This Chinese turtle urinates through its mouth (sort of)

By Ed Yong | October 11, 2012 4:00 am

The Chinese soft-shelled turtle looks like someone glued the snout of a pig onto the face of a fish, with the texture of a scrotum for good measure. But its bizarre appearance pales in comparison to an even more bizarre, and newly discovered, habit: it expels waste through its mouth.

When the turtle breaks down proteins in its liver, it ends up with an abundance of nitrogen, which it expels from its body in the form of urea. Humans are the same—we get rid of urea in the form of urine, via our kidneys.  But the soft-shelled turtle has an altogether different route.

It’s well-adapted to life in the water, and lives in salty swamps and marshes. But Yuen Ip from the National University of Singapore noticed that when the turtle emerges from water, or is stranded on land during dry spells, it will plunge its head into puddles. While submerged, it rhythmically expands and contracts its mouth. Ip found that the turtle gets rid of most of its urea through its mouth rather than its kidneys, via gill-like studs in its mouth. It can breathe and get rid of waste through the same structures.

[Update: Note the comments below. Urine is more than just dissolved urea, so while the stuff the turtle spits out is similar, it’s not quite the full deal.]

It’s not hard to get a hold of Chinese soft-shells in east Asia – they’re raised on several farms and millions are sold in markets as pets or food. Ip bought some for research instead, picking them up from a wet market in Singapore’s China Town. He put them in dialysis, placing them in water and collecting the urine from their rear ends. He also measured the total levels of urine in the surrounding water.

Ip found that just 6 percent of the urea that the turtles produces was ejected via their kidneys. The rest came out through the mouth. When Ip gave the turtles a chance to dip their heads in water, he found that they can sit there sucking, swilling, and spitting out the liquid for up to 100 minutes. This oral route gets rid of urea between 15 and 50 times faster than the kidneys.

The turtle’s mouth contains rows of tiny studs that are thought to behave like gills, which supplement the animal’s lungs when it stays underwater for protracted bouts. Ip showed that these same studs are also littered with urea transporters – proteins that flush urea out of cells. These proteins are closely related to those found in the kidneys of other turtles. But it’s absent in the Chinese soft-shell’s kidneys; instead, it’s all over the inside of its mouth. And as a result, the level of urea in the turtle’s saliva is a “phenomenal” 250 times greater than that in its blood.

Fish share the same trick – they too can get rid of urea through their gills. But why is this turtle the only other back-boned animal that does so? Ip thinks the answer lies in its salty habitat. To urinate through the normal kidney route, the turtle would need to constantly drink more water to make up for what it lost. And because the surrounding water is so salty, they would soon build up toxic levels in their bodies (especially since reptile kidneys are atrocious at getting rid of unwanted salts).

The Chinese soft-shell has evolved to deal with this problem through oral urination. It doesn’t need to drink anything. It just gargles some surrounding water and spits out its waste.

Reference: Ip, Loong, Lee, Ong, Wong & Chew. 2012. The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney. J Exp Biol http://dx.doi.org/10.1242/jeb.068916

Images by Monika Korzeniec

CATEGORIZED UNDER: Animal behaviour, Animals, Plants

Comments (11)

  1. I wonder then, do also sea turtles expel part (or most) of their urea via such a mechanism? They also live in salty water. Does anybody know?
    -wsa

  2. I’ve already had one debate on Twitter this morning about whether “urinates through mouth” is correct. Yes, urea is not synonymous with urine. It’s a substance that is dissolved in water to make urine. I think I’m pretty clear in the piece which is which, at different points through the study.

    The turtle is excreting urine in the normal way through its kidneys. In its mouth, it’s excreting urea. But then it’s also swallowing & swilling water, in which that urea dissolves, and then spitting this out again. This is so functionally indistinct from “urinating through its mouth” that I think the description fits.

  3. Hi Ed. If you don’t mind, I’ll just copy our Twitter exchange here and then post a little extra:

    @thonoir: urea =/= urine. Urea is a waste product produced by metabolising proteins. It is highly soluble and is found /in/ urine.

    @edyong209: Paper repeatedly refers to urine though

    t: only re: renal route. Orally excreted urea dissolves in water. It doesn’t pass urine orally.

    e: Except it kind of does. Takes water in, swills, urea dissolves, spits. The villi secrete urea. What it spits is basically urine.

    t: I disagree. Urine is produced in the bladder and excreted via the urinary system. Orally excreted & dissolved urea =/= urine. (…) The two solutions aren’t synonymous. ‘Oral urination’ is, respectfully, incorrect.

    While it could be argued that the two processes are indeed ‘functionally indistinct’ in that they deal with the elimination of waste via a liquid medium, this is a superficial and misleading analogy. The physiological process which produce urea and urine are both very different as indeed are the substances themselves. Urea is largely nitrogenous waste which in this instance is mostly secreted in the form of ammonia, while urine, which is secreted by the kidneys, contains urea in addition to salts, proteins, enzymes, blood cells and numerous other waste products.

    The notion of ‘oral urination’ may make for a better headline (no snarkiness intended) but it is fundamentally inaccurate.

  4. ChasCPeterson

    This is so functionally indistinct from “urinating through its mouth” that I think the description fits.

    No it’s not. Maybe it would be if urine production was only about nitrogen excretion, but that’s absurdly oversimplified. Urea is just one of many solvents in urine. Making urine in the kidneys (and, in turtles, modifying it further in the bladder and/or cloaca) serves a variety of other homeostatic functions, including water balance, osmotic regulation, and balances of sodium, potassium, calcium, pH, etc. etc. etc.

    I look forward to seeing the paper. Did they also ammonia production? If an animal is relying on non-renal routes of nitrogen excretion, it might be able to save the energy of urea production altogether.

  5. SoftshellKeeper

    Softshells are quite different from other turtles and have evolved more advantages to staying below the surface, with adaptations like their snorkel nose and their measured ability to absorb oxygen through their skin while submerged. This is a very interesting adaptation and I’m also curious if they were eliminating just urea or other waste byproducts like ammonia as well.

  6. Paul R. Potts

    Turtles are very strange creatures. For another odd variation, a turtle that breathes through its rear end (well, sort of), do a search for the “Fitzroy River Turtle.”

  7. @Chas + Anthony – Gotcha. Thanks for comments.

  8. lkr

    okay guys, this is a complete pissing contest.

    And I’m with Ed….kiss this turtle and you get a golden shower.

  9. Heteromeles

    @Paul: Yes, breathing through anal “gills” and excreting through mouth “gills,” but no evidence (yet) that one species does both.

    Good grief. Can I suggest that whatever genes control development of secretory cells, there are some major mutations in that area of turtle genomes, possibly with some transposons or similar mechanisms that causes these weird developments? Too bad turtle genomics isn’t on the radar outside the systematists.

    Also, I keep thinking about TetZoo’s infamous Turtle Sex Organ post, and wondering what else they use their penises for.

  10. I wonder if this feature is also present in other softshell turtle species; Pelodiscus isn’t the only species known to live in habitats with brackish water.

  11. DavidB

    Makes a change from politicians, who prefer to talk out of their arse…

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