Acoustic allometry revisited: morphological determinants of fundamental frequency in primate vocal production

Maxime Garcia,Daniel L Bowling,Jacob C Dunn,W Tecumseh Fitch,Christian T Herbst

doi:10.1038/s41598-017-11000-x

Abstract

A fundamental issue in the evolution of communication is the degree to which signals convey accurate (“honest”) information about the signaler. In bioacoustics, the assumption that fundamental frequency (fo) should correlate with the body size of the caller is widespread, but this belief has been challenged by various studies, possibly because larynx size and body size can vary independently. In the present comparative study, we conducted excised larynx experiments to investigate this hypothesis rigorously and explore the determinants of fo. Using specimens from eleven primate species, we carried out an inter-specific investigation, examining correlations between the minimum fo produced by the sound source, body size and vocal fold length (VFL). We found that, across species, VFL predicted minimum fo much better than body size, clearly demonstrating the potential for decoupling between larynx size and body size in primates. These findings shed new light on the diversity of primate vocalizations and vocal morphology, highlighting the importance of vocal physiology in understanding the evolution of mammal vocal communication.

Highlights

A fundamental issue in the evolution of communication is the degree to which signals convey accurate (“honest”) information about the signaler
The prediction of a correlation between body size and fo relies on two main assumptions5: 1) body size directly determines the size of the larynx and the length of the vocal folds, and 2) that the resting vocal fold length has a direct influence on fo
Ordinary least squares (OLS) regression showed a significant positive relationship between log vocal fold length (VFL) and log body size (r2 = 0.35, β = 1.26, t = 2.51, P = 0.03; Fig. 1a), which was confirmed by the phylogenetic generalized least squares (PGLS) regression (r2 = 0.52, λ = 1.00, t = 3.44, P = 0.007)

Summary

Introduction

A fundamental issue in the evolution of communication is the degree to which signals convey accurate (“honest”) information about the signaler. Contemporary understanding of vocal production in mammal communication has benefited greatly from adopting the source-filter theory of vocal production[4,5,9,10] According to this framework[11], originally developed for human speech and later applied to animal communication, a sound is produced by the vibrating vocal folds within the larynx (the sound source) and their vibration rate determines the fundamental frequency (hereafter fo) of the acoustic signal. The prediction of a correlation between body size and fo relies on two main assumptions5: 1) body size directly determines the size of the larynx and the length of the vocal folds (as vocal folds in mammalian larynges extend from the thyroid cartilage to the arytenoids14), and 2) that the resting (i.e., unstretched) vocal fold length (hereafter VFL) has a direct influence on fo Biomechanical theory corroborates the latter condition, predicting that longer focal folds produce lower fo[15]. The former assumption has been challenged, given that www.nature.com/scientificreports/

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