Classifying Chimpanzee (Pan troglodytes) Landscapes Across Large-Scale Environmental Gradients in Africa

Primates are sometimes categorized in terms of their habitat. Although such categorization can be oversimplistic, there are scientific benefits from the clarity and consistency that habitat categorization can bring. Chimpanzees (Pan troglodytes) inhabit various environments, but researchers often refer to forest or savanna chimpanzees. Despite the wide use of this forest-savanna distinction, clear definitions of these landscapes for chimpanzees, based on environmental variables at study sites or determined in relation to existing bioclimatic classifications, are lacking. The robustness of the forest-savanna distinction thus remains to be assessed. We review 43 chimpanzee study sites to assess how the landscape classifications of researchers fit with the environmental characteristics of study sites and with three bioclimatic classifications. We use scatterplots and principal components analysis to assess the distribution of chimpanzee field sites along gradients of environmental variables (temperature, rainfall, precipitation seasonality, forest cover, and satellite-derived Hansen tree cover). This revealed an environmental continuum of chimpanzee study sites from savanna to dense forest, with a rarely acknowledged forest mosaic category in between, but with no natural separation into these three classes and inconsistencies with the bioclimatic classifications assessed. The current forest-savanna dichotomy therefore masks a progression of environmental adaptation for chimpanzees, and we propose that recognizing an additional, intermediate forest mosaic category is more meaningful than focusing on the ends of this environmental gradient only. Future studies should acknowledge this habitat continuum, place their study sites on the forest-savanna gradient, and include detailed environmental data to support further attempts at quantification.