Alpha male Guyanan red howler monkey responses to nocturnal and diurnal loud calls

Abstract

Our understanding of the communication system of howler monkeys is hampered by our ability to collect acoustic and behavioral data across their entire period of activity. It is not surprising that most studies are restricted to the daylight period despite knowledge that this species is also active at night. Our goal was to describe vocalization patterns, call structure, and test the function of diurnal and nocturnal loud calls of the Guyanan red howler monkey. To be able to study 24-h cycle call emissions, we used 20 passive acoustic monitoring devices deployed in the territory of four different troops of howlers. To test the function of diurnal and nocturnal loud calls, we set up a broadcast experiment and recorded the behavioral response of alpha males to the playbacks. There is a significant difference between some spectral properties of nocturnal and diurnal howls (duration, standard deviation, median frequency, skewness, kurtosis, and flatness). The alpha males responded differently to nocturnal and diurnal playbacks. They approached more often, vocalized more, and never evaded when presented with nocturnal call playbacks. We found significant differences between approach latency, evade latency, and soft call latency. Howler monkeys responded differently to the playbacks, which suggest that nocturnal and diurnal loud calls serve different functions. We found that nocturnal calls triggered more aggressive responses than diurnal calls.