The span of correlations in dolphin whistle sequences

TL;DR

This study investigates the extent of long-range correlations in dolphin whistle sequences, revealing significant correlations up to the 4th previous whistle, which suggests complex communicative structures beyond simple random models.

Contribution

It demonstrates that dolphin whistles exhibit statistically significant long-range correlations, challenging simple random or non-linguistic explanations for Zipf's law in dolphin communication.

Findings

Correlations extend back to the 2nd previous whistle (global test).

Correlations extend back to the 4th previous whistle (local test).

Results are inconsistent with simple random or non-linguistic models.

Abstract

Long-range correlations are found in symbolic sequences from human language, music and DNA. Determining the span of correlations in dolphin whistle sequences is crucial for shedding light on their communicative complexity. Dolphin whistles share various statistical properties with human words, i.e. Zipf's law for word frequencies (namely that the probability of the $i$th most frequent word of a text is about $i^{-\alpha}$) and a parallel of the tendency of more frequent words to have more meanings. The finding of Zipf's law for word frequencies in dolphin whistles has been the topic of an intense debate on its implications. One of the major arguments against the relevance of Zipf's law in dolphin whistles is that is not possible to distinguish the outcome of a die rolling experiment from that of a linguistic or communicative source producing Zipf's law for word frequencies. Here we show that statistically significant whistle-whistle correlations extend back to the 2nd previous whistle in the sequence using a global randomization test and to the 4th previous whistle using a local randomization test. None of these correlations are expected by a die rolling experiment and other simple explanation of Zipf's law for word frequencies such as Simon's model that produce sequences of unpredictable elements.