No evidence that maximum fundamental frequency reflects selection for signal diminution in bonobos

Acoustic allometry consists of looking at how an organism's body size scales with the characteristics of its vocalizations. A typical finding based on this framework is that across mammals body size is reflected in the fundamental frequency (f_o) of vocalizations, whereby lower f_o indicates larger body size [1]. This relationship holds owing to the fact that vocal fold length generally scales with body size [2]. Cross-species comparisons allow for the identification of interesting outliers from the body size-f₀ regression [3]. Such cases are of particular relevance as they can provide insight into the selective forces potentially driving deviations from standard allometric principles [2]. In a recent study in Current Biology, Grawunder et al.[4] argue that selective pressure for higher f₀ has led to the evolution of shorter vocal folds in bonobos than in chimpanzees. Thus, they claim, vocal fold length has evolved independently of body size in bonobos for the purposes of signal diminution (i.e., reducing the impression of body size that they advertise through their calls). However, considering both the existing literature and their own data, this conclusion does not appear to be supported for several reasons.