Why the Frog's Right Hand Cleans the Frog's Stomach
A personal take on science
Recently, in reporting about Sims, Andrews, and Young's curious research paper on "Stomach Rinsing in Rays," I remarked on their sentence:"In frogs and toads, stomach eversion is accompanied by 'gastric grooming' with the right hand to wipe away vomitus."
The authors are specific about it being the RIGHT hand. No mention is made of a frog using its left hand. "But can they be certain about this?," I asked, "Is it always the right hand? I vowed not to rest until I found out.
Now I Can Rest
I enjoyed your cute little piece on whether frogs are really right-handed or not. For the record, the serious scientific answer is that frogs have to do this task of stomach wiping with their right hand simply because the upchucked stomach always hangs out of the right side of the mouth.
It would be entertaining, I admit, if this stomach orientation were cultural, but it is not. It is simply the result of the asymmetry of the stomach in all amphibians and the relative lengths of the mesenteries that hold the stomach in place within the abdomen. The mesenteries are shorter on one side than the other and consequently pull to the stomach toward the right side when it comes out the frog's mouth. This point is mentioned in the short note that I published with Tomio Naitoh in Nature about five years ago (and cited in the paper on stomach rinsing