Yawn Length Predicts Brain Weight & Cortical Neuron Number in Mammals

Yawning has been described as perhaps the least understood common human behavior. Now, arguably the world’s leading expert on yawning, Andrew C. Gallup, Ph.D., of the State University of New York (SUNY) at Oneonta, together with colleagues, has reported evidence that the mean and variance in yawn length are robust predictors of brain weight and cortical neuron number in mammals. In accordance with this, primates were found to generally have longer and more variable yawn durations compared with those of other mammals. In the new report, published online on October 4, 2016 in the Royal Society Biology Letters, the researchers state that the combined effects (mean and variation in yawn duration) “represent a striking scaling between brain and behavior,” with neither body size nor anatomical structures specific to yawning (cranium and mandible) driving these effects. As examples, they noted that gorillas, camels, horses, lions, walruses and African elephants all have shorter average yawns than humans. Furthermore, the authors noted that “having a larger skull does not necessitate more variable motor pattern duration. Instead, differences in yawn duration appear to be specifically linked to interspecies variation in brain size and complexity, with cortical neuron number being the most significant factor.” The authors noted that previous research has suggested that yawning is an adaptation to enhance intracranial circulation and brain cooling, which in turn could promote cortical arousal and state change.
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