Abstract (eng)
Yawning is a basic mechanism, ubiquitous among vertebrate species. Yet, its function remains a much-debated topic. It has been suggested that yawning serves a cooling function for the brain. In line with this hypothesis, previous studies on mammals have found a positive correlation between both inter- and intra-specific variation in mean yawn duration and neural anatomy, with brain weight and total and cortical neuron numbers indeed providing a robust prediction of yawn duration. So far, however, these studies were run on relatively small samples, and have been restricted to mammals only. To investigate whether these results can be generalized across mammals and endothermic vertebrates, I examined the yawn duration of a large sample of mammal and bird species. Therefore, 811 yawns of 507 individuals across 60 mammalian species and 669 bird yawns from 271 individuals across 46 avian species were coded, and the duration of this action pattern was linked to brain measures of these species. These analyses showed that total neuron number, pallium neuron number and brain weight of both mammals and birds are highly significant positive predictors of the mean yawn duration. Consequently, my results provide support for the brain-cooling hypothesis, as variation in neural anatomy seems to be directly proportional to the yawn duration that may be necessary for cooling it. Moreover, these data suggest that yawn duration can be used as a marker of interspecific differences in brain measures across vertebrates.