Brevity is not a universal in animal communication: evidence for compression depends on the unit of analysis in small ape vocalizations

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.