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Ants have something like a single fertile female to several males. The same applies to bees, and larger animals - elephants too. Is the sex ratio universally skewed in favour of the female? What animal species exhibit this ?

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  • $\begingroup$ You mean for animals, right? You get all sorts of crazy sex ratios (and crazy sexes!) going on in plants. $\endgroup$ – Oreotrephes Jul 20 '13 at 12:11
  • $\begingroup$ So just to be clear, you're looking for animals who have sex ratios of male:female <1:>1 in the population, right? Sex ratios within broods, as opposed to within populations can behave strangely. $\endgroup$ – Oreotrephes Jul 20 '13 at 12:22
  • $\begingroup$ @Oreotrephes: Yep. Thanks. Just updated the title $\endgroup$ – Everyone Jul 20 '13 at 12:59
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It is advantageous to have a skew towards more males. Males can be expendable as they are only required briefly for impregnation and can then focus on providing resources for the mother and young and protection. This breaks down when this is no longer the case, for example in humans we are closer to a 50/50 divide.

This article notes that in aphids, there are twice as many females as males, the same is true for daphnia. I don't think the reason behind this is known. In reptiles, the female to male ratio can depend on temperature of incubation thus this can lead to strong fluctuations.

An interesting principle is Fisher's principle:

Fisher’s principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher’s argument in his 1967 paper on “Extraordinary sex ratios” as follows, given the assumption of equal parental expenditure on offspring of both sexes. Suppose male births are less common than female. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. Therefore the genes for male-producing tendencies spread, and male births become more common. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio. Source

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    $\begingroup$ At least in some species, aphids have nested parthenogenic vivipary, that is, a female may be pregnant with a female child who is pregnant. Of course, this can result in rapid population growth, and if selected for would explain a higher female ratio. $\endgroup$ – Oreotrephes Jul 20 '13 at 16:55
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In rock hyraxes, similar to elephants, the sex ratio of male to females are between 2:3 and 1:3. Since subadult males are pushed from the population by the territorial male just before they become sexually active. When looking at adult hyraxes, however, the females can outnumber the males by quite some margin.

Recent studies had shown an adult sex ratio of ♂1:♀17 & that the male to female ratio, in populations situated in residential areas, does not show such a big difference.

https://www.ncbi.nlm.nih.gov/pubmed/24899114

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