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Overtone-Based Pitch Selection in Hermit Thrush Song: Unexpected Convergence with Scale Construction in Human Music

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Overtone-Based Pitch Selection in Hermit Thrush Song: Unexpected Convergence with Scale Construction in Human Music Overtone-based pitch selection in hermit thrush song: Unexpected convergence with scale construction in human music Emily L. Doolittlea,1, Bruno Gingrasb,1, Dominik M. Endresc,d, and W. Tecumseh Fitchb,2 aDepartment of Music, Cornish College of the Arts, Seattle, WA 98121; bDepartment of Cognitive Biology, Faculty of Life Sciences, University of Vienna, Vienna A-1090, Austria; cTheoretical Neuroscience Group, Philipps University of Marburg, 35032 Marburg, Germany; and dSection for Computational Sensomotorics, Hertie Institute for Clinical Brain Research, Center for Integrative Neuroscience, Bernstein Center for Computational Neuroscience, and University Clinic Tübingen, 72076 Tübingen, Germany Edited by Dale Purves, Duke University, Durham, NC, and approved October 8, 2014 (received for review April 3, 2014) Many human musical scales, including the diatonic major scale the syrinx (in songbirds, the medial and lateral labia) generate prevalent in Western music, are built partially or entirely from a periodic source signal that is filtered by the air column within intervals (ratios between adjacent frequencies) corresponding to the trachea and mouth and then emitted to the environment. small-integer proportions drawn from the harmonic series. Scientists These principles are important in formulating various alternative have long debated the extent to which principles of scale generation hypotheses considered below. in human music are biologically or culturally determined. Data from Naturalists have long wondered whether birdsong could be animal “song” may provide new insights into this discussion. Here, said to have musical properties (11–13). However, early studies by examining pitch relationships using both a simple linear regres- on pitch selection tended to be anecdotal, based on a small sample sion model and a Bayesian generative model, we show that most size, or lacking in analytical rigor. Two more recent studies spe- songs of the hermit thrush (Catharus guttatus) favor simple fre- cifically comparing pitch selection in bird song and human musical quency ratios derived from the harmonic (or overtone) series. Fur- scales concluded that birdsong does not make preferential use of thermore, we show that this frequency selection results not from musical intervals found in commonly used Western musical scales physical constraints governing peripheral production mechanisms (14, 15). However, because these studies each only examined one but from active selection at a central level. These data provide the species [the white-throated sparrow (Zonotrichia albicollis)andthe most rigorous empirical evidence to date of a bird song that makes nightingale wren (Microcerculus philomela), respectively], a conclu- use of the same mathematical principles that underlie Western and sion that birdsong in general does not exhibit musical properties many non-Western musical scales, demonstrating surprising conver- seems premature. Indeed, other studies have shown preferential “ ” gence between human and animal song cultures. Although there use of consonant intervals in tropical boubou shrikes (Laniarius is no evidence that the songs of most bird species follow the over- aethiopicus)(16)andmusicianwrens(Cyphorhinus arada)(17), tone series, our findings add to a small but growing body of research although in the first case no rigorous statistical analysis was showing that a preference for small-integer frequency ratios is not presented. unique to humans. These findings thus have important implications Here, we investigated songs of the hermit thrush (Catharus for current debates about the origins of human musical systems and guttatus), a medium-sized North American songbird whose fa- may call for a reevaluation of existing theories of musical conso- mously “musical”-sounding song has attracted the attention of nance based on specific human vocal characteristics. ornithologists and musicians alike (18) but has not yet been subjected to detailed pitch analysis. Its songs are composed of music | birdsong | overtones Significance any human musical scales, including the diatonic major Mscale prevalent in Western music, are built partially or entirely from intervals (ratios between adjacent frequencies) The song of the hermit thrush, a common North American corresponding to small-integer ratios drawn from the harmonic songbird, is renowned for its apparent musicality and has series (1). A long-running debate concerns the extent to which attracted the attention of musicians and ornithologists for more than a century. Here we show that hermit thrush songs, principles underlying the structure of human musical scales de- like much human music, use pitches that are mathematically rive from biological aspects of auditory perception and/or vocal related by simple integer ratios and follow the harmonic series. production or are historical cultural “accidents” (2–4). The songs Our findings add to a small but growing body of