Blackwell Publishing

Adaptations in sexual reproduction - Why is a 50:50 sex ratio so prevalent in nature?


Local mate competition

Fisher's theory assumes that there is 'population-wide' competition for mates. Hamilton (1967) first pointed out that the sex ratio will become biased in species in which an individual competes against only a limited part of the rest of its population. The condition is called local mate competition. The pyemotid mites are an extreme case. In this group, brothers inseminate their sisters while they are still inside their mother. A mother therefore only needs to produce one or two males in a brood to inseminate all her daughters; the sex ratio is female biased. In Pyemotes ventricosus , for instance, the sex ratio of a brood contains on average four males and 86 females. The males are not competing against the whole population for mates; they compete only against their brothers, and a mother can reduce the fruitless competition by producing fewer sons.

Dominant females produce more males

In polygynous species, a small number of males do a disproportionate amount of the breeding. Many males die before reaching the breeding stage and even then some males fail to breed. We saw above that, in Fisher's theory, a female cannot gain by producing more males, even if only a minority of them survive to breed. By producing more males, the average female would be as likely to produce more of the failures as successes.

However, there is a circumstance in which it can be advantageous to bias the sex ratio. It is when a female 'knows' that she is not an average female, and that her offspring will not be average sons and daughters. Consider the case of the red deer Cervus megalocerus (a herd of which are shown opposite), which has been studied in detail by Clutton-Brock et al . on the island of Rhum, off Scotland. The breeding system is polygynous, as females (called hinds) live in groups, which are defended by a single male. Within a group of hinds, there is a dominance hierarchy. The more dominant hinds are better fed and stronger, and they can better provide for their offspring. Their offspring in turn grow up to be stronger than average. In these circumstances, as Trivers and Willard pointed out, there can be selection on higher ranking females to produce more sons, and on lower ranking females to produce more daughters. A high ranking female can expect to produce successful sons, and a low ranking female to produce sons that fail to breed. Females can only make use of this fact if they in some sense know.

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