Fish Yawn Too: Why Did Animals Evolve This Ancient Behavior?
Humans yawn when they are drowsy, and so do dogs, cats, and monkeys. None of this seems particularly surprising. But once we broaden the view to vertebrates as a whole, yawning becomes much more puzzling. Birds yawn, salamanders yawn, and even fish living entirely underwater perform similar movements. If the purpose of yawning were simply to take a deep breath and replenish oxygen, the existence of yawning in fish would immediately complicate that intuitive explanation.
In mammals, a yawn typically begins with a gradual opening of the mouth accompanied by deep inspiration. This is followed by a brief period of peak muscular contraction, after which the jaw closes and expiration follows. Fish do not yawn in exactly the same way, but they also perform a stereotyped sequence in which the mouth gradually opens, remains maximally gaped for a short period, and then rapidly closes. Despite the similarities among these yawn-like behaviors across vertebrates, it remains unclear whether yawning in fish, amphibians, and mammals is fully homologous in evolutionary terms, and there is no reason to assume that it serves exactly the same function in every group.
One early explanation proposed that yawning increases oxygen supply. The problem is that experiments in humans that altered the concentrations of oxygen and carbon dioxide in inhaled air failed to change yawning frequency in the way this hypothesis predicts. Researchers exposed participants to air containing elevated carbon dioxide. If yawning were a response to inadequate gas exchange, intended to increase oxygen intake or eliminate excess carbon dioxide, then higher carbon dioxide levels should have induced more yawning. No clear increase was observed. Conversely, having participants breathe 100% oxygen did not suppress yawning.
Human fetuses also yawn before birth. Yawn-like stereotyped mouth-opening movements can already be observed relatively early in gestation, even though the fetus is not breathing air through its lungs and receives oxygen primarily through the placenta. This observation further weakens the idea that the main purpose of yawning is simply to increase oxygen uptake.
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