Absolute auditory thresholds in monkeys and humans

Abstract

To determine the limitations imposed by the auditory system on the processing of species-specific vocalizations we have conducted a variety of psychoacoustic tests with different monkey species. This paper summarizes measurements of absolute auditory thresholds for three monkey species—the Japanese macaque (Macaca fuscata), de Brazza's monkey (Cercopithecus neglectus), and the vervet (Cercopithecus aethiops)—and four humans tested in the same setting. Human and monkey threshold contours assumed the same general shape but were displaced from one another along the frequency axis: humans were more sensitive than the monkeys below 2 kHz but less sensitive above this frequency. Also, whereas the monkey hearing range extended to 45 kHz, the humans' hearing cutoff in the vicinity of 16 kHz. Previous comparisons of the sensitivity of Old World monkeys have suggested that all species have essentially identical threshold contours. However, we found a small but consistent sensitivity difference for high frequency signals among the species we tested. Frequencies below 22.6 kHz were equally audible to all three species but the vervets were consistently more sensitive (on the order of 3 to 12 dB) to frequencies above 22.6 kHz. Interestingly, this sensitivity difference correlates closely with variations among the species' head sizes and, therefore, interaural distances. These findings are thus consistent with previous reports that close-set ears are highly correlated with enhanced high-frequency sensitivity and they fit nicely with Masterton's et al. [J. Acoust. Soc. Am. 45, 966–985 (1969)] theory that such sensitivity is probably related to selective pressures for sound localization. [Work supported by NSF and NIH.]