Popular Science: Why do we yawn (longer than birds)?
Yawning is a stereotypical activity that we cannot willingly control; therefore, we can’t stop yawning even during a lecture in a dark, hot room. The hot room is precisely what is needed to trigger a yawn, given that the most probable function of this phenomenon is to cool our brain. Long muscle contraction and long inhale draw warm blood away from the skull and pump cooler blood in. This theory also foreshadows the connection between the duration of the yawn with the brain size and number of neurons, because a larger brain needs more intense cooling. In this study, the link has been confirmed for birds and mammals.
However, there is one major difference between birds and mammals: the latter yawn much longer, although the brain size of both is similar. This in fact only verifies the hypothesis that yawning is a mechanism of thermoregulation. Bird blood is 2 °C warmer than that of mammals; hence, it exchanges heat faster. Birds also have beaks which serve as an effective thermoregulatory organ, owing to their direct connection to nasal and oral cavities. The size of the beak also plays an important role, which was confirmed by comparing the results for toucans (birds with a large beak) and owls or parrots – birds with small beaks. Toucans lose as much as 400% of produced heat through their beak. The production of heat is also crucial. Bird neurons are considerably less energetically expensive – on average – than mammal neurons. For example, a pigeon’s neuron produces three times less heat than an average mammal neuron. Accordingly, birds yawn significantly less than mammals, notwithstanding the similar brain size both have.
The exception to the rule the naked mole rat (Heterocephalus glaber). These rats yawn almost as much as jaguars, but their brain is 300 times smaller. This small rodent is exceptional in many ways; for instance, it has only a limited ability to regulate its body’s temperature. These rats mainly use the temperature of their surroundings, shaking or yawning for thermoregulation. Naked mole rats neither have sweat glands, nor produce more saliva or pant while hot. Yawning is therefore the only way left for them to cool down.
The entire study suggests that yawning, as a reflex, is common to all mammals and birds and can be traced back to at least their common ancestor. The connection between the duration of a yawn and the brain size, with other thermoregulatory factors taken into account, confirms the theory that yawning functions as a cooling mechanism, principally 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