AP Biology class blog for discussing current research in Biology

Yawning and Brain Size


Recently, scientists discovered a correlation between yawning and brains: the longer the average duration of a specie’s yawn, the bigger that specie’s brain size,  as measured by brain weight and total number of cortical neurons.

The study was conducted on 109 individuals from across 19 different species, including cats, humans, mice, camels, and more. The investigators found that the duration of yawns was shortest in mice, who averaged 0.8 seconds, and longest in humans, who averaged 6.5 seconds. The scientists plan on investigating whether this correlation holds true amongst individual members of a species.

The study was created in response to the ideas set forth in Gallup’s 2007 paper on the thermoregulatory theory of yawning, one of the strongest theories about why we yawn (we do not yet definitively know the biological purpose of yawning). The thermoregulatory theory indicates that yawning cools down the brain in homeotherms via three potential mechanisms. But whether or not this brain-cooling is simply a side effect or the primary function of yawning is up for debate.

Based on Gallup’s paper, the investigators of this study hypothesized that longer yawns would produce greater physiological responses, in terms of blood flow and circulation to the brain– which would be evolutionarily necessary for species with larger, more complex brains.

There are other theories about why we yawn, such as a 2014 paper stating that yawning stimulates cerebrospinal fluid circulation, which in turn increases species’ alertness. A common theory that yawning increases blood oxygen levels has largely been disproved. How would such alternate theories have different implications for the discovered correlation between yawning and brain size?

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  1. oxergin

    I think that yawning is a very interesting unknown in the world of science. I think that it is also important to note that a species brain size has no affect of the animals level of intelligence. People may assume that the longer a animal yawns, the smarter it is in comparison to other species. That is not the case, the size of the brain does not determine intelligence.

  2. evansymes

    Everyone knows that seeing other people yawning makes you more likely to feel the need to yawn yourself. I wonder if reading about other people yawning has the same effect. It didn’t happen while I was reading this though because nothing gets my heart pumping like some bioquakes blog action. I think its interesting that we don’t know the biological function of yawning. I wonder whether its social, to indicate to others without formal language that we’re tired. Maybe its a vestige mechanism we inherited from some previous ancestor species that served some purpose to them. I think it’s interesting that we don’t know so much, especially about our own biology. Similar to this is that scientists don’t actually know why we sleep. It’s true, here is the link.

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