Dependence of the Sex Ratio at Birth on the Age of the Father

Abstract

SZILARD'S conjecture about the effect of paternal age on the sex ratio, which his figures for man support¹, might perhaps be capable of further confirmation in horses. Unlike the ages of man and wife, those of mare and stallion are uncorrelated, and both are commonly bred into old age. I investigated the effects of paternal age on foal longevity², which is also germane to Szilard's theory, and found no appreciable decline in the vigour of the fillies of old stallions, judged by life-spans in the “General Stud Book”—no records of colt longevity are available. My material gives some figures for the sex ratio, though the numbers are too small to do more than suggest the absence of any gross age effect. The ratio of males to females was exactly 100.00 for the 446 live progeny born during 1885–96 of a group of stallions known to be more than twenty years old at covering, against 100.47 for all foals in the same period. Masculinity in thoroughbred horses is less than in man, and the ratio has been reasonably stable for a century or more (1853–68 R = 100.32; 1869–80 R = 99.18; 1885–96 R = 100.47; 1925–40 R = 99.31).