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Abstract
The study of animal vocal signals can either focus on the properties of distinct vocal elements or address the signal as a whole. Although some attention has been given to the continuous progression patterns of bird songs, such patterns in mammalian vocalisations have been largely overlooked. We examined temporal changes in structural and acoustic parameters in male rock hyrax songs. We found a gradual increase in call frequency and amplitude towards the song ending, as well as an abrupt increase in bout syntactic complexity, peaking in the last quintile of a song. In musical terms, such a pattern can be described as a crescendo (amplitude increase) with a terminal climax. In Western music, crescendos are used to maintain attention and direct the listeners towards a memorable highpoint of the musical piece. This structure may have an analogous function in animal communication, recruiting audience attention towards the climactic and potentially most informative part of the signal. Our playback experiments revealed that hyrax males tend to reply more to songs with a climactic ending, indicating that this progression pattern is important for hyrax communication. We suggest that animal vocal communication research can benefit from adding musical concepts to the analysis toolbox.
Highlights
Animal acoustic signals often form long and complex sequences[1], traditionally most of the research on animal song structure has focused on classifying the signal into distinct units
There is an increasing interest in such signal properties and several attempts have been made to assess the continuous parameters of vocalisation[3,4,5,6], this approach is still far from being a common practice in animal vocalisation research
Out of the 140 songs used, 60% showed an ascending progression pattern towards the song ending, 30.7% of songs did not show significant change, and 9.3% of songs showed a significant decrease in bout duration and/or entropy rate towards the song ending
Summary
Animal acoustic signals often form long and complex sequences[1], traditionally most of the research on animal song structure has focused on classifying the signal into distinct units (syllables) This allowed easy quantitative analysis of repertoire size, singing versatility and specific component production[1, 2]. In the thrush nightingale (Luscinia luscinia) a song’s temporal structure resembling an accelerando-like rhythm acceleration was found to progress through several song phrases towards a glissando (high-pitch sweep) finish This gradual build-up towards culmination, manipulation of rhythmic timing and amplitude, could function to maintain the attention of receivers[2] and avoid signal habituation[14]. Siamang (Hylabates syndactylus) mated pairs produce long and stereotyped duets that include multiple different phrases and male-female transitions[17]
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