Inter-group encounters and male incursions in Colobus vellerosus in central Ghana

Inter-group encounters can I - reflect between-group contest competition for access to limiting, defensible resources, 2 - allow individuals to gather information on the surrounding groups to assess mating or dispersal opportunities or to assess resistance to a possible transfer or takeover, 3 - allow individuals to convey information to the other group about one's quality/status. We present data on inter-group encounters and male incursions (where one or several mates encounter a reproductive unit) in Colobus vellerosus at the Boabeng Fiema Monkey Sanctuary in Ghana to assess whether they are compatible with these three functions, and in the case of the limiting resource defense hypothesis, to assess the resource competed for. Our study group was observed for 343 hours over 61 days between June and November 2001, and interacted with three neighboring groups in 47 encounters. Inter-group encounters were more frequent than male incursions (34 vs 13) and lasted longer (median: 45 vs 9 minutes). Most encounters included aggression towards individuals in the opposing group (39/47). Males were aggressors as well as targets of male aggression in a majority of encounters. Females were targets of male aggression in over half of encounters, while being aggressive in only a few of them. We describe three cases of male aggression towards immatures during male incursions, and one case of female transfer following one of these male incursions. Two extra-group mating attempts were observed during inter-group encounters. Most encounters took place in quadrats that were important food patch (>5% of feeding time), although the two quadrats that had the hi best proportion of feeding did not hold any encounters. Inter-group encounters were more likely to occur in the feeding context than male incursions. The intensity of male aggression was not influenced by whether the encounter took place in the feeding context or not. Our data suggest that direct and indirect male mate defense is the most likely motivation for encounters under the limiting resource defense hypothesis. Female resource defense may occasionally occur in C. vellerosus, but does not explain the majority of encounters. Encounters in general, but in particular nonaggressive encounters and male incursions are probably a mean by which individuals can assess individuals of neighboring groups for mating, transfer or takeover possibilities, as well as convey information about their own status or quality.