Abstract
Nighttime studies are underrepresented in ecological research. Even well-known behaviors, such as the loud call of howler monkeys, are rarely studied at night. Our goal was to help fill this knowledge gap by studying the 24 h vocal behavior of the Guianan red howler monkey (Alouatta macconnelli) and to compare the acoustic structures of howling bouts made during the day to those made at night. We used passive acoustic monitoring coupled with automatic acoustic detection to study three groups of howlers over three months in the Viruá National Park, Roraima, Brazil. The automatic classifier we built detected 171 howling bouts with a 42% recall rate and 100% precision. Though primarily diurnal, howlers vocalized mainly at night. Greater vocal activity before nautical twilight might be associated with territorial and resource defense behaviors, with howlers calling from roosting sites before starting their daily routines. We also found that during the day, howling bouts were longer and had lower harmonic-to-noise ratios, lower frequencies, and more symmetric energy distributions than bouts at night. Our study adds to growing evidence that passive acoustic monitoring and automatic acoustic detection can be used to study primates and improve our understanding of their vocal behavior.
Highlights
Our goals were: (1) to investigate whether passive acoustic monitoring (PAM) and automatic acoustic detection of calls might be useful to monitor the vocal behavior of the species in a biodiversity hotspot in northern Brazil where automatic detection may be problematic due to saturated soundscapes, and (2) to determine if the acoustic structure of howling bouts made during the day were different from those made at night
While previous research on the Guianan red howler monkey described different types calls in their repertoire
[15], a detailed and systematic description of their vocal behavior calls in their repertoire [15], a detailed and systematic description of their vocal behavior and howling bouts waswas lacking
Summary
Howler monkeys (Alouatta spp.) emit the most powerful primate vocalization in the Neotropics, which may outperform all animals worldwide in both call duration and amplitude per body size [1]. These loud calls are thought to have multiple functions [2]. Despite the large interest in these remarkable loud calls, several species of howler monkeys still lack acoustic characterization of their calls [2,6,7]. Even less is known about the nocturnal vocal activity of howlers because most studies focus on their diurnal behaviors [1,2,5]
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