Chinese Soft Shelled Turtle
Chinese soft shelled turtles urinate through their mouth. Credit: The Journal of Experimental Biology  / Ip Yuen Kwong

Chinese soft shelled turtles urinate through their mouth, researchers from the National University of Singapore have found.

The discovery was made while studying the urea excretion of the turtles. Researchers found that only six percent of the total urea that the animals produced was excreted through kidneys.

"Purchasing turtles from the local China Town wet market and immersing them in water for six days, we measured the amount of urea that passed into the turtles' urine and found that only 6% of the total urea that the animals produced was excreted through the kidneys," said Ip Yuen Kwong, researcher at the National University of Singapore

Then researchers removed the turtles from the water and dipped their heads into water for nearly two hours. They calculated the excretion rate of urea through the mouth by measuring the amount of urea that accumulated in the water.

The researchers were stunned to find that turtles excrete more through the mouth and the excretion rate was 50 times higher than the excretion rate through the kidney, according to the findings published in The Journal of Experimental Biology.

To ascertain how the turtles urinate through their mouth, the researchers studied the structure of their mouth. They found that the the turtle mouth contains a specialised class of protein transporters that helps expel the waste.

To find out how proteins help expel waste through their mouth, the researchers decided to test the effect of phloretin, an antioxidant, on the turtle's ability to excrete urea. When the turtles were supplied phloretin in their puddle of water, they were unable to excrete urea from their mouth when they submerged their head. Phloretin blocks the essential proteins which make them urinate through their mouth.

Finally, researchers checked to see if the turtles have any specific gene in their mouth that helps in urea excretion, and found evidence of the mRNA gene. They say that the mRNA gene is necessary to produce the essential urea transporter, allowing the reptiles to excrete urea through the mouth.