Singing Patterns of White-Cheeked Gibbons (Nomascus sp.) in the Annamite Mountains of Laos

Abstract

Studying the structure and patterns of vocalizations in primates is important to understand their evolution, responses to changing environments, and social behavior, and to design efficient vocalization-based survey techniques. Gibbons (Hylobatidae) are small arboreal apes known for their stereotyped songs. Data on their vocalization patterns remain limited, especially for crested gibbons (Nomascus spp.). We recorded songs of white-cheeked gibbons (Nomascus sp.) heard from listening posts at a site in Nakai–Nam Theun National Protected Area, central-eastern Laos, during a cold dry and a warm dry season, from November 2013 to May 2014, along with weather data. Males typically initiated the duet songs at a mean time of 06:14 h. Duet song bouts lasted a mean of 16 min. Song bout length was relatively constant throughout the study but decreased with the occurrence of wind. On a given day, the first male song started at a mean of 12 min before sunrise and all songs ceased at a mean of 13 min after sunrise. The start time of the first song relative to sunrise did not differ significantly between the two seasons but we found a significant negative relationship between the start time of the first song and both time of sunrise and temperature. The mean number of song bouts heard was significantly larger during the cold dry season than the warm dry season, and we found a significant negative relationship between the number of song bouts heard and temperature. Song rates also increased during days with no rain relative to rainy days. Our findings provide a preliminary understanding of song patterns in Nomascus spp. and can aid the planning and interpretation of surveys that rely on documenting their calls.