Evidence of social learning across symbolic cultural barriers in sperm whales

TL;DR

The paper investigates whether sperm whales exhibit social learning across socio-cultural boundaries by analyzing fine-grained vocal structure in codas. It introduces a subcoda-tree framework that uses discretized inter-click intervals (dICIs) and variable-length Markov chains (VLMCs) to capture rhythmic micro-variations within codas and define vocal style. Applying the method to Atlantic Dominica data and Pacific Ocean data shows that vocal styles partition whales into clans consistent with prior repertoire-based clans, and that non-ID codas become more similar across sympatric clans, implying cross-clan vocal learning. These findings support a two-component vocal identity (repertoire plus style) and provide a general framework for comparing communication systems and inferring cultural transmission in animals.

Abstract

We provide quantitative evidence suggesting social learning in sperm whales across socio-cultural boundaries, using acoustic data from the Pacific and Atlantic Oceans. Traditionally, sperm whale populations are categorized into clans based on their vocal repertoire: the rhythmically patterned click sequences (codas) that they use. Among these codas, identity codas function as symbolic markers for each clan, accounting for 35-60% of codas they produce. We introduce a computational method to model whale speech, which encodes rhythmic micro-variations within codas, capturing their vocal style. We find that vocal style-clans closely align with repertoire-clans. However, contrary to vocal repertoire, we show that sympatry increases vocal style similarity between clans for non-identity codas, i.e. most codas, suggesting social learning across cultural boundaries. More broadly, this subcoda structure model offers a framework for comparing communication systems in other species, with potential implications for deeper understanding of vocal and cultural transmission within animal societies.

Figures (4)

• Figure 1: Statistical modeling of subcoda structure in sperm whales. A Sperm whale communication consists of rhythmic sequences of clicks, called codas. A coda is specified by a sequence of inter-click intervals (ICIs). Codas are classified into types based on their rhythmic pattern, which can have various degrees of regularity (e.g., 4R2 vs 1+3). B Social groups of sperm whales employ specific vocal repertoires: the set of coda types they use and their associated usage frequencies. As an illustration, we show those of the EC1 and EC2 clans from the Dominica dataset bermant2019deep. Only the most numerous coda types are shown: the rest of the vocal repertoires consists of more coda types with residual frequencies. C The subcoda structure can be modeled by considering rhythmic variations within codas of the same coda type. To do so, codas are represented as sequences of discrete inter-click intervals (dICIs), by discretizing absolute ICIs into discrete bins, which can then be considered akin to symbols (e.g. A, B, C, ...), providing a tokenization of codas. Different instances of a single coda type can correspond to slightly different dICI sequences. The resulting dICI sequences are modeled using variable-length Markov chains, which can be represented as subcoda trees. These trees are built starting from the root node. Each node represents a dICI, and a dICI is a child of another in the tree if its presence in the affects the output probability of the parent, that is the probability of the parent depends on whether the child node has been observed previously in the sequence; see the Methods for a thorough explanation. These trees can be built for an individual speaker or for a group of speakers (here represented by colors), and capture the statistical properties of rhythm variations within codas and in the transitions between those. In other words, the tree captures a vocal style---how they say what they say. The vocal styles of different groups of sperm whales can be quantitatively compared by calculating a distance between their subcoda trees, which is a statistical distance between the dICI output distributions.
• Figure 2: Vocal style recovers social structure of vocal clans in Dominica sperm whales.A We show the similarity of vocal style, measured as subcoda tree-distance, among social units within a vocal clan (within, darker color shade) and between two clans (between, lighter color shade). We used the manual clan assignments from gero2016socially as ground truth. Vocal style is more similar within clans than between clans. B We show the hierarchical clustering of social unit subcoda trees. Each leaf corresponds to a social unit, and the colors below show their known clan assignments. The clustering recovers the two-clan structure observed in past work gero2016socially.
• Figure 3: Vocal style recovers social structure of vocal clans in Pacific Ocean sperm whales.A We show the similarity of vocal style, measured as subcoda tree-distance, between coda samples within a vocal clan (within, darker color shade) and between a clan and all others (between, lighter color shade). We used the vocal clans identified in hersh2022evidence. Vocal style is more similar within a clan than between clans. B We show the hierarchical clustering of subcoda trees. Each leaf corresponds to a coda sample, and the colors below show their vocal clan assignments (based on coda usage) from hersh2022evidence. We find generally good overlap between the groups obtained from clustering vocal style and those from vocal repertoire, with the exception of the Short clan (red) that is somewhat mixed with the Palindrome (orange) and Rapid Increasing (yellow) clans.
• Figure 4: Clan overlap influences non-ID coda vocal styles Comparing the similarities of different VLMC models fit for each Pacific Ocean clan for both ID and non-ID coda samples. The y-axis represents the measured distance between the subcoda trees, and the x-axis shows the geographical clan overlap (as calculated in hersh2022evidence ; see "Measuring clan overlap" in our Methods for details). Each point represents a pairwise comparison between two clans. The effect of overlap on ID coda vocal style similarity is minimal and non-significant while the opposite is true for non-ID codas: overlapping clans produce non-ID codas with a more similar vocal style. The VLMC distances are also typically much greater for ID codas than for non-ID codas. Note that while these results are visually opposite to those reported in Hersh et al. hersh2022evidence, they support the same final conclusions (see "Identity and non-identity codas show different trends" for details).