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

Abstract

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.

Environmental variability supports chimpanzee behavioural diversity

“…We considered savannah habitats as more ecologically variable than forested habitats given they have more pronounced seasons and greater fluctuation in climate 37 . Although this dichotomization likely underestimates the full spectrum of environmental variation 38 , we constrained the classification to these two categories since the shift from a closed and wet, to an open and arid habitat has been emphasized repeatedly as a critical tipping point for human evolution [19][20][21] . We classified any community where fragmented, tropical, lowland, wet, or montane forest is present as forest and all others as savannah woodland (see Methods for additional details).…”

Deciding Where to Sleep: Spatial Levels of Nesting Selection in Chimpanzees (Pan troglodytes) Living in Savanna at Issa, Tanzania

“…The spatiotemporal changes in the availability of water and food in savanna habitats seem to influence their selection of sites for nesting (Ogawa et al 2014). With the exception of small parts of evergreen vegetation, the majority of the landscape in savanna habitats is occupied by deciduous vegetation types where tree height is lower, tree trunk is smaller, and trees exist in overall lower densities and have a discontinuous canopy in comparison to more forested habitats (Abbot et al 1997;Hutley and Setterfield 2008;van Leeuwen et al 2020;). These physical characteristics of trees in deciduous vegetation types may have forced the chimpanzees in our study to select sites with overall high trees and tall and large trees for nesting.…”

Introduction to the Special Issue on Savanna Chimpanzees

“…One aspect of contextualizing savanna sites is by categorizing/classifying habitats. Three papers in this special issue (Pennec et al 2020, van Leeuwen et al 2020, and Wessling et al, 2020 seek to objectively quantify (habitat) components of savannamosaic landscapes for Pan species. While dense forest characterizes most landscapes with bonobos (e.g., Hickey et al 2012), Pennec et al 2020 characterize the habitat of a group of bonobos living in a mosaic of savanna and forest.…”

“…Characteristic differences such as lower forest cover (van Leeuwen et al 2020), lower tree densities, and lower arboreal basal area (Wessling et al 2020) may lead to constraints on the arboreal nest-building and sleeping habits of Pan. Two articles in this special issue examine the physical characteristics of nesting sites to elucidate potential selective pressures involving sleep in dry and open savannas.…”

“…We also are fortunate to have a contribution from Pennec et al (2020) to provide us with a comparative perspective for the bonobo (Pan paniscus). The descriptor "savanna chimpanzees" is somewhat imprecise yet does suggest that these apes are not found in habitats dominated by some form of forest cover (van Leeuwen et al 2020). Rather, the habitats they occupy, as implied by the use of "savanna," are more open; may include woodland, grassland; and are often labeled mosaic habitats (Hunt and McGrew 2002).…”