The active space of blue monkey and grey-cheeked mangabey vocalizations

Abstract

The audible distance of 11 primate vocalizations uttered by blue monkeys, Cercopithecus mitis, and grey-cheeked mangabeys, Cercocebus albigena, and the human utterance ‘hey’ were determined experimentally. Calculations were based on measurements of (1) sound power of vocal signals (Brown: Bioacoustics, in press), (2) the attenuation rates of sound of different frequencies in East African forests (Waser & Brown: Am. J. Primatol., 1986, 10, 135–154), and (3) sensitivity of conspecific listeners to vocal signals presented in forest noise. Calculations were made of the active space, the area over which a call is audible, and the expected number of recipients of signals in nature. Masked thresholds for test vocalizations ranged from 21·1 dB for the mangabey ‘staccato bark’ call to 41·3 dB for the blue monkey ‘boom’ vocalization. The audible distance of the test signals ranged from 79 m for the blue monkey ‘chirp’ call to 1951 m for the mangabey ‘chorused grunt’ vocalization. Calls could be grouped into short- and long-range signals. The audible distance of primate long-range calls varied between 2·4 and nine times that of a typical yell given by human subjects. The active space of the test signals ranged from 1·4 to 1031·8 ha. The mean active space of monkey long-range calls (445·4 ha) was more than an order of magnitude greater than the loudest human yell. The average blue monkey long-range call was audible for 870 m, while the average mangabey long-range call was audible for 1800 m. The typical mangabey home range is four times that of the blue monkey, and in both species the average long-range call had an audible distance twice the diameter of the median home range of each species.