Temporal but Not Acoustic Plasticity in Hybrid Howler Monkey (Alouatta palliata x A-pigra) Loud Calls

The relatively fixed vocal structure of nonhuman primates stands in stark contrast to humans. However, primate vocal-plasticity studies are particularly limited by ethical and logistic constraints. As an alternative approach, we take advantage of a confirmed howler hybrid zone (Alouatta palliata x A. pigra) to compare the effects of genetic ancestry and experience on vocal variation. Deviations from a tight phenotype-ancestry correlation can indicate potential plasticity. We also tested whether temporal features (e.g., syllable number, calling rate) show more plasticity than acoustic features (e.g., peak frequency, bandwidth) which might be morphologically constrained. Using 29 microsatellite markers, most hybrid male subjects fell at the extremes of the genetic ancestry distribution, consistent with the entire population's distribution. We then analyzed 182 howling bouts and 231 loud call recordings from 33 male hybrid and purebred subjects from sympatric and allopatric populations. Acoustic features of hybrid calls clustered with calls from genetically similar purebred males, and calls from the only genetically intermediate hybrid in our sample fell between the two acoustic extremes, suggesting a strong effect of ancestry. However, temporal features tended to converge in sympatry (e.g., vocalizations from purebred A. palliata in the hybrid zone were more pigra-like than vocalizations from allopatric A. palliata, and indistinguishable from sympatric palliata-like and intermediate hybrid vocalizations). Our study shows that, at least for temporal features, primate vocal plasticity can extend beyond intrapopulation variation, consistent with a growing body of research suggesting that primate vocalizations may be more flexible than traditionally assumed.