When a species of soft-shelled turtle in China piddles in puddles, it does so through its mouth—the first evidence of an animal doing so, a new study says.
The findings could also have stomach-churning implications for humans with kidney failure, scientists say.
Researchers at the National University of Singapore noticed Pelodiscus sinensis turtles would stick their heads into puddles of water and wiggle their tongues, but they weren't drinking.
Study leader Yuen K. Ip and colleagues also knew that the soft-shelled turtle had structures similar to gills inside its mouth, which had previously been thought to help the turtle breathe—but did not actually function as gills.
"However, I saw a controversy here," Ip said via e-mail. "If the turtle has lungs, why would it need to submerge its head in water [to breathe]?"
Tracking Turtle Pee
To find out, the researchers purchased live turtles from a local market and kept them in water for six days. Only 6 percent of the urea—the main ingredient in animal urine besides water—produced by the turtles ended up in the urine from their hind ends.
After the researchers removed the reptiles from the water but provided them with a puddle, the turtles dipped their heads and, using the water puddles as a mouth rinse, spat out 50 times more urea than was present in the mouth discharge. The urea travels through the reptiles' bloodstreams to their mouths, so it's not technically urination.
The team also found that the turtles carry a gene that produces a specialized protein that helps expel urea. The gene was expressed in their mouths, not their kidneys.
The reptiles live in brackish water, which makes the mouth pee a clever adaptation.
If the turtles expelled their urea the traditional way—a process that requires a lot of water—they'd need more water to stay hydrated.
This would lead to what Ip called a "a vicious cycle of imbibing more seawater"—like us, the turtles would have to drink more saltwater to lower the salt in their blood, because reptiles can't expel salt in their urine. This process would continue until the saltwater proved fatal.
Instead, simply rinsing out the mouth without drinking any salty water helps keep the creatures healthy.
"The ability to excrete urea through the mouth instead of the kidney might have facilitated P. sinensis and other soft-shelled turtles to successfully invade the brackish and/or marine environment," Ip said.
Turtle-Pee Research May Help People
Ip's research interests tend toward "what we can learn from the animal world to resolve biomedical problems," he added.
In other words, the turtle with a mouth full of wee could someday help humans who have undergone kidney failure. Currently, human patients with failed kidneys must undergo dialysis to remove waste from their bloodstream. (Explore an interactive of the human body.)
"Hypothetically," Ip said, "if active urea-excretion mechanisms can be expressed in the mouth of a patient with kidney failure, urea excretion can still occur, through rinsing the mouth with water—just like the soft-shelled turtle."
For now, though—perhaps to the relief of some patients—the idea is just a tinkle in Ip's eye.
The turtle-pee study appears in the November 1 issue of the Journal of Experimental Biology.