Chinese Soft-Shelled Turtles Expel Urea Through Mouths

The Chinese soft-shelled turtle (Pelodiscus sinensis) excretes urea, the waste product of urine, from its mouth, scientists report Oct. 11, 2012, in the Journal of Experimental Biology. Photo courtesy: Serene Lee

Soft-shelled turtles from China can essentially expel pee from their mouths, researchers say.

This odd ability may have helped them invade salty environments, researchers explained.

Scientists investigated the Chinese soft-shelled turtle (Pelodiscus sinensis). These turtles are often found in brackish swamps and marshes. Intriguingly, these reptiles submerge their heads in puddles on dry land, even though they are air-breathers that mainly depend on their lungs for oxygen, making them unlikely to breathe underwater.

Some fish excrete urea, the main waste product found in urine, out their gills. The researchers speculated these turtles might excrete urea out their mouths when they dunk their heads into water, seeing as the reptiles possess strange gill-like projections there.

The investigators bought soft-shelled turtles from the local Chinatown in Singapore and measured how much urea the reptiles excreted in their urine by sticking plastic tubes onto their hindquarters. Surprisingly, the researchers discovered the water the turtles were kept in held more than 15 times the urea that was in their urine.

The scientists then kept the turtles in dry boxes and gave them a puddle they could dip their heads into. They found the reptiles could immerse their heads underwater for up to 100 minutes and could excrete up to about 50 times more urea through their mouths than from their rear ends. In addition, when the researchers injected the turtles with urea, they found saliva levels of urea were 250 times greater than in the blood.

"It is generally accepted that the kidney is responsible for the excretion of urea in vertebrates, except fish," said researcher Yuen Kwong Ip, a molecular physiologist at the National University of Singapore. "Contrary to this common notion, our results suggest that the mouth can be a major route of urea excretion in soft-shelled turtles."

The scientists conjecture that Chinese soft-shelled turtles excrete urea through their mouths instead of with their kidneys because of their salty environment.

"Soft-shelled turtles are often found in brackish water or even the sea," Ip told LiveScience.

Peeing requires drinking freshwater to wash out urea, but saltwater is not safe to drink. Instead, all these turtles need to do is rinse their mouths out with local water, avoiding the problems that come with drinking saltwater.