| Summary: | Sperm whale vocalizations are among the most intriguing communication systems in the animal kingdom. Traditionally, sperm whale codas, or groups of clicks, have 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 likely actively controlled by whales and potentially meaningful in this communication system. We uncover previously unobserved recurrent 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 introduce the source-filter analysis of sperm whale codas and argue that they are on many levels analogous to human vowels and diphthongs: 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: the a-coda vowel and i-coda vowel. Both coda vowels are possible on different traditional coda types. Our discovery thus suggests that spectral (filter) properties are independent of the source properties (number of pulses and timing). We also show that sperm whales have diphthongal patterns on individual codas: rising, falling, rising-falling and falling-rising formant patterns are observed. Finally, we control for whale movement and present several pieces of evidence suggesting that the observed patterns are not artifacts, but are actively controlled by sperm whales. We also show that the two coda vowels (the a-vowel and i-vowel) are actively exchanged by sperm whales in dialogues. These uncovered patterns suggest that spectral properties have the potential to add to the communicative complexity of codas independent of the traditionally analyzed properties.
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