Longevity of English Thoroughbred Horses 37 



Each life history normally ends in an entry that the animal 

 was shot, died, was exported, or was disposed of (put out of 

 stud, sold, given away as barren). Some end in unexplained 

 disappearance from the record or in "no further return". 

 Each unaccounted absentee was sought by name in each 

 volume to the end of a 32-year period from its year of foaling, 

 and for two volumes thereafter to allow for corrections. 



The forms used in the terminal entries are "died", "shot", 

 "sold", "sent abroad", and qualifications of these, e.g. "broke 

 leg and shot", "died after foaling" (the last of these implies 

 only that the mare had foaled, not that foaling was the cause 

 of death). The entries therefore discriminate between losses 

 to the record, deaths due to accident, animals destroyed, and 

 deaths presumed to be due to natural causes ; but of animals 

 entered as "died" only those dying at or after foaling, and of 

 animals shot only those which met with an accident, are 

 usually distinguished. Deaths not attributed to accident or 

 shooting account for 1,009 out of the total 2,742 lives in the 

 cohort samples. Only these, which include deaths at or after 

 foaling, were scored as "natural" (d^) in computing the age- 

 dependent mortality, all other losses, and all animals dying at 

 imprecise ages, being scored as lost to the record (a^). Where 

 the last surviving individual is lost to the record, it is assumed 

 to have died in that year. 



Life-tables from 4 years of age were then prepared by cal- 

 culating the yearly mortality under this convention, with the 

 usual correction for losses. Details of the calculation have 

 been given elsewhere (Comfort, 1958a). Standard errors of the 

 expectation of life were obtained by Irwin's (1949) approxima- 

 tion. 



Results and Discussion 



General form of the survival curve 



Survival curves drawn for the 2,742 thoroughbred mares in 

 samples A and B, and the tabulated figures from which they 



