Contextual and combinatorial structure in sperm whale vocalisations$1:
Our results demonstrate that sperm whale vocalisations form a complex combinatorial communication system: the seemingly arbitrary inventory of coda types can be explained by combinations of rhythm, tempo, rubato, and ornamentation features. Sizable combinatorial vocalisation systems are exceedingly rare in nature; however, their use by sperm whales shows that they are not uniquely human, and can arise from dramatically different physiological, ecological, and social pressures.
These findings also offer steps towards understanding how sperm whales transmit meaning. In some organisms with combinatorial codes, such as honey bees (Apis sp.), the constituent features of the code transparently encode semantics (e.g., direction and distance to food sources). Further research on sperm whale vocalisations may investigate if rhythm, tempo, ornamentation, and rubato function similarly, directly encoding whales’ communicative intents. Alternatively, one of the key differentiators between human communication and all known animal communication systems is duality of patterning: a base set of individually meaningless elements that are sequenced to generate a very large space of meanings. The existence of a combinatorial coding system-at either the level of sounds, sound sequences, or both-is a prerequisite for duality of patterning. Our findings open up the possibility that sperm whale communication might provide our first example of that phenomenon in another species.