Biologists from the University of Singapore have discovered that a certain Chinese soft-shelled turtle, well-adapted to living at the bottom of brackish (salty) muddy swamps, coming to the surface to breathe the air, would immerse their heads in puddles when their swamps dry up. To you and I, such an observation might not cause even the slightest curiosity, but to Y. K. Ip and his team, it was curiosity that led them to discover a truly unique animal - a turtle that pees through its mouth.
Chinese soft-shelled turtle: image via jeb.biologists.org
How was this discovered?
The researchers purchased live Chinese soft-shelled turtle, Pelodiscus sinensis, from a local market and kept them in water for six days. Testing the water, they found that only a very small percent of the urea that the turtles produced was excreted through the kidneys. After removing the turtles from water, the turtles were provided simply with puddles deep enough for them to bury their heads.
The turtles submerged their heads occasionally, sometimes for long periods, the longest being 100 minutes. Testing the water afterward, the scientists found that up to 50 times more urea had been deposited there than in waters where the turtles had been totally submerged. Subsequent testing after injecting urea into the turtles showed that the levels of urea in the saliva were 250 times greater than the levels in the turtles' blood.
But why did these turtles urinate through their mouths?
Upon analyzing the turtles' cDNA, they found a urea transporter gene similar to a gene that other animals carry, but that these P sinensis, uniquely, carry the gene in their mouths. These genes are beneficial to the soft-shell Chinese turtle, an adaptation that helps it avoid drinking the salty water of their brackish habitats. In these waters, drinking the salt water would become so difficult for turtles and other reptiles to excrete that they would likely die from ingesting it.
sources: National Geographic, Journal of Experimental Biology (1), (2)