The developmental asynchrony hypothesis for sex ratio manipulation.

This paper presents the hypothesis that developmental asynchrony between embryos of both sexes, on the one hand, and offspring development and responsiveness of maternal reproductive organs, on the other, may result in significant sex ratio biases at birth in mammals, and in birds. It is argued that the developmental asynchrony hypothesis may account for a significant proportion of findings on sex ratio skews, including the inconsistency of evidence and the lack of successful selection attempts. This mechanism could be used by the maternal organism adaptively to alter the sex ratio by control of the relative time of insemination within the oestrous cycle or by alteration of reproductive hormone levels around the time of ovulation and early embryonic development. However, the developmental asynchrony hypothesis may also imply that many of the sex ratio biases can be viewed as by-products of constraining selection on control of fertility and fecundity and on sexual dimorphic growth rates.