As brains get bigger yawns get longer

New paper this week by Andrew Gallup and colleagues from SUNY Oneonta in Biology Letters. OK first of all, did you all know that one of the proposed functions of yawning is brain cooling?! I always thought it was increasing the amount of oxygen in the brain (debunked in 1987!), but some research has shown that yawning increases circulation to the skull to cool the brain down, arousing the brain and waking you up. In this week's paper the authors went on youtube and recorded the length of yawns of 24 different mammal taxa including foxes, elephants, camels, hedgehogs, walruses, and marmosets. They even have this gem of a quote: "one video with multiple yawning clips from a litter of kittens was excluded owing to the inability to distinguish between individuals". For each of these species they then used previously published data on brain size, encephalization quotient (how much larger the brain is than it is expected to be given the animal's body size), and number of cortical neurons. They found a strong correlation each of these brain size measurements and yawn duration. Primates have particularly high relative brain sizes and cortical neuron counts and accordingly had significantly longer yawn durations than other mammals. Interestingly, neither body size, skull size, nor jaw size predict yawn duration. The authors propose that longer yawns may be necessary to arouse larger brains, although the mechanism by which this might occur needs further study. What is novel about this paper is that there is consistent species variation in yawn length, and this correlation with brain size may be a potential explanation.