Seminomadic ranging in a population of black colobus (Colobus satanas) in Gabon and its ecological correlates

We observed ranging patterns in a black colobus population in the Foret des Abeilles, Central Gabon. We compared the results with those for two other populations (Lope Reserve in Central Gabon, Douala-Edea in coastal Cameroun) in order to estimate the extent of specific variability in population structure and ranging behavior and to identify ecological factors influencing interpopulation differences. The mean number of monkeys in a group is 17 and all groups had a multimale structure. Home ranges were large, and the fact that they continued to increase with increasing number of observations indicated that the monkeys did not frequently resample their habitat. Ranging patterns varied according to the staple food consumed: distances travelled daily increased with increasing seed intake and decreased with increasing leaf intake. The ranging patterns of groups of Colobus satanas are seminomadic. Up to 6 or 7 groups shared the same space, and two group ranges could overlap by as much as 65-75%. Comparisons with the two populations previously studied showed that group size and group structure are broadly similar At all three sites, black colobus were mainly seed eaters and ate large amounts of leaves in the season of fruit shortage. In the Foret des Abeilles, however group home ranges were much larger than at other sites, and this population had the lowest density. This is correlated with vegetation composition and with harshness of the main dry season. This colobus population appears limited by both recurring food shortage in the dry season and episodic periods of seed shortage resulting from irregular fruiting of the dominant family Caesalpiniaceae. Their seminomadic ranging would constitute the least costly strategy to cope with the low carrying capacity of their habitat.