50 A. Comfort 



between the longevity of greys in the early and late years of 

 the sample. Large factors of selection may well operate — at 

 many periods grey horses appear to have been selectively 

 exported and they show a high proportion of losses to the 

 record. Most of the apparent reduction in their expectation of 

 life is due to early deaths, the expectations at 10 and 15 years 

 being 10-96 and 3-85 years in greys, as against 11-17 and 

 2 • 93 years in the whole of sample A. Causes of death are not 

 given, and there is consequently no information about the 

 incidence of melanomas, to which grey horses are sometimes 

 subject (McFadyean, 1933). 



Longevity of stallions 



The Stud Book does not contain records of stallions com- 

 parable to those of mares. Three different estimates of 

 thoroughbred stallion longevity have been obtained, with the 

 help of other records — two are based (unlike the mare studies) 

 on cross-sectional samples, and the third is a longitudinal 

 study of the earliest age group for which a list of names could 

 be had. All three leave a good deal to be desired, but they 

 give some provisional indications of the rate at which the 

 expectation of life declines with age in entire males. 



Cross-sectional samples were taken of (a) all the animals 

 listed in volumes 1 and 2 (1910 and 1913) and (b) all the 

 animals listed in volume 5 (1921) of the Register of Thorough- 

 bred Stallions (excluding the appendix). Thirty-three animals 

 from sample (a) were still alive in 1921 and figure in both 

 samples. The cohort sample was obtained by taking, from the 

 lists of sires of brood mares in volumes 17 and 18 of the 

 General Stud Book, all the stalhons (180) foaled in 1880-84 

 inclusive. Life-tables were prepared by calculating age speci- 

 fic death rates from death and disposal records in the obituary 

 and export lists of the Register, the General Stud Book, and 

 the lists of premium stallions of the National Hunters and 

 Light Horse Society. 



