Mammal Sounds and Motivation-Structural Rules: A Test of the Hypothesis

Abstract

We tested if mammal sounds given in aggressive and fearful/friendly contexts showed acoustic design characteristics in accordance with the Motivation-Structural (MS) rule hypothesis (Morton, 1977). Our data were taken from literature and consist of 76 aggressive sounds and 75 fearful/friendly sounds from 50 species of mammals. Aggressive sounds are of low-frequency and wide bandwidth and thus support MS rule predictions. Fearful/friendly sounds show a trend toward conforming to MS rule design expectations (high-frequency and tonal) but exhibit considerable variation. This variation may be due to the existence of acoustic parameters other than frequency or bandwidth that might convey motivation information in fearful/friendly contexts. In addition, fear and friendliness represent two very different motivation states in mammals and MS rules may not apply to both.