Network analysis reveals underlying syntactic features in a vocally learnt mammalian display, humpback whale song

Abstract

Vocal communication systems have a set of rules that govern the arrangement of acoustic signals, broadly defined as ‘syntax’. However, there is a limited understanding of potentially shared or analogous rules across vocal displays in different taxa. Recent work on songbirds has investigated syntax using network-based modelling. This technique quantifies features such as connectivity (adjacent signals in a sequence) and recurring patterns. Here, we apply network-based modelling to the complex, hierarchically structured songs of humpback whales ( Megaptera novaeangliae ) from east Australia. Given the song's annual evolving pattern and the cultural conformity of males within a population, network modelling captured the patterns of multiple song types over 13 consecutive years. Song arrangements in each year displayed clear ‘small-world’ network structure, characterized by clusters of highly connected sounds. Transitions between these connected sounds further suggested a combination of both structural stability and variability. Small-world network structure within humpback songs may facilitate the characteristic and persistent vocal learning observed. Similar small-world structures and transition patterns are found in several birdsong displays, indicating common syntactic patterns among vocal learning in multiple taxa. Understanding the syntactic rules governing vocal displays in multiple, independently evolving lineages may indicate what rules or structural features are important to the evolution of complex communication, including human language.

Shannon entropy as a robust estimator of Zipf's Law in animal vocal communication repertoires

“…Any communicative channel must increase in complexity if it is to contain more information, but complex communication places higher cognitive and motor demands on the animal, both for meaningful signal production, and accurate signal interpretation (Fedurek et al., 2017; Seyfarth & Cheney, 2010). There is considerable evidence that many animal species balance these trade‐offs according to empirical laws of information encoding, such as the Menzerath‐Altmann law (Gustison & Bergman, 2017; Gustison et al., 2016; Heesen et al., 2019), Zipf's law of brevity/abbreviation (Demartsev et al., 2019; Semple et al., 2010) and Zipf's law of least effort (this study; Allen et al., 2019; Ferrer‐i‐Cancho & McCowan, 2009). In itself, conforming to such laws does not indicate that an animal possesses true linguistic abilities, rather, that the evolutionary history of the species has had to contend with conflicting pressures of information content and cognitive‐motor constraints.…”

Song Morphing by Humpback Whales: Cultural or Epiphenomenal?

“…Different phrases or components of phrases called “subphrases,” morphed in different ways and at different rates. Nevertheless, they described all of these changes as appearing “to follow set rules of progressive change.” Other more rapid changes in humpback whale songs that occur across years (referred to as “song revolutions,” see Noad et al, 2000 ; Allen et al, 2018 ) may also involve morphing of phrases ( Garland et al, 2017 ; Allen et al, 2019 ). However, because revolutions were identified based on comparisons of symbolic transcriptions of songs rather than through direct acoustic comparisons of units, it is difficult to evaluate whether new phrases were morphs of earlier phrases.…”

Animal Linguistics: A Primer

“…String edit distance methods (e.g., Kershenbaum et al, 2012;Kershenbaum & Garland, 2015) can allow long sequences of signals to be compared to detect underlying structure. Hierarchical structure can be investigated using a number of different computational tools, like entropy estimators (Suzuki, Buck, & Tyack, 2006), network analyses (Allen et al, 2019), Markovian processes (Sainburg et al, 2019) and quantification of clustering events (Kello et al, 2017), for example. performed by the recipient that results in cessation of signalling by the signaller.…”