Zoologger: The turtle that urinates through its mouth

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Pelodiscus sinensis
Habitat: Throughout Southeast Asia, though demand for farmed turtles has impacted the wild population

Everyone knows the feeling of desperately needing to go to the loo. Bouncing from one foot to the other, you hare around in search of an appropriate facility, praying all the while that it won't be occupied.

When the Chinese soft-shelled turtle needs to urinate, its approach is a little different. It goes in search of a puddle, and dunks its head under the surface.

That's because the Chinese soft-shelled turtle is unique in the animal kingdom: it urinates through its mouth.

Chinese soft-shelled turtles are most commonly encountered as food in upscale restaurants. They're widely farmed in several Southeast Asian countries. Even as embryos, they are remarkable: a turtle embryo can move to the warmest spot in its egg when it has yet to develop limbs.

In the wild, they live in swamps and marshes where the water is often brackish: salty, but not as salty as seawater. They spend plenty of time out of water, especially in summer, though they often stick their heads into puddles.

Fingers in mouths

The linings of their mouths are covered with tiny, finger-shaped protrusions, discovered in the late 19th century. It turned out that they allow the turtles to breathe underwater: they increase the surface area of the mouth over which oxygen and carbon dioxide can be exchanged. The turtles can also take in other chemicals, such as sodium, through the protrusions.

But that's only part of the story, according to Alex Yuen Kwong Ip of the National University of Singapore. He thinks the turtles only started breathing underwater to make it easier to urinate through their mouths.

Working with Shit Fun Chew of Nanyang Technological University in Singapore, Ip kept four turtles in tanks of water for six days. Each turtle had a tube attached to its cloaca, where urine would normally exit the body. Ip sampled the tank water regularly, and collected any urine passed through the cloaca. Just 6 per cent of the turtle's urea came out of the cloaca. The rest turned up in the tank water.

Ip also tried restraining the turtles on land. When he placed a bucket of water in front of them, the turtles plunged their heads in for between 20 and 100 minutes. They held the water in their mouths for a while, then spat it out ? at which point the urea concentration in the water increased.

Urea transporter

Proteins in the turtle's mouth lining actively transport urea out of its bloodstream. Ip managed to identify one such transporter, and found that it was only present in the mouth lining, not in the kidney.

"I know of no other animals that can excrete urea through the mouth," says Ip. Most fish excrete through their gills, and some amphibians and lungfish may excrete through their skin, but the Chinese soft-shelled turtle's oral habit is almost certainly a one-off. Without the tiny protrusions in the animal's mouth, this could never work efficiently.

Ip thinks oral urination helped the turtles colonise brackish waters. To excrete urea through their cloaca via their kidneys, the turtles would need to drink a lot of water to flush it through. That would mean taking in a lot of salt, which would be difficult to get rid of. Rather than drinking the brackish water, the turtles' habits allow them to simply rinse their mouths with it.

As yet there is no record of the turtles washing their mouths out with soap and water.

Journal reference: Journal of Experimental Biology, DOI: 10.1242/jeb.068916

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