We will look now at a behaviour that can be maintained for a variety of reasons, most of which are related to reproduction, and that is sex-biased dispersal. This occurs in many species when one sex is more likely than the other to disperse between populations. In mammals, females are usually philopatric, meaning that they tend to remain at, or return to, their natal site for breeding, whereas males often disperse from their birthplace and never return. The opposite is true in birds, with males more likely to be philopatric and females more likely to disperse, although exceptions to the rule can be found in both groups.
One possible explanation for sex-biased dispersal is described by the resource-competition hypothesis (Greenwood, 1980), which predicts that the sex remaining at its natal site will be the one that benefits most from home-ground familiarity. In birds this will often be males, who can benefit from site familiarity when acquiring and defending territories. In mammals, females may benefit most from knowledge of a particular area because they may be able to produce more young if they are familiar with local food resources. A second possible explanation is summarized by the local mate competition hypothesis (Perrin and Mazalov, 1999), which proposes that individuals disperse so that they will not have to compete with their relatives for mates, thereby increasing their inclusive fitness. Alternatively, sex-biased dispersal may be explained by the inbreeding avoidance hypothesis (Pusey, 1987), which is based on the idea that the sex that incurs the greatest cost from inbreeding is more likely to disperse.
Deciding which hypothesis provides the most appropriate explanation for sex-biased dispersal in a particular species can be difficult, in part because relevant hypotheses may not be mutually exclusive. In addition, reasons for dispersal may change over time, depending on a variety of factors such as environmental conditions or the density of a local population. The biggest contribution of molecular ecology to this area of research has been through the quantification of sex-biased dispersal, and we shall look now at four ways in which molecular data can be used to contrast the dispersal patterns of males and females.
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