LAUSR

Small-scale comparisons help us to understand how habitat features and food availability impact primate abundance. This is particularly useful at sites without human impacts, as it allows for the investigation of the natural factors influencing nesting patterns and great ape abundance. We provide a small-scale study of sympatric great ape nests in an unlogged old-growth forest without poaching activities. We conducted a line transect survey (52 km total effort) around Mbeli Bai, a forest clearing in Nouabale–Ndoki National Park, Congo, applying on-site nest decay rates and assigning nest builder using logistic regression. We found a high occurrence of monodominant Gilbertiodendron dewevrei at Mbeli Bai (34%) that correlates with low great ape densities at Mbeli Bai. Chimpanzees (Pan troglodytes troglodytes) showed a preference for nesting in trees in closed canopy monodominant forest. We found a low percentage of gorilla (Gorilla gorilla gorilla) nests in mixed species forest (35%) and a higher percentage in trees (64%) compared to other study sites. However, generalized additive models found higher gorilla nest encounter rates in mixed species forest with dense understory than in monodominant forest and open understory. We found no indication of higher gorilla densities close to Mbeli Bai than elsewhere, and line transect estimates were lower than the number of gorillas revealed from direct observations. There were substantial differences between our findings and those for nearby sites, demonstrating the utility of small-scale comparisons to further understand the factors determining chimpanzee and gorilla densities within and between sites and the limitations of nest surveys.