Abstract
Evidence for compression, or minimization of code length, has been found across biological systems from genomes to human language and music. Two linguistic laws—Menzerath's Law (which states that longer sequences consist of shorter constituents) and Zipf's Law of abbreviation (a negative relationship between signal length and frequency of use)—are predictions of compression. It has been proposed that compression is a universal in animal communication, but there have been mixed results, particularly in reference to Zipf's Law of abbreviation. Like songbirds, male gibbons (Hylobates muelleri) engage in long solo bouts with unique combinations of notes which combine into phrases. We found strong support for Menzerath's Law as the longer a phrase, the shorter the notes. To identify phrase types, we used state-of-the-art affinity propagation clustering, and were able to predict phrase types using support vector machines with a mean accuracy of 74%. Based on unsupervised phrase type classification, we did not find support for Zipf's Law of abbreviation. Our results indicate that adherence to linguistic laws in male gibbon solos depends on the unit of analysis. We conclude that principles of compression are applicable outside of human language, but may act differently across levels of organization in biological systems.
Highlights
Brevity is not a universal in animal communication: evidence for compression depends on the unit of analysis in small ape vocalizations Dena J
Human language has been considered unique among communication systems as it contains semantics, wherein sounds are combined in unique ways to confer meaning [2], but there are cases wherein animal vocalizations have been linked to particular behavioural contexts [3] and semantics in non-human communication remains a topic of debate [4]
We identified putative phrase types within male solos using affinity propagation clustering, and verified cluster solutions based on silhouette coefficients
Summary
Mueller’s Bornean gibbons (H. muelleri; hereafter gibbons) are pair-living hominoids found on the island of Borneo [40]. Both mated and unmated males engage in early morning solos [71] typically between the hours of 5.00–7.00 local time [72], with the solo performances of mated and paired males being indistinguishable to the human observer [71]. The smallest possible unit is termed a ‘note’ [73], and we defined ‘phrases’ as sequences of notes that are separated by breaks that are 2 s in duration or more, following Inoue et al [41]. The majority of analyses of gibbon solos have focused on quantifying patterns in various note types [29,41,49,51], but a few have done phrase-level analyses [52,64]
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