Old world monkey vocalizations: adaptation to the local habitat?

Abstract

Twenty-one representative vocalizations of two species of rain forest monkeys (blue monkeys, Cercopithecus mitis, and grey-cheeked mangabeys, Cercocebus albigena) and two species of savanna monkeys (vervet monkeys, Cercopithecus aethiops, and yellow baboons, Papio cynocephalus) were broadcast in both rain forest and savanna habitats. Broadcast signals were re-recorded at distances of 12·5 and 100 m, digitized, and analysed on a supercomputer to measure the magnitude of distortion of the vocal repertoires of the savanna and forest monkeys in both the ‘appropriate’ and ‘inappropriate’ habitats. Independent distortion analyses were conducted in the time and frequency domains. The results showed that (1) distortion was strong following transmission distances of only 12·5 m in both habitats, (2) distortion scores were further increased, but only by a modest amount, when transmission distance was increased (3) distortion scores were greater in the savanna habitat than in the rain forest habitat, (4) rain forest monkey calls were distorted less in the ‘appropriate’ rain forest habitat than in the ‘inappropriate’ savanna habitat, but (5) savanna monkey calls were similarly distorted in both habitats. The results were consistent with the idea that the rain forest environment is generally more favourable for high-fidelity sound propagation, and that selection for reduced distortion has more strongly influenced the physical form of the vocal repertoire of the two rain forest species than that of the two savanna species.