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.
Highlights
IntroductionDespite the fact that animal calls fail to reach the complexity and production flexibility of human language, there are numerous anatomical (Fitch and Reby 2001; Fitch and Suthers 2016; Fitch 2018) and neural analogies (Jurgens 2009; Fitch and Suthers 2016; Fitch 2018) as well as fundamental communication principles shared between human and animal communicational systems (Marler 1998; Pika et al 2018)
Our results demonstrate a clear distinction between male and female hyrax vocal repertoires, with females producing more call types and using the shared call types in different proportions to those of males (Fig. 2)
It has been suggested that the higher vocal variability in red-capped mangabey (Cercocebus torquatus) females is related to their stronger social integration in comparison to males (Bouchet et al 2010)
Summary
Despite the fact that animal calls fail to reach the complexity and production flexibility of human language, there are numerous anatomical (Fitch and Reby 2001; Fitch and Suthers 2016; Fitch 2018) and neural analogies (Jurgens 2009; Fitch and Suthers 2016; Fitch 2018) as well as fundamental communication principles shared between human and animal communicational systems (Marler 1998; Pika et al 2018). A related “principle of least effort” (Hailman et al 1985) predicts an approximately monotonic relationship between word probability of occurrence and its rank of use (the frequency of a word decays as a power law of its rank; Cancho and Sole 2003) The reasoning behind these relationships is related to the economic compromise between information transfer and the articulatory effort associated with the production of sound (Hailman et al 1985; Manning and Schutze 1999; Cancho and Sole 2003). This is well demonstrated in the process of language evolution, as longer words are often shortened by speakers when in frequent use (Hernandez-Fernandez et al 2016; e.g., the word “television” has been abbreviated to TV)
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