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The Sex Ratio in the Mule¹

Oklahoma A. & M. College

Abstract

In the early discussions pertaining to the sexes of hybrids the observers recorded the opinion that males predominate. The writers, however, frequently neglected to distinguish between mammals and birds. This together with the fact that males do occur more numerously than females among hybrid birds, may account in part for the view that males are in excess in hybrid mammals as well.

The sex records for the mule here reported were collected through the cooperation of jack owners, county agents, vocational agricultural teachers and animal husbandry men. The records include the foaling seasons for 1929 to 1932. Foaling records for 1416 mules (627 males and 789 females) sired by 98 different jacks were secured. The sex ratio is, therefore, 44.28±0.89 per cent males, or in round numbers 80 males to 100 females. This is 5.41 per cent lower than the sex ratio commonly reported for the horse, which is 49.69 per cent males. The difference is 6 times greater than its probable error and is considered to be distinctly significant.

¹ Paper No. 149 from the Department of Genetics, Wisconsin Agricultural Experiment Station. Published with director's approval.