Unusual urination in turtle: through the mouth

Source: nationalgeographic.com, photo by Monika Korzeniec
A turtle species native to China has an unusual way to shed the waste product urea: by rinsing its mouth. In most animals, urea is excreted in the urine. In other words, this turtle would seem to pee through its mouth.
The Chinese soft-shelled turtle (Pelodiscus sinensis) lives in brackish water. Individuals of this species had regularly been observed dipping their heads into water and moving their tongues, but not drinking. It was already known that the turtles have a gill-like structure in the mouth, but as the animals have lungs, these gills are not used for breathing.

For the study, soft-shelled turtles were bought at the market and kept in water. This made it possible to the discover that only 6 per cent of the produced urea – besides water, the main ingredient of urine – was excreted in urine via the cloaca.

The animals were removed from the water but given access to a puddle. The scientists observed the turtles dunking their heads in the water and spitting out large amounts of urea. The turtles were using the water in the puddle as a mouth rinse, and the concentration of urea they spat out was 50 times higher than that normally found in their mouths. Technically speaking, this isn’t urination (which goes via the kidneys and the bladder). The scientists discovered a gene that produces a specialised protein for excreting urea. This gene is active in the mouth, not the kidneys.

Oral excretion is a good adaptation to the brackish water in which the turtles live. Urinary excretion of urea requires a great deal of water. And drinking large amounts of brackish water would mean taking in large amounts of salt, which in turn would need to be excreted. The animals can stay healthy by rinsing with, not drinking, the salty water.

The study was published in the Journal of Experimental Biology
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