By TOMOHIRO YAMAMOTO/ Staff Writer
December 4, 2023 at 08:00 JST
Yawn-like behavior in a wild Indo-Pacific bottlenose dolphin (Provided by Mie University)
TSU--Dolphins yawn, even in the wild seas, a study showed.
A group of researchers with Mie University here analyzed 1,816 hours of video recordings and spotted free-ranging Indo-Pacific bottlenose dolphins yawning underwater around Mikurajima island in the Izu island chain south of Tokyo.
The study, led in part by Akiko Enokizu, a student at the Mie University Graduate School of Bioresources, was a follow-up to the team’s research project that found captive common bottlenose dolphins yawning at the Minami-Chita Beach Land aquarium in Aichi Prefecture．
The video recordings, owned by Mikurajima island’s tourism association, showed that dolphin yawning is not “captive-specific,” the team said.
In 94 cases, the dolphins exhibited open-mouth behavior, but most of those movements were directed toward other dolphins.
However, in five cases, the action showed specific characteristics of yawning: a slow and wide opening of the mouth, maximum gaping, and a quick closing of the mouth.
The five cases all occurred in the morning and usually when the dolphins were resting.
Some of the yawning episodes in the captive common bottlenose dolphins were followed by increased levels of activity, the researchers said.
The team also saw a captive dugong yawning at Toba Aquarium in Mie Prefecture, usually when it appeared drowsy.
Yawning has also been seen in monkeys, birds, frogs and other species. Those animals are believed to yawn to inhale oxygen that reaches their brains and to chase away drowsiness.
Dolphins and dugongs emerge on the water surface to breathe because they cannot do so underwater. The team’s studies show that the animals’ underwater yawning does not involve breathing.
“A wide opening of the mouth in itself could arouse activity,” said team member Tadamichi Morisaka, a professor of cetacean research with the Mie University Graduate School of Bioresources.
Morisaka said the definition of yawning should be modified and called for future studies focusing on open-mouth movements.
He said he plans to do comparative studies on whales and other species.
Enokizu is currently a visiting researcher with Tokyo Metropolitan University.
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