Longevity of English Thoroughbred Horses 53 



Conclusions 



The main use of the study has been in providing a survival 

 curve for a large mammal, sufficiently detailed to be used in 

 criticizing hypotheses about lifespan determinants, for which 

 maximum age-records are unsuitable. More such curves are 

 badly needed. By the criteria needed in dealing with a matter 

 as theoretically important as the supposed paternal age effect 

 the study was too small and the chance of systematic biases, 

 especially in losses, too large for convincing subdivision by 

 parental ages, though the data were better than those on 

 which such effects have sometimes been claimed. The 

 literature of mammalian parental effects on longevity is con- 

 tradictory, and has been reviewed elsewhere (Miner, 1954; 

 Comfort, 1956). Yerushalmy (1939) found an increased still- 

 birth rate in babies with very old or very young fathers, but 

 the difficulty arising from correlation between ages of spouses 

 (Sonneborn, 1957) affects the value of such data. Our study 

 was confined to parental age effects on longevity; paternal 

 age has been held to affect other characters of stock perform- 

 ance, particularly by Russian breeders (e.g. Zamyatin et at., 

 1946; Isupov, 1949; Ponomareva and Spitskaya, 1953; 

 Pospelov, 1952; Eidrigevits and Polyakov, 1953; Barton, 1951 ; 

 Frankland, 1955). Man and the horse, since they continue to 

 breed into old age, are clearly the mammals in which such an 

 effect on longevity is most likely to be demonstrable; our 

 figures do not bear out the suggestion that old stallions have 

 short-lived offspring, but it might still be desirable to examine 

 the stillbirth rate in the mares which they covered. This, 

 unfortunately, could not be done from stud-book records. 



The small but positive correlation of filial with parental 

 longevity is in accord with Beeton and Pearson's (1901) work, 

 and with Haldane's (1949) interpretation of it — if heterozygos- 

 ity is an important correlate of vigour, the sib-sib correlation 

 of lifespan should be larger than that between generations. 

 Perhaps the most striking feature of the study is the smallness 



