Male chimpanzees produce a species-typical long distance call known as the pant hoot. Males give acoustically similar pant hoots when calling together during choruses, but the process by which this vocal convergence takes place is unclear. Three potential mechanisms might account for call matching. First, vocal convergence could represent the passive effect of male chimpanzees sharing a common affective state during choruses. Second, matching could result through the active modification of calls. Such modification might occur if males modeled their calls after those produced by their current chorusing partner. Alternatively, actively mediated convergence would result if males mimicked the calls given by high-ranking males. We examined acoustic variation in calls given during choruses and while alone to test these three hypotheses. Results showed that a similar call type is not given repeatedly during successive choruses. In addition, low-ranking individuals did not produce calls that matched those of the alpha male. Chorused calls produced by two dyadic pairs were more similar to each other than they were to calls delivered during other choruses. The higher ranking, yet dependent, member of one of these dyads appeared to give calls that converged on those produced by the other individual. These data are consistent with the hypothesis that male chimpanzees accomodate each other vocally through the active alteration of their calls during choruses. We suggest that a similar form of accomodation takes place in animals from a wide range of other taxa and that the process serves to maintain and strengthen social bonds between individuals.