Dolphins breathe through their blowholes, not through their mouths. That's a well-established fact you'll find in any book describing dolphin biology and anatomy. But one little dolphin in New Zealand clearly hasn't done the reading. May we present the first documented case of a mouth-breathing dolphin.

Image: Steve Dawson/University of Otago

Stephen Dawson and his team of researchers from the University of Otago first observed the strange behaviour off New Zealand's Banks Peninsula back in 2014. A Hector's dolphin (Cephalorhynchus hectori), easily recognised by a small "tattoo" lesion near its blowhole, would jut out of the water at a steep angle while opening its mouth – as if gulping down air.

The same dolphin was seen on several occasions over the years, with researchers finally capturing its wacky surfacing behaviour on video in December of 2015. Close analysis of the footage showed that the animal never opened its blowhole when at the surface to breathe – instead, it seemed to suck in air through its mouth.

"[An] inspiration noise was clearly audible as the dolphin opened its mouth," noted Dawson and his team in an article published in Marine Mammal Science, detailing this abnormal behaviour. 

Could it be that the dolphin was, against everything everyone thought they knew, breathing through its mouth? "[T]he published literature states, and the prevailing view among marine mammalogists was, that this was impossible," notes Dawson in an interview with Hakai Magazine.

The progression of mouth breathing with the blowhole closed throughout. Image: Stephen Dawson/Marine Mammal Science

Dolphins evolved a breathing hole on the top of their head to make breathing in air while submerged less dangerous. During the course of evolution, the dolphin larynx changed its shape to form a kind of plug that reaches into the nasal cavity. This bizarre plug, sometimes called a goosebeak or epiglottic spout, tunnels right through the oesophagus and lodges snugly in the air passage leading to the blowhole, resulting in "complete separation of the respiratory and digestive tracts".

Scientists have long known that this laryngeal plug can be moved out of the way. Veterinarians needing to reach into a dolphin's stomach to retrieve accidentally swallowed objects will often fight against the plug as the dolphin tenses its muscles to keep it in place. Sometimes after a dolphin has been anaesthetised, it will take a couple of breaths through its mouth after the ventilation tube has been removed, before its respiratory muscles come back online to stick the plug back in position. And then there are examples of dolphins that choked to death when they shifted their laryngeal plug aside to swallow large fish. In one case, a pilot whale choked to death when a fish tried to escape through its blowhole.

Even though dolphins seem to have some muscular control over their larynx, it was widely believed that they couldn't – or at least wouldn't – attempt to move their laryngeal plug (and risk drowning) in order to breathe through their mouths. According to experts, shifting the plug is a risky behaviour that is used only as a last resort.

So why is this Hector's dolphin doing it?

Researchers aren't really sure. There might be something wrong with the muscles controlling its blowhole. Or perhaps there is a tumour or foreign object (maybe even parasites) stuck in its nasal passage. Whatever the problem, it seems the dolphin has lived this way for at least three years. So the ailment isn't new and the mouth-breathing workaround is likely permanent – and quite effective. "[The animal is] in great condition and looks absolutely normal, except for this weird breathing behaviour," Dawson tells New Scientist.

Whatever the reason behind this respiratory quirk, we can no longer claim that dolphins don't breathe through their mouths. One scrappy little Hector's dolphin has taken care of that. 

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Top header image: Stephen Dawson/Marine Mammal Science