Introduction

Sex-biased dispersal

Dispersal is a common feature of bird and mammal societies. Philopatry, however, is generally assumed to be the most advantageous situation, because it assures knowledge of the home range and offers better opportunities for cooperation with relatives. Several benefits of foregoing these advantages have been proposed (better mating opportunities, including inbreeding avoidance; reduction of within-group competition; coercion) (Greenwood 1980; Pusey & Packer 1987a; Moore 1993). The balance between the costs and benefits of dispersal obviously differs between the sexes, because dispersal is often highly sex biased. Most bird species show a female bias in dispersal, whereas most mammal species show a male bias (Greenwood 1980; Dobson & Jones 1985; Liberg & Schantz 1985). The best explanation proposed so far for such a strong sex bias is inbreeding avoidance.

In mammals it is generally assumed that the costs of dispersal are higher for females than for males, because loss of a range or allies affects female reproductive success more than male reproductive success, explaining the prevalence of male-biased dispersal (Waser et al., 1986; Pusey & Packer 1987a; Clutton-Brock 1989a). Female dispersal, however, is found in some mammal species and a considerable number of primate species (Moore 1984; Clutton-Brock 1989a; Strier 1994).

Costs and benefits of dispersal

Different costs and benefits of female philopatry and dispersal can be distinguished. Females in gregarious animals such as diurnal primates can remain in their natal range or in their natal group.