Human vowels consist of a sequence of glottal pulses produced by vocal folds. Whale codas consist of a sequence of clicks produced by vibrating phonic lips, which play a role similar to the human vocal folds [15]. In human languages, the frequency of glottal pulses corresponds to pitch—closely spaced glottal pulses give rise to a higher pitch, while more widely spaced pulses give rise to a lower pitch. In linguistics, tone refers to pitch as recruited to express linguistic meaning. Many languages use tone to distinguish between different words. For example, in Mandarin Chinese, the following four words differ only in their tonal contour, while having the same consonants and vowels [21]: high and level tone ma ‘mother’, rising tone má ‘hemp’, falling-rising tone ma ‘horse’ and falling tone mà ‘scold’. The coda types can therefore be compared to human tone: ‘regular’ coda types can be compared to level tones, codas with ‘increasing’ ICIs to falling tones and codas with ‘decreasing’ ICIs to rising tones. (However, our analogy has a limit: while in human languages, different tones can be associated with different meanings, the meanings conveyed by sperm whale codas have not been established.) In figure 1, the ‘F0’ (fundamental frequency) of each coda is represented with a blue line.

Beguš et al. [15] show that different coda vowel qualities can be instantiated on the same coda types and propose that coda type and coda quality are orthogonal [15]. This points to another parallelism between the sperm whale communication system and human language, as tone and vowel quality are often similarly orthogonal. For example, in Mandarin Chinese, the falling–rising tone may appear on any vowel, e.g. ma ‘horse’, ma ‘rice’ and ma ‘smear’. Orthogonality, in this case, is used to describe the independent mechanisms of production between the traditional timing or source features and the vocalic or filter features. In other words, the rate of vocal fold or phonic lip vibration can be independent of the shape of the resonant body (the vocal tract or the distal air sac), and both vowel types surface on several traditional coda types. However, while the production can be independent, there can still exist distributional patterns, where a vowel quality is more frequent on certain tones or some coda vowels are more common on certain traditional coda types. Our paper builds on Beguš et al.’s [15] findings and reveals further complexities within the system of sperm whale vocalizations.