Eviction and dispersal in co-operatively breeding banded mongooses (Mungosmungo)

The mode by which individuals disperse, and the cost of dispersal, are of g reat importance in attempts to understand variation in reproductive skew in animal societies. In this paper we report detailed information on dispersa l and pack formation in banded mongooses Mungos mungo. Six pack fission eve nts were recorded among 11 packs over 22 months. Pack fission occurred unde r two distinct circumstances. First, groups of individuals were evicted fro m their natal group as a result of intense aggression from other group memb ers. A small fraction of group members was responsible for most of the aggr ession. Both sexes helped to attack and evict individuals from the group, a nd both males and females were driven out of their natal groups en masse. T he second mode of pack fission occurred when groups of same-sex individuals left their natal group voluntarily to join dispersing individuals of the o pposite sex, thereby forming new packs. Dispersing groups were more frequen tly involved in fights with rival packs of mongooses compared to establishe d groups, and in one instance these fights seemed to be responsible for sev ere injury and increased mortality among members of a dispersing group. The observations of eviction provide one line of evidence that the presence of subordinates is sometimes detrimental to dominants, contrary to the assump tions of concession models of reproductive skew.