Subspecies rhythmic patterns. r indicates rhythm ratios of consecutive inter-hit intervals (IHIs) within drumming bouts produced by the two chimpanzee subspecies (nratios = 1,815;neastern ratios = 547;nwestern ratios = 1,268) fromn= 371 drumming bouts (neastern drumming bouts = 162;nwestern drumming bouts = 209). The yellow line is a simulated distribution of rhythm ratios from a uniform random distribution of inter-hit intervals. Rhythm ratios produced by eastern chimpanzees are indicated in blue; rhythm ratios produced by western chimpanzees are indicated in brown. Both distributions differ significantly from the random distribution and from each other. On-integer ratio ranges are highlighted using dashed lines. The central peak of the western distribution on 1:1 indicates use of isochronous inter-hit intervals (i.e., isochronous drumming), while the peaks at the sides of the eastern distribution indicate the use of alternated shorter and longer inter-hit intervals within drumming bouts.
