Environmental variability supports chimpanzee behavioural diversity

Abstract

Large brains and behavioural innovation are positively correlated, species-specific traits, associated with the behavioural flexibility animals need for adapting to seasonal and unpredictable habitats. Similar ecological challenges would have been important drivers throughout human evolution. However, studies examining the influence of environmental variability on within-species behavioural diversity are lacking despite the critical assumption that population diversification precedes genetic divergence and speciation. Here, using a dataset of 144 wild chimpanzee (Pan troglodytes) communities, we show that chimpanzees exhibit greater behavioural diversity in environments with more variability - in both recent and historical timescales. Notably, distance from Pleistocene forest refugia is associated with the presence of a larger number of behavioural traits, including both tool and non-tool use behaviours. Since more than half of the behaviours investigated are also likely to be cultural, we suggest that environmental variability was a critical evolutionary force promoting the behavioural, as well as cultural diversification of great apes.

Fig. 1. Distance of chimpanzee communities to Pleistocene forest refugia.

The green areas depict the Pleistocene forest refugia as described by Maley, purple lines show straight-line distances calculated between the center point of a chimpanzee community to the limit of the nearest forest refuge, and dots represent a unique chimpanzee community in a predominantly forest (blue) or savannah woodland (orange) habitat. Chimpanzee geographic range plotted according to the IUCN 2018.

Fig. 2. The probability of occurrence for 31 chimpanzee behaviours per community as a function of environmental variability.

These behaviours are more likely to occur when chimpanzees live in habitats a further away from Pleistocene forest refugia, with b greater precipitation seasonality, and c a predominantly savannah woodland landscape. The size of the circles in plots a and b indicates the sample size, or number of chimpanzee communities per value of the predictor where the total n = 144 chimpanzee communities. The coloured areas depict the 67, 87, and 97% credible intervals centred on the mean predicted posterior distribution (a, b dashed line, c horizontal line) for the probability of occurrence across all 31 behaviours.

Fig. 3. Effects of all three environmental variability predictors on chimpanzee behavioural diversity.

We tested precipitation seasonality, savannah woodland versus forest habitat, and distance to Pleistocene refugia on the probability of occurrence for 31 chimpanzee behaviours as well as various subsets of the data to verify robustness of the results. The plot shows the mean of the marginal posterior distribution (dots) and the 67, 87, and 97% credible intervals centred on the mean (coloured areas).