The Structure and Function of Male Thomas Langur Loud Calls

Abstract

This study has addressed the acoustical structure of male loud calls and their function in Thomas langur social organisation. Thomas langurs are medium sized primate that lives in Sumatra, Indonesia. Ome of the characteristics of this species is the loud call of males. Several functions for loud calls have been suggested such as mate-defence, resource-defence and mate-attraction. In this study playback studies were applied to determine that loud calls function in between-group spacing and resource-defence. The function of loud calls in mate-attraction was not demonstrated directly, but an analysis of long-term data showed that females often choose newly adult males when the transfer between groups. These newly adult males have loud calls of shorter duration than adult males that already have associated with females and have offspring. This indicates that females may use loud calls as a cue to choose newly adult males. Since newly adult males are better in protecting offspring against infanticidal extra-group males and predators, it is functional for females to choose these males. In addition, it was shown that males show individual variation in loud call characteristics and that calls also differ between behavioural context, such as dawn chorus or alarm.