

 indices, that the different indices may produce curves which are 

 made up differently. 



Comfort: What was the maximum age for the police horses ? 



Kershaw: Fifteen to twenty years. 



Comfort: These thoroughbred mares live a good deal longer. My 

 figures refer to breeding practice during the last century. The maxi- 

 mum age is lower today, particularly in mares from commercial 

 studs, which are put out at 23-24 years, but rather less so in stallions. 

 The number of stallions which "fall dead" and the number of them 

 recorded as having died of old age is large, even now. They are kept, 

 if they were famous animals, very much as pets of the establishment. 

 One of the common, and I suppose the most enviable, terminal 

 entries, is "fell dead after serving a mare". 



