Nest count is not an appropriate method to estimate abundance of gorillas and chimpanzees where both species live sympatrically. To apply an alternative method that could estimate their abundance separately, we examined dung count for gorillas on line transects. In the northern part of Moukalaba-Doudou National park, Gabon, we conducted a survey of gorilla dung piles (DPs) along the 11 line transects of 44.3 km in total length. First, we counted and marked all encountered dung piles and estimated DP density by the distance sampling method. After two days, we walked the same transects and checked whether the marked DPs were still recognizable, in order to calculate a daily dung disappearance rate. Using DP density, daily dung disappearance rate and the defecation frequency extrapolated from the other western gorilla populations, we calculated the density of gorilla. DP density was estimated at 102.3 dps/ km2. Dung piles of gorillas are easily discriminated from those of chimpanzees, so dung pile density is considered as a good indicator of gorilla abundance. However, individual density derived from DP density, daily dung disappearance rate and defecation rate seemed to be signifi cantly overestimated. Precise information on dung decay duration, age/sex difference in defecation rate, dietary effects on defecation rate, and group-level DP production frequency are needed for a reliable individual density estimate. No signifi cant difference was found in the encounter frequency of dung piles between the home range of our habituated gorilla group and those of adjacent areas, suggesting an overall high density of gorillas in the northern part of the Park.
