Researchers have discovered that the Chinese soft-shelled turtle passes waste through its mouth.

The Chinese soft-shelled turtles (Pelodiscus sinensis) are a delicacy in many parts of Asia. The turtles can be seen dunking their heads in small puddles of water, despite the fact that they use lungs to breath. For a long time, many researchers had tried to find why they behave this way.

Ip Yuen Kwong, from the National University of Singapore (NUS), is one of the people who went all the way to find the reason behind the turtle's strange behavior. After closely studying Chinese soft-shell turtles in a lab, Professor Ip along, with his colleague Shit Fun Chew of Nanyang Technological University in Singapore, found that the turtle passed waste through its mouth and for a turtle that lives in brackish waters; this strategy is the best one that it has got to adopt to the environment.

The researchers put a set of turtles in water for six days. They found that the turtles had released just about 6 percent of the total urea produced in the body via the kidneys. When the researchers let the turtles near small puddles of water, they found that the turtles expelled 50 times more urea through their mouths in the puddles of water than through their cloaca, a small posterior opening that the amphibian uses to remove urine and feces.

The research team then injected urea in the turtle's body and measured the levels of urea in the blood and saliva of the turtle. They found that the level of urea in the saliva was 250 times greater than the level of urea in the blood.

"Throughout [the study] period, the urea excretion rate through the mouth was significantly greater, 15- to 49-fold, than that through the cloaca. These results indicate for the first time that [mouth tissue] processes and rhythmic [throat] movements were involved in urea excretion in P. Sinensis," said Professor Ip, reports BBC.

Professor Ip says that the tiny, finger-like protrusions in the turtle's mouth help it to breath in water, make sodium and excrete urea.

"We were greatly surprised by our novel results because it is generally accepted that the kidney is responsible for the excretion of urea in vertebrates - except fish. Contrary to this common notion, they suggest that the mouth can be a major route of urea excretion in soft-shelled turtles," Professor Ip told the BBC.

Researchers aren't sure why the turtle urinates through its mouth. they say that it is probably because of the environment around the creature has high levels of salt. "Since the buccopharyngeal [mouth and throat] urea excretion route involves only rinsing the mouth with ambient water, the problems associated with drinking brackish water... can be avoided," said the research team. 

The study was published in the Journal of Experimental Biology.