On the occurrence of nocturnal and diurnal loud calls, differing in structure and duration, in red howlers (Alouatta seniculus) of French Guyana.

Abstract

Structural patterns and 24-hour rhythms of the loud calls of Alouatta seniculus were studied in a wild population living in primary rain forest in French Guyana. These loud calls are low-pitched noisy hoarse sounds with an upper frequency limit around 3,000 Hz. Their spectral energy is concentrated in two clusters, each of which includes two dominant frequency bands. Loud calls comprise one or several vocal units with alternating crescendos and decrescendos. Five different types of units can be distinguished. Depending on their duration, loud calls can be roughly divided into two classes: long and short calls. Long calls (mean duration 3.28 min) include three phases. After the end of the calls, one or two blowing noises of weak amplitude may be heard. Short calls (mean duration 11 s) are frequently uttered in clusters of several calls given by the same troop at intervals of a few seconds to a few minutes. Observations carried out over the 24-hour cycle show that the calling rate is higher at night, with a peak around dawn and that long calls are more frequent at night, while short calls are mainly heard during the day. Most long calls are emitted from the sleeping sites and carry great distances. They probably play a role in inter-troop communication. Despite their intensity, short calls are used for short-distance communication. The occurrence of two different loud calls in Guyanan howlers and the amount of night calling in these monkeys are discussed with reference to studies of other howler populations.