Yawning is a widespread, involuntary behavior observed across many animal species, often associated with transitions or changes in alertness. While humans commonly associate yawning with tiredness or boredom, the behavior involves a distinctive mouth opening. This raises an interesting question: do fish also exhibit a similar behavior, and if so, what might it signify for them?
Do Fish Truly Yawn?
Fish exhibit a behavior that visually resembles yawning, characterized by a wide opening of the mouth. Some species stretch their mouths impressively before rapidly closing them; for instance, a frogfish can expand its mouth up to 12 times its original volume. This mouth-gaping behavior has been observed in various fish species, including sabretooth blennies, Port Jackson sharks, and common goldfish.
While the movement looks similar to a human yawn, its underlying purpose in fish often differs. Scientists describe the fish “yawn” as a gradual mouth gaping, held open for at least three seconds, followed by a quick closure. This distinct action differentiates it from regular breathing, where the mouth opens and closes more rapidly.
Why Fish “Yawn”
The reasons behind this “yawning” behavior in fish are varied and often differ significantly from human motivations. One proposed function is jaw realignment, particularly after feeding or extending the mouth to forage. Some fish, like the grey morwong, may “yawn” to reposition their jaws after pushing their mouths into the sand for food, ensuring their feeding apparatus remains in optimal condition.
The behavior can also serve as a form of communication or display. For example, a male cardinalfish might yawn more frequently and widely during courtship as spawning approaches. In aggressive contexts, fish may “yawn” as a territorial display to warn off intruders, exposing teeth or making themselves appear larger.
Another hypothesis suggests “yawning” can be a response to environmental conditions or a way to maintain gill health. Observations link increased “yawning” to low oxygen levels or poor water quality, indicating fish might be struggling to absorb enough oxygen. This mouth-gaping action might also help dislodge parasites or debris from gills, or signal to cleaner fish that it is safe to enter the mouth for cleaning.
Comparing Fish and Human Yawning
While fish and humans both exhibit a behavior termed “yawning,” the underlying mechanisms and evolutionary purposes are largely distinct. In humans, yawning is often associated with fatigue, boredom, or stress, and some theories suggest it helps regulate brain temperature or promotes alertness. It can also be socially contagious, where seeing or hearing someone yawn can trigger a yawn in another individual, a phenomenon linked to empathy and social synchronization.
Conversely, fish “yawning” rarely indicates tiredness or boredom. Instead, it appears to serve more practical functions related to their aquatic environment and survival. Fish do not breathe air or regulate body temperature in the same way humans do, so the brain-cooling or oxygen intake theories for human yawning do not directly apply. The primary functions in fish revolve around jaw maintenance, territorial displays, or gill flushing.
Despite these differences, there are some intriguing parallels. Both fish and human yawning involve a wide mouth opening and can be linked to changes in physiological state or preparation for action. Recent research has even identified contagious yawning in zebrafish, where fish were more likely to “yawn” after watching another fish yawn, suggesting a form of social synchronization even in a cold-blooded species. This indicates that while the specific triggers and outcomes differ, the fundamental concept of a widespread, reflex-like mouth-gaping behavior is conserved across diverse vertebrate groups.