Reaffirming the Loma Mountains National Park in Sierra Leone as a critical site for the conservation of West African chimpanzee (Pan troglodytes verus).

The population of West African chimpanzees (Pan troglodytes verus) is declining rapidly mostly due to the impact of human activities and habitat loss. Sierra Leone harbors the third largest population of this subspecies, recently reclassified as Critically Endangered in the 2016 IUCN Red List. Population monitoring provides crucial data for planning and evaluating conservation and management policies. Therefore, to assess the status of the population size inhabiting the Loma Mountains National Park (LMNP) in Sierra Leone, we performed a nest count survey and estimated chimpanzee density and abundance using distance sampling. In total, 34 × 2-km-line transects were surveyed, with transects being systematically distributed across the LMNP area (288.5 km ). Concurrently, we compiled environmental data, which were used to model nest distribution and infer the most relevant environmental and anthropogenic drivers of the observed nest abundances. We encountered 10.03 nests/km and estimated a density of 3.47 ± standard error (SE) 0.92 individuals/km (i.e., 1002 ± SE 266 individuals in total). Compared to the figures obtained from a systematic literature review, our results suggest that the density and abundance of chimpanzees in the LMNP is among the highest across Africa. Contrary to expectation, no specific anthropogenic features predicted nest distribution and abundance. However, the nest distribution model indicated preference for elevated and steep areas covered by closed evergreen forest, which could be an indication of human avoidance. Based on these results, we highlight the value of LMNP for the conservation of the chimpanzees in Sierra Leone and the urgent necessity of guarantying long-term funding for this park's management to ensure the survival of this critically endangered subspecies in West Africa.