The perception of vocal signals by blue monkeys and grey-cheeked mangabeys

Old World forest primates may need to signal vocally over long distances in noisy habitats. Perceptual experiments focused on the audibility of signals in noise were conducted in the laboratory using standard psychoacoustical methods. Six human listeners, two blue monkeys (Cercopithecus mitis), and two grey-cheeked mangabeys (Cercocebus albigena) served as subjects. Based upon measurements of East African rain forests (Brown and Waser 1984) testing was conducted in the presence of simulated rain forest noise, or in the quiet. Signals consisted of pure tones 2-seconds in duration ranging in frequency from 63 Hz to 32 kHz, or recordings of vocalizations. Test vocal signals for monkey subjects were the blue monkey chirp, pyow, trill, ka, ka-train, grunt and boom calls; the mangabey chorused grunt, soft grunt, gobble and staccato bark calls. Human listeners were tested with the consonant-vowel stop consonants: ba, pa, da, ta, ga, ka; the two-syllable words: baseball, lifeguard, vessel and leisure; and the sentence "Can you hear me?" The results showed that mangabeys exhibit an audibility function nearly indistinguishable from that for blue monkeys (Brown and Waser 1984). Compared to terrestrial open country monkeys, these arboreal rain forest monkeys possess enhanced low-frequency sensitivity which coupled with mechanisms specialized for low-frequency vocal production may function as a long-distance communication system. Testing with vocalizations in the presence of a masking noise revealed that monkey calls were about 10 dB more audible in noise than were human speech sounds. No differences were found between the audibility of graded or discrete calls. Testing for species-specific abilities was conducted with a subsample of six calls. While differences were not found when testing was conducted in the quiet, in the presence of forest noise, monkeys were about 3.4 dB more sensitive to the calls of conspecifics than to the calls of sympatric species. These findings support the idea that differences in the audibility of the vocalizations of related species is principally due to acoustical differences between signals, yet more subtle receptive specializations may have evolved as well. Hence selection may have acted on both receptive and productive mechanisms to promote vocal communication under adverse environmental conditions.