Group leaving in mountain goats: are young males ousted by adult females?

Emigration of young males from the natal social group is common in wild mammals. Dispersal has some potential costs, such as increased predation risks while travelling in unfamiliar terrain, but is thought to provide several benefits to dispersing individuals, such as inbreeding avoidance (Dobson and Jones, 1985; Wolff, 1993). In goat-antelopes (Rupicaprini tribe, subfamily Caprinae, family Bovidae), adult males are either solitary or in small bachelor groups for most of the year; young males usually remain within the natal nursery group (made up of adult females, young of the year and subadults of both sexes) until they reach sexual maturity at 2–4 years of age (Chadwick, 1977; Hamr, 1984). Sexual segregation of adults is common in ungulates including rupicaprins (Lovari and Cosentino, 1986; Clutton-Brock et al., 1987). For Mountain goats Oreamnos americanus, young males may leave nursery groups spontaneously or may be expelled by the aggressive behaviour of adult females (Holmes, 1988). Conversely, young females stay in their natal group (Chadwick, 1977). Adult females tend to be the most aggressive sex-age group in this species (Risenhoover and Bailey, 1985), which appears to have a greater frequency of aggressive interactions than other ungulates (Fournier and Festa-Bianchet, 1995). Because Mountain goats are very aggressive, it is reasonable to suspect that aggression (possibly stimulated by competition over food resources) may be involved in the emigration of young males (Festa-Bianchet and King, 1984; Anderson, 1989). Therefore, the aim of our study was to assess whether adult females may force young males from the group.