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Abstract
Mammals usually produce approximately equal numbers of
sons and daughters, but there are exceptions to this
general rule, as has been observed in ruminant ungulate
species, where the sex allocation hypothesis of Trivers
and Willard has provided a rational evolutionary
underpinning to adaptive changes in sex ratio. Here, we
review circumstances whereby ruminants and other mammalian
species, especially rodents and primates, appear able to
skew the sex ratio of their offspring. We also discuss
some of the factors, both nutritional and non-nutritional,
that potentially promote such skewing. Work from our
laboratory, performed on mice, suggests that age of the
mother and maternal diet, rather than the maternal body
condition per se, play directive roles in controlling sex
ratio. In particular, a diet high in saturated fats but
low in carbohydrate leads to the birth of significantly
more male than female offspring in mature laboratory mice,
whereas when calories are supplied mainly in the form of
carbohydrate rather than fat daughters predominate. As the
diets fed to the mice in these experiments were
nutritionally complete, and since litter sizes did not
differ between treatments, dietary inadequacy seems not to
be the cause for sex ratio distortion. A number of
mechanisms, all of which are testable, are discussed to
provide an explanation for the phenomenon. We conclude the
review by discussing potential implications of these
observations to human medicine and agriculture.
Key words:
Embryo •
Early development •
Fertilization •
Trophoblast •
Uterus
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