Males and females often differ in their liability to disperse. Differences are especially strong in some insects, where it is the male that is usually the more active disperser. For example, in the winter moth (Operophtera brumata), the female is wingless whilst the male is free-flying. In a seminal paper, Greenwood (1980) contrasted the sex-biased dispersal of birds and mammals. Amongst birds it is usually the females that are the main dispersers, but amongst mammals it is usually the males. Evolutionary explanations for a sex bias have emphasized on the one hand the advantages of a sex bias in its own right as a means of minimizing inbreeding, but also that details of the mating system may generate asymmetries in the costs and benefits of dispersal and philopatry in the two sexes (Lambin et al., 2001). Thus, in birds, competition for territories is typically most intense amongst males. They, therefore, have most to gain from philopatry in terms of being familiar with their natal habitat, whereas the dispersing (and often monogamous) females may gain from exercising a choice of mate amongst the males. In mammals, the (often polygamous) males may compete more often for mates than for territories, and they therefore have most to gain by dispersing to areas with the largest number of defensible females.
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