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. 2021 May 6;4:503. doi: 10.1038/s42003-021-02019-y

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© The Author(s) 2021

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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Fig. 1 — a Video sequence of a yawn from both a mammal (Banded mongoose) and a bird (Emu), which demonstrate the fixed action pattern as described by Barbizet¹, including the twitching of the eyes in the banded mongoose and the closing of the nictitating membrane in the emu. b Images of several species in our sample mid-yawn. Above: Mammals, from left to right: Chimpanzee (Pan troglodytes), Ring-tailed lemur (Lemur catta), Southern sea lion (Otaria flavescens), Lion (Panthera leo), and Capybara (Hydrochoerus hydrochaeris); below: Birds, from left to right: Greylag goose (Anser anser), Common raven (Corvus corax), Snowy owl (Nyctea scandiaca), Diamond dove (Geopelia cuneata), and Budgerigar (Melopsittacus undulatus).