Estimating primate densities using home range and line transect methods: Acomparative test with the black colobus monkey Colobus satanas

The line transect method is one of the main methods used to estimate primat e densities. Several protocols have been proposed to analyze the data recor ded under this method but none of them have been widely accepted since ther e is a considerable controversy about their respective accuracy. In this st udy, densities of the black colobus monkey Colobus satanas calculated using eight different protocols were compared with the actual density given by t he home range method. Rates of polyspecific associations were also compared . The two most accurate estimates of group density (under- or overestimatio n < 10%) were yielded by the protocol that used the maximum transect-to-ani mal distance and by that of using a fixed distance of 100 m. These protocol s, however, underestimated individual density because counts performed from transects underestimated by 23% the average group size. The six other prot ocols overestimated group density by 20 - 195%. Factors that could explain these overestimation were discussed. Because histogram of sighting frequenc y showed several classes of distances with no records and because groups ha ve been detected as far as 160 m, we suggested that the uneven topography o f the study site increased the variability of the sighting distances. Combi ned with a relatively low number of sightings (n =23). this did not allow t o identify a sharp detection distance. Rates of polyspecific association fo und with the two methods were similar. We recommend to investigate the infl uence of the topography for bias in density estimates when using the line t ransect method.