Selectivity in buttress drumming tree properties among chimpanzees (Pan troglodytes schweinfurtii) of the Waibira community in Budongo Forest, Uganda

Wild chimpanzees drum on tree buttresses during dominance displays and travel, generating low-frequency sounds that are audible over distances of more than 1 km. Western chimpanzees (Pan troglodytes verus) in the Nimba Mountains of Guinea selectively choose trees and buttresses when drumming, potentially based on their resonant properties, suggesting that these chimpanzees are optimizing their drumming signals. We investigated whether male eastern chimpanzees (Pan troglodytes schweinfurthii) from the Waibira community in the Budongo Forest, Uganda, also show preferences in tree and buttress choice, exploring whether selectivity is a species-wide feature. We tested chimpanzee preferences for tree species and diameter, number of buttresses, and buttress area and width, by comparing trees and buttresses used in drumming bouts with nearby unused trees and buttresses. Waibira chimpanzees drummed preferentially on two tree species: the tropical hardwood Cynometra alexandrii and the softwood Chrysophyllum albidum. Chimpanzees selected trees with a larger diameter over nearby trees with a smaller diameter, and buttresses were more likely to be used for drumming if they had a larger area or larger width. These results suggest that chimpanzees in the Waibira community select trees and buttresses based on physical properties, most likely related to acoustically relevant characteristics. These findings support the argument that buttress drumming is a goal-directed behavior and contributes to our understanding of chimpanzees' use and optimization of their long-distance acoustic signals.