Rhythmically repeated patterns of pulsed vocalizations in wild narwhals (Monodon monoceros)

Abstract

Sequences of vocalizations are indicative of communicative complexity. However, relative to birds and terrestrial mammals, the extent to which marine mammals use vocal sequences is not well understood. I provide the first investigation into vocal sequences in narwhals (Monodon monoceros), a gregarious Arctic cetacean.

Eight female narwhals were fitted with animal-borne recording devices, resulting in one of the largest datasets of narwhal acoustic behaviour to date. I used a combination of visual and quantitative classification procedures to rigorously demonstrate stereotyped organizational properties of subjectively defined sequence types. Next, acoustic characteristics were used to generate coarse inferences regarding patterns of sequence use across individuals. Finally, I used generalized linear models (GLMs) to assess the behavioural and acoustic contexts under which sequences were produced.

I identified two types of sequences: “paired” patterns, consisting of combinations of two stereotyped click-based calls, the pair of which were often repeated in rapid succession. While these calls were rare, I found multiple subtypes that were predominantly associated with recordings from specific tags. I secondly identified “burst pulse series”, temporally stereotyped sets of short burst-pulses which themselves were combined into repetitive vocalization events. I found few links between sequence use and behaviour, though burst-pulse series were more likely to be produced in periods when other vocalizations were heard, suggesting possible use in social contexts.

These findings extend the set of odontocetes which are known to use sequences of vocalizations. Both sequence types show rhythmic repetition of lower-level patterns, suggestive of hierarchical organizational principles. Furthermore, paired patterns constitute the first evidence of multi-unit sequences in the family Monodontidae. I propose that further inquiry into vocal sequences in narwhals and other understudied marine mammals is warranted.