research of nonhuman animals, such as birds or whales, potentially offer showing that a preference for small-integer ratio intervals is a valuable perspective on this debate. On the one hand, features not unique to humans and are thus particularly relevant to the of human music that are culturally bound, or dependent on ongoing nature/nurture debate about whether musical pre- specific characteristics of the human voice or auditory system, dispositions such as the preference for consonant intervals are should be absent in animal vocalizations. On the other hand, biologically or culturally driven. aspects of human music observed in the vocalizations of other species seem likely to be partially determined by general physical Author contributions: E.L.D. and W.T.F. designed research; E.L.D. and W.T.F. performed or biological constraints rather than solely by cultural practices. research; E.L.D., B.G., and D.M.E. analyzed data; and E.L.D., B.G., D.M.E., and W.T.F. wrote Such shared features would complement recent research sug- the paper. gesting that common motor constraints shape both human song The authors declare no conflict of interest. and that of some bird species (5). This article is a PNAS Direct Submission. The physical principles underlying vocal production in song- Freely available online through the PNAS open access option. birds are well understood (6–10) and do not differ fundamentally 1E.L.D. and B.G. contributed equally to this work. from those of other vertebrates. Sound is produced by tissue 2To whom correspondence should be addressed. Email: [email protected]. vibrations in the syrinx, a bird-specific organ located at the base of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the trachea. Flow-driven vibrations of fleshy membranes within 1073/pnas.1406023111/-/DCSupplemental. 16616–16621 | PNAS | November 18, 2014 | vol. 111 | no. 46 www.pnas.org/cgi/doi/10.1073/pnas.1406023111 Downloaded by guest on September 26, 2021 elements (the smallest unit of song construction, seen as con- A 6000 tinuous uninterrupted traces on spectrograms) that may exhibit either a variable pitch, such as trills and slides, or a stable pitch— pure, non-frequency-modulated, “flutelike” sounds. These stable sounds, which we refer to as “notes” (Fig. 1), are characterized by strong fundamental frequencies and very weak higher harmonics, 4000 making them ideally suited for an analysis of pitch relationships (15). Males typically sing 6–10 different song types, defined as nearly identical sequences of elements, durations, and frequen- cies. In a number of early- and mid-20th-century studies, hermit Frequency (Hz) 2000 thrush song was variously attributed with use of major, minor, and pentatonic scales (19, 20) and claimed to follow the overtone series (21). However, these early studies again suffered from small sample sizes and anecdotal reporting and were not based on rigorous acoustic analysis. More recent hermit thrush studies have 0 focused on regional differences and song-type ordering, rather 0 0.25 0.5 0.75 1 1.25 1.5 than pitch selection (22, 23). Time (s) Here we tested the overtone hypothesis, which predicts that the frequencies of the individual song notes are integer multiples B 6000 (harmonics) of an implied (but not actually sung) base frequency (hereafter fi). This hypothesis seems plausible because, unlike some previous claims, it does not attribute human-specific music- theoretical concepts to hermit thrush song. Moreover, the sub- 4000 jective impression of trained musicians listening to hermit thrush songs (played at one-sixth of the original speed to shift the speed and frequency of the songs into a range more suitable for human hearing) was that most notes indeed seemed to follow an over- tone series (see Fig. 2 and Audio File S1 for the corresponding Frequency (Hz) 2000 COGNITIVE SCIENCES sound example). However, determining whether a set of notes PSYCHOLOGICAL AND are harmonics of a frequency not present in the set requires a rigorous procedure to estimate and evaluate fi. To this end, we used two different statistical approaches, an ordinary least- squares regression model and a generative Bayesian estimator. 0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 Both approaches were used to test the hypothesis that a song is Time (s) { { an exchangeable sequence of frequencies that are integer mul- { Overtone Stacked tiples of some implied fi, versus the null hypothesis that songs are Ordered notes generated by drawing frequencies out of a random log-normal series overtones distribution (see Materials and Methods for details). By using a Fig. 2. Frequency distribution of a hermit thrush song compared with an Bayesian approach in addition to the least-squares regression overtone series. (A) Notes of a hermit thrush song. (B) The same notes rear- model we evaluate whether our analyses represent a rigorous test ranged in ascending order to show how they correspond to overtones 3, 4, 5, of our overtone hypothesis and not simply a post hoc explanation and 6 of an overtone series fitted to the frequencies corresponding to these notes (the complete stacked overtone series is shown on the right). 8000 that minimizes an error measure by “memorizing” the data.
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