Rhythm of the Deep: Two-Tier Combinatorial Structure in Sperm Whale Codas Revealed by Acoustic Unit Induction

arXiv:2606.16084v2 Announce Type: replace-cross Abstract: Sperm-whale codas are conventionally described as recurring click-count and timing patterns. We show instead that their waveforms contain a two-tier combinatorial acoustic organization. Recurring click units combine with inter-click rhythm to form coda units, and recurring coda units exhibit additional sequence-level dependence under a different acoustic carrier. Using 1,483 recordings, eight families of frozen audio encoders induce click and coda inventories. Held-out transfer, matched nulls, destructive waveform counterfactuals, expert timing baselines, and explicit abstention separate supported structure from shared encoder shortcuts. Click-token composition predicts induced coda identity with a median normalized mutual-information lift of 0.380, including when events are detected without published click times or counts. Stable click order is weak, whereas rhythm predicts coda-representation distance after the exact click-token multiset is fixed. At the second tier, coda tokens show 0.132 bits of incremental lag-2 dependence under the prespecified categorical estimator. Two expert-inspired rhythm representations do not recover the positive component. Coda identity is also more tempo-stable than click identity. These results change the acoustic description of codas from a single prescribed rhythm inventory to layered waveform organization. The controlled-induction framework provides a general method for discovering and falsifying combinatorial structure in under-annotated audio.

Source: arXiv cs.CL | 2026-08-04