Contextual and combinatorial structure in sperm whale vocalisations
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Abstract
Sperm whales ( Physeter macrocephalus ) are highly social mammals that communicate using sequences of clicks called codas. While a subset of codas have been shown to encode information about caller identity, almost everything else about the sperm whale communication system, including its structure and information-carrying capacity, remains unknown. We show that codas exhibit contextual and combinatorial structure. First, we report previously undescribed features of codas that are sensitive to the conversational context in which they occur, and systematically controlled and imitated across whales. We call these rubato and ornamentation. Second, we show that codas form a combinatorial coding system in which rubato and ornamentation combine with two context-independent features we call rhythm and tempo to produce a large inventory of distinguishable codas. Sperm whale vocalisations are more expressive and structured than previously believed, and built from a repertoire comprising nearly an order of magnitude more distinguishable codas. These results show context-sensitive and combinatorial vocalisation can appear in organisms with divergent evolutionary lineage and vocal apparatus.
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Second, we evaluated whether sequences of codas reflect longer-term trends. To do so, we collected coda triples of the same discrete coda type, and measured the correlation between tempo drift across adjacent pairs. We found a significant positive correlation, compared to a null hypothesis that drift between adjacent pairs is uncorrelated (test: Spearman’s rank-order correlation (two-sided), r(2586) = 0.57, p = 2e−220, 95% CI= [0.54, 0.60], n = 2588). Thus, rubato is distributed across sequences of multiple codas.Finally, we evaluated whether rubato is perceived and controlled by measuring whales’ ability to match their interlocutors’ coda durations when chorusing. We measured the average absolute difference in duration between (1) pairs of overlapping codas from different whales, and (2) pairs of non-overlapping codas of the same …
Second, we evaluated whether sequences of codas reflect longer-term trends. To do so, we collected coda triples of the same discrete coda type, and measured the correlation between tempo drift across adjacent pairs. We found a significant positive correlation, compared to a null hypothesis that drift between adjacent pairs is uncorrelated (test: Spearman’s rank-order correlation (two-sided), r(2586) = 0.57, p = 2e−220, 95% CI= [0.54, 0.60], n = 2588). Thus, rubato is distributed across sequences of multiple codas.Finally, we evaluated whether rubato is perceived and controlled by measuring whales’ ability to match their interlocutors’ coda durations when chorusing. We measured the average absolute difference in duration between (1) pairs of overlapping codas from different whales, and (2) pairs of non-overlapping codas of the same discrete coda type. Durations are significantly more closely matched for overlapping codas (0.099s on average) than would be expected under a null hypothesis that chorusing whales match only discrete coda type (which would give a drift of 0.129s on average) (test: permutation test (one-sided), p = 0.0001, n = 908; see Supplementary Section 6).
I wonder if calculating the autocorrelation of coda durations might be a nice complementary measure here. Autocorrelation could give you a sense of the time scale over which the rubatos decay and, seemingly, might also provide a sense for the timescale of longer-term trends.
Similarly, I wonder if cross-correlation might be useful for comparing the information quantity shared with interlocutors? The correlation value would be interesting, in addition to any patterns of temporal lag between codas. It might be a comprehensive metric for comparing the similarities of codas over time (as opposed to just looking at overlapping codas).
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