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. 2020 Apr 30;10:7332. doi: 10.1038/s41598-020-64323-7

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Spectral regularities are a characteristic feature of fast amplitude modulated vocalizations (fAMVs). (A and B) Show the spectrograms and waveforms of one example fAMV and one slow modulated vocalization (sAMV, A and B, respectively). (C and D) Are the spectra of the same two example vocalizations. Note that the observed spectra (200-Hz frequency resolution) and “smooth” spectra are represented. Smooth spectra were calculated using a 5-point moving average. The difference between the observed and the smooth-spectra was used for harmonic-to-noise difference (HND) calculations. (E) Histograms of HND for fAMVs and sAMVs. The p-value of a Wilcoxon ranksum test and the d size-effect metric are provided. (F) Spectral autocorrelograms for the example fAMV and sAMV shown in (C and D). Note that spectral regularities (i.e. peaks spaced at regular distances) occurred in the example fAMV. In this example, the median inter-peak distance (MPID) was 1.6 kHz. (G) Median spectral autocorrelograms for all fAMVs and sAMVs studied (25th and 75th percentiles shown as shaded areas). Note the side- peaks occurring in the population of fAMVs. (H) Histogram of MPID for the fAMVs in which more than one peak could be detected in the spectral autocorrelogram. Median and interquartile range are (iqr) given.