Overtone-based pitch selection in hermit thrush song: Unexpected convergence with scale construction in human music

Abstract

Many human musical scales, including the diatonic major scale prevalent in Western music, are built partially or entirely from intervals (ratios between adjacent frequencies) corresponding to small-integer proportions drawn from the harmonic series. Scientists have long debated the extent to which principles of scale generation in human music are biologically or culturally determined. Data from animal "song" may provide new insights into this discussion. Here, by examining pitch relationships using both a simple linear regression model and a Bayesian generative model, we show that most songs of the hermit thrush (Catharus guttatus) favor simple frequency ratios derived from the harmonic (or overtone) series. Furthermore, we show that this frequency selection results not from physical constraints governing peripheral production mechanisms but from active selection at a central level. These data provide the most rigorous empirical evidence to date of a bird song that makes use of the same mathematical principles that underlie Western and many non-Western musical scales, demonstrating surprising convergence between human and animal "song cultures." Although there is no evidence that the songs of most bird species follow the overtone series, our findings add to a small but growing body of research showing that a preference for small-integer frequency ratios is not unique to humans. These findings thus have important implications for current debates about the origins of human musical systems and may call for a reevaluation of existing theories of musical consonance based on specific human vocal characteristics. music | birdsong | overtones M any human musical scales, including the diatonic major scale prevalent in Western music, are built partially or entirely from intervals (ratios between adjacent frequencies) corresponding to small-integer ratios drawn from the harmonic series (1). A long-running debate concerns the extent to which principles underlying the structure of human musical scales derive from biological aspects of auditory perception and/or vocal production or are historical cultural "accidents" (2-4). The songs of nonhuman animals, such as birds or whales, potentially offer a valuable perspective on this debate. On the one hand, features of human music that are culturally bound, or dependent on specific characteristics of the human voice or auditory system, should be absent in animal vocalizations. On the other hand, aspects of human music observed in the vocalizations of other species seem likely to be partially determined by general physical or biological constraints rather than solely by cultural practices. Such shared features would complement recent research suggesting that common motor constraints shape both human song and that of some bird species (5).The physical principles underlying vocal production in songbirds are well understood (6-10) and do not differ fundamentally from those of other vertebrates. Sound is produced by tissue vibrations in the syrinx, a bird-specific organ located at the b...

A biological rationale for musical consonance

“…Finally, several studies have explored consonance in nonhuman animals, so far with inclusive and sometimes perplexing results (54)(55)(56)(57)(58). For example, male hermit thrush songs comprise tones with harmonically related fundamental frequencies, despite the fact that their vocalizations do not exhibit strong harmonics (58).…”

“…For example, male hermit thrush songs comprise tones with harmonically related fundamental frequencies, despite the fact that their vocalizations do not exhibit strong harmonics (58). The authors argue that rather than attraction to conspecific vocalization, small-integer ratios may be more easily remembered or processed by the auditory system, an idea for which there is also some support in humans (50).…”

Preface to the Special Issue on Animal Music Perception

“…However, the universals we find across cultures, such as discrete pitches and the use of pitch intervals with simple interval ratios (Brown & Jordania, 2011;Burns, 1981;Carterette & Kendall, 1999), are sometimes also found in isolation in other species (e.g., Cator, Arthur, Harrington, & Hoy, 2009;Doolittle & Brumm, 2013;Doolittle, Gingras, Endres, & Fitch, 2014). Because pitch plays a central role in the discussion of human music universals, it is the focus of the current special issue on animal music perception (see preface in this issue for an introduction).…”

Interval singing links to phenotypic quality in a songbird

“…Among the very few bird species analyzed so far, the melodic songs of the hermit thrush (Catharus guttatus) are composed of flute-like sounds of stable pitch, whose frequency relationships form small-integer ratios based on the harmonic (overtone) series (5). Similarly, musician wrens (Cyphorhinus arada) use intervals with small-integer frequency ratios of 1:2 (octave), 2:3 (fifth), and 3:4 (fourth) (6).…”

“…Furthermore, if the precision in the use of intervals and singing performance has been under selection imposed by receivers, the receiver also requires the sensory and neuronal capacities to judge the precision of the sung intervals. On the basis of the mentioned theoretical and empirical studies (2,3,5,6), not only for the producer but also for the receiver of a signal aiming to deduce information about phenotypic qualities of a singer, the most logical and the only natural reference intervals should thus be based on small-integer frequency ratios derived from the harmonic series. Most important, such a yardstick does not require the use of human-specific concepts of music theory or practice (5), as in the many scales used in Western and non-Western music based on forms of tempered tuning and invented for the needs of performing orchestral music and modulating in all keys.…”