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Mathieu Douhard, Eric Baubet, and Marlène Gamelon: Read the article
Mammalian adult females generally live longer than males in the wild, but it is poorly known whether sex differences in prenatal mortality occur, and if so, in what direction. Douhard et al. found a higher embryonic mortality for females than for males in a wild boar population.
The battle of the sexes is occurring within the female body. Previously it was thought that sex ratio at conception and prenatal mortality (embryos missing the implantation window) are biased toward males. Therefore, male embryos were thought to be in excess at conception and experience higher prenatal mortality. A new study by Mathieu Douhard, Eric Baubet, and Marlène Gamelon on French wild boar (Sus scrofa) challenges these classical predictions in wild mammals.
In mammals, prenatal sex ratio prior to implantation is difficult to study. Embryos experiencing prenatal mortality are often reabsorbed by the mother’s body, making it impossible to know if and how many eggs were ovulated. However, female wild boars do not reabsorb the corpus luteus, the outer shell that remains in the mother’s ovary after releasing a mature egg. The authors determined sex ratio of prenatal mortality by dissecting wild boar ovaries, counting the corpora lutea, and comparing this number to the sex ratio of fetuses. Sex ratio at conception was determined using litters with no embryonic mortality. These techniques yielded shocking results.
This study found that sex ratio at conception was not only balanced, but that female rather than male embryos experienced higher prenatal mortality, contradicting classical predictions. The female bias in prenatal mortality was especially skewed in larger litters and was independent of food availability or the mother’s body mass. The authors hypothesize that female embryos develop slower than males after conception, therefore missing the narrow implantation window. This study opens the doors to questioning long-standing predictions of prenatal sex ratio and embryonic mortality and the implications it may have on prenatal competition.
Julia Dovi is a master’s student in the Department of Ecology and Evolution at Stony Brook University. She is passionate about research on animal responses to stress associated with climate change and the genetic mechanisms underlying changes in behavior. She enjoys traveling, playing the piano, and hitting the dance floor.
