Vowels and Diphthongs in Sperm Whales

The sperm whale communication system, consisting of groups of clicks called codas, has been primarily analyzed in terms of the number of clicks and their inter-click timing. This paper brings a new dimension to the study of sperm whale communication — spectral properties — and argues that spectral properties are not an artefact of movement, but likely actively controlled by whales, exchanged in dialogues, and potentially meaningful in this communication system. We discover formant structure in whale codas and uncover previously unobserved spectral patterns that are orthogonal to the traditionally analyzed properties. We present a visualization technique that allows us to describe several previously unobserved patterns. We argue that codas are on many levels analogous to human vowels and diphthongs and can be conceptualized in terms of the source-filter theory: vowel duration and pitch correspond to the number of clicks and their timing (traditional coda types), while spectral properties of clicks correspond to formants in human vowels. We identify two recurrent and discrete coda-level spectral patterns that appear across individual sperm whales and across traditional coda types: the a- and i-coda vowels. We also show that sperm whales have diphthongal patterns on individual codas: rising, falling, rising-falling and falling-rising formant patterns are observed. These uncovered patterns suggest that spectral properties have the potential to add to the communicative complexity of codas independent of the traditionally analyzed properties.