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.