The “Law of Brevity” in animal communication: Sex-specific signaling optimization is determined by call amplitude rather than duration

Abstract

The efficiency of informational transfer is one of the key aspects of any communication system. The informational coding economy of human languages is often demonstrated by their almost universal fit to Zipf's “Law of Brevity,” expressing negative relationship between word length and its usage frequency. Animal vocal systems, however, provided mixed results in their adherence to this relationship, potentially due to conflicting evolutionary pressures related to differences in signaling range and communicational needs. To examine this potential parallel between human and animal vocal communication, and also to explore how divergent, sex‐specific, communicational settings affect signaling efficiency within a species, we examined the complete vocal repertoire of rock hyraxes (Procavia capensis). As male and female hyraxes differ in their sociality levels and male hyraxes vocal repertoire is dominated by sexual advertisement songs, we hypothesized that sex‐specific vocal repertoires could be subjected to different signaling optimization pressures. Our results show that the sexes differ in repertoire size, call usage, and adherence to coding efficiency principles. Interestingly, the classic call length/call usage relationship is not consistently found in rock hyraxes. Rather, a negative relationship between call amplitude and call usage is found, suggesting that the efficiency of the vocal repertoire is driven by call amplitude rather than duration. We hypothesize that, in contrast to human speech that is mainly intended for short distance, the need for frequent long‐range signaling shapes an animal's vocal repertoire efficiency according to the cost of call amplitude rather than call length. However, call duration may be a secondary factor affecting signaling efficiency, in cases where amplitude is under specific selection pressures, such as sexual selection.

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

“…Therefore, it is possible that we did not find support for Zipf's Law of abbreviation because selective forces which oppose compression, and maximize transmission in noisy environments, were more important in shaping male solo phrase organization. As Demartsev et al [39] royalsocietypublishing.org/journal/rsos R. Soc. Open Sci.…”

“…But, Zipf's Law of abbreviation was not found in the vocal repertoire of two new world monkeys: common marmosets (Callithrix jacchus) and golden-backed uakaris (Cacajao melanocephalus) [37], ravens (Carvus corax) [38] or the full body gestures of chimpanzees [28] which may be related to the small repertoire sizes in these animals, or differences in function or context of use [37]. A recent study on rock hyraxes (Procavia capensis) found support for Zipf's Law of abbreviation in male but not female vocal repertoires, and also found a negative relationship between call amplitude and call usage, and the authors propose that for long-distance communication, costs of call amplitude may be more important than call duration [39].…”

Adherence to Menzerath's Law is the exception (not the rule) in three duetting primate species

“…Three laws, in particular, have garnered a substantial amount of interest: Zipf's Law (wherein the most commonly used word will occur approximately twice as often as the next most common word [8]), Zipf's Law of Abbreviation (elements used more frequently in a communication system are shorter [8]) and Menzerath's Law (the greater the whole, the smaller its constituents [9]). There has been increasing interest in applying these statistical laws outside of human language [10][11][12][13][14][15][16][17][18][19][20][21]. These statistical laws reflect compression, and it has been proposed that compression is a universal principle not just in vocal communication but in behaviour more broadly [1].…”

“…The authors attributed lack of conformity to Zipf's Law in one chickadee species (but not the other) to differences in social complexity [ 14 ]. One of the first-documented tests of Zipf's Law of Abbreviation was also in black-capped chickadees [ 14 ], and subsequent analyses showed adherence to Zipf's Law of Abbreviation in dolphin surface behaviours [ 25 ], Formosan macaque vocalizations [ 16 ], the short-range vocalizations of four bat species [ 17 ], a subset of chimpanzee gestures [ 12 ], male (but not female) rock hyrax vocalizations [ 18 ], penguin vocalizations [ 19 ], and in note [ 20 ] but not phrase usage [ 21 ] in male gibbon solos. Adherence to Zipf's Law of Abbreviation was not seen in the vocal repertoire of two New World monkeys (common marmosets and golden-backed uakaris [ 15 ]), ravens [ 6 ] or the full-body gestures of chimpanzees [ 12 ].…”

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.…”