Professor Rudolf Meier
Srivathsan, A., R. Meier (2011) Proboscis monkeys (Nasalis larvatus (Wurmb, 1787)) have unusually high-pitched vocalizations. The Raffles Bulletin of Zoology, 59(2): 319-323.
Its abstract as follows:
According to qualitative descriptions, proboscis monkeys have high frequency vocalizations, but no technical analysis of the calls is available because the species is rare, difficult to study in the field, and few zoological institutions have breeding populations. Here we analyse the high pitch vocalizations of captive proboscis monkeys in the Singapore Zoological Gardens. The monkeys produce very high frequency tonal vocalizations ranging from 1.4 kHz to 6.8 kHz. The calls are produced during acts of aggression or used as alarm calls and the pitch is higher than expected based on a comparative study for such a large primate. We discuss that the high frequency may be an adaptation for predator avoidance (crocodiles) and/or communication in dense vegetation.