Reduced geographic variation in roars in different habitats rejects the acoustic adaptation hypothesis in the black‐and‐gold howler monkey (Alouatta caraya)

Abstract

Vocalizations used for long‐range communication must disperse without significant structural changes to be decoded by receivers. The acoustic adaptation hypothesis (AAH) holds that, since acoustic signals are influenced by the habitat in which they disperse, sounds will possess specific structural characteristics to diminish sound degradation. Additionally, vocalizations can also be influenced by genetics, anatomy, and/or cultural aspects. Here, we tested the AAH through quantitative comparisons of roars in four allopatric populations of the black‐and‐gold howler monkey (Alouatta caraya) across an environmental gradient from open to closed, in northeastern Argentina. At each site, we obtained good‐quality recordings from three adult males, also between July and November 2013, conducted vegetation surveys (measuring tree density, canopy closure, and vertical structure), evaluated potential masking of roars by gathering environmental sound samples, and assessed sound attenuation of a synthetic tone at three different distances: 10 m (landmark reference distance), 50, and 100 m. We also tested the alternative hypothesis that acoustic properties of roars could be explained by population genetic divergence. Our results did not support the AAH. Although our four study sites were significantly different in vegetation structure, conforming to an open‐to‐closed gradient, roars of A. caraya were not different among populations. Likewise, although environmental sound differed between sites, we found no evidence of environmental sound affecting the acoustic properties of roars. The attenuation of the synthetic tone was only near significant at 100 m distance between both extreme sites from the environmental gradient. The four A. caraya study populations grouped into three genetically differentiated clusters. Since roar features were independent from population genetic clustering, we reject the genetic hypothesis too. The combination of high amplitude and low peak frequency of roars, coupled to small home range size and extensive overlap between neighboring groups, allows roars to keep their communication value across habitats without need of specific environmental tuning.

Passive acoustic monitoring of the diel and annual vocal behavior of the Black and Gold Howler Monkey

“…In this sense, reliable long-term monitoring methods would be a useful tool for scientists and wildlife managers aiming to monitor the BGHM as well as other primates. The utterance of loud calls with little variability, even among regions (Holzmann & Areta, 2020), suggests that automated signal recognition should be a reliable tool for monitoring the BGHM (Heinicke et al, 2015). Indeed, a previous assessment using automated detection of the calls of the Mantled Howler Monkey (Alouatta palliata) showed that the species was automatically detected in 35 of the 43 recordings (81.3%) in which it was present (Aide et al, 2013).…”

“…Our recall rate refers to the percentage of recordings that the species was automatically detected, while previous studies of primates used number of calls.Although this precludes us from making direct comparisons, our findings, together with those obtained byAide et al (2013), provide strong evidence of the effectiveness of automated signal recognition software for detecting the presence of howler monkeys. The effectiveness of automated software for monitoring howler monkeys might be partly explained by the long and loud calls of the genus (da Cunha et al, 2015), as well as limited variability of calls among populations(Holzmann & Areta 2020). Indeed, the few instances when the BGHM was detected by the observer but not by Kaleidoscope Pro, in our study, were cases in which the species was presumably calling at long distances from the recorder, according to the low sound pressure level of these calls detected in the spectrograms (Cristian Pérez-Granados personal observation).…”