Sex differences in length of gestation in mammals.

Abstract

Observations on length of gestation of males and females are presented for man, the guinea-pig, sheep, pig and rabbit; from the literature, data are also available for the cow, horse, camel and goat. Pregnancy is longer for males than for females in the cow, horse and possibly the sheep and camel; it is longer for females than for males in man and possibly the guinea-pig. No definite conclusion is reached about the pig and rabbit, in which litter-size is normally large, or about the goat, in which the number of observations on record is small. The sex differences in length of gestation are reflected in the sex ratios of offspring born after different periods of gestation, and in the percentage distribution of the sexes by duration of gestation. The difference in length of gestation can be attributed to a sex difference in foetal weight in man, but not in the guinea-pig or in species such as the cow, in which gestation is longer for males than for females. It is suggested that in the cow the longer male gestation may be due to the fact that the proportion of males conceived is higher if mating takes place early, rather than late, in heat. (The only alternative explanation is a sex difference, attributable to something other than weight of foetus, in the period between fertilization and birth.) In man the earlier birth of males is apparently due to their greater weight, attributable wholly or in part to a sex difference in rate of prenatal growth. This observation does not exclude the possibility that in man, as in the cow, the proportion of males conceived is higher if mating occurs early in the cycle, since the method of recording length of gestation (from onset of menstruation) gives no information about time of mating.