The animals at Schönbrunn Zoo are part of the world’s most comprehensive yawning study. This is a cooperative effort including researchers from Austria, the Czech Republic, Switzerland, the USA and The Netherlands. Margarita Hartlieb, who conducted this research in the framework of her Master degree at the University of Vienna, was one of the main authors.
Several years ago the so-called brain cooling hypothesis was proposed. The contraction of muscles surrounding the oral cavity when breathing in cool air promotes the flow of cooler blood to the brain. According to the hypothesis underlying this study, animals with larger brains and more neurons, i.e. higher brain activity, would need to yawn longer to gain the same cooling effects.
Yawning is an important neurophysiological function. This study was also the first to examine yawning in birds.
Yawning is apparently conserved throughout many animal groups. Accordingly, its evolutionary origins must go back to the common ancestors of birds and mammals, potentially even further back in history.
In this study the researchers analyzed videos of yawning mammals and birds. The recordings stem either from various online sources, were made available by colleagues or by zoological gardens, or were taken by the study authors themselves in zoos and research institutions. Zoos and their enormous species diversity can play a significant role in studies such as these. This information would be almost impossible to collect in the wild.
Overall, the lengths of 1291 yawns were evaluated in relation to the brain size of the respective animal species. The study encompassed 697 different animals from 101 species (55 mammal and 46 bird species).
This study established a clear correlation of brain mass and neuron number with the duration of yawning. The results were valid for both mammals and birds and confirm the hypothesis that yawning developed as a cooling mechanism for the brain.
Massen, J.J.M., Hartlieb, M., Martin, J.S. et al. Brain size and neuron numbers drive differences in yawn duration across mammals and birds. Commun Biol 4, 503 (2021). https://doi.org/10.1038/s42003-021-02019-y