Musical qualities in the songs of African clawed frogs: phylogenetic signals and laryngeal mechanisms
Columbia University in the City of New York, Biological Sciences, USA
University of California at Berekely, Department of Environmental Science, Policy and Management, USA
Of Niko Tinbergen's four questions for ethology, the evolutionary origins of behaviors have proven the most challenging. We have approached this question by comparing the qualities of courtship songs across species of African clawed frogs. Male advertisement songs in the three major clades of Xenopus are readily distinguishable by temporal and spectral qualities. The temporal features of advertise songs diverge markedly within a clade suggesting a role in species identification (Tobias, Evans and Kelley, 2011). In contrast, we report here that certain spectral features of songs are conserved within clades but differ between clades. Each click in male advertisement songs includes two, simultaneously produced dominant frequencies. It is the ratio of the higher (DF2) to the lower (DF1) frequency that is clade-characteristic and these ratios form consonant musical intervals. The ratios for the three major clades are 1.33 (perfect fourth musical interval, F), 1.21 to 1.25 (minor to major thirds, T) and 2.00 (octaves, O). Within the F clade, while DF2 and DF1 differ between species as might be expected from genetic drift, the ratios do not. Thus frequency ratios are under stringent selection. Using the vox in vitro preparation (Tobias and Kelley, 1986), in which the isolated vocal organ produces clicks following nerve stimulation, we determined that DF2 and DF1 (and the resulting DF2/DF1), match frequencies and ratios present in actual songs. These acoustic characters are thus intrinsic features of the vocal instrument. Using laser vibrometry and simultaneous sound recording in this preparation, we determined that sounds are initiated by one laryngeal cartilage component, the arytenoid disks, while clade-specific ratios are due to the presence of elastic cartilage, another laryngeal component. Disrupting elastic cartilage shifts the spectral features of clicks while disrupting hyaline cartilage has no effect on frequency but does shorten click duration. Darwin (1871) famously observed “As neither the enjoyment nor the capacity of producing musical notes are faculties of the least direct use to man in reference to his ordinary habits of life, they must be ranked among the most mysterious with which he is endowed.” That courtship songs of non-human species, such as birds, whales and gibbons, also evince musical qualities suggests that musical features might contribute to reproductive success. The presence of characteristic, DF2/DF1 ratios in songs from all three major Xenopus clades suggests an ancient reproductive advantage for consonant musical intervals.
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Tobias, M. and Kelley, D. 1987. Vocalizations of a sexually dimorphic isolated larynx:
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Tobias,M.L., Evans, B. and Kelley, D.B. 2011. Evolution of advertisement calls in African
clawed frogs. Behaviour 148, 519 - 549.
Tenth International Congress of Neuroethology, College Park. Maryland USA, USA, 5 Aug - 10 Aug, 2012.
Invited Symposium (only for people who have been invited to a particular symposium)
(2012). Musical qualities in the songs of African clawed frogs: phylogenetic signals and laryngeal mechanisms.
Front. Behav. Neurosci.
Tenth International Congress of Neuroethology.
01 May 2012;
07 Jul 2012.
Prof. Darcy B Kelley, Columbia University in the City of New York, Biological Sciences, New York, NY, 10027, USA, firstname.lastname@example.org