The Sex Ratio in Mules and Other Hybrid Mammals

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Abstract
The data are collected and discussed concerning the sex ratios among various hybrids of the Equidae and the Bovidae. Miscellaneous hybrids are also mentioned, including those from different families of cavies, mice, goats, sheep, pigs, rabbits, swine, monkeys, etc. The fetal sex ratios, sterility, criteria of hybridity and fertility of the mule are also discussed. In general, [female] hybrids exceed [male][male] in number; in mammals [male][male] appear to be conceived more often than [female][female]. It is suggested that the fetal death toll generally falls heavier on the [male] and that this is due to the action of at least 3 factors: (1) sex-linked lethal genes in some instances; (2) since the Y-chromosome apparently does not in general carry genes, the [male] may receive fewer dominant genes than the [female] and therefore may be actually weaker in some cases; (3) evidently maleness imposes a higher rate of metabolism than femaleness and this may become a deleterious force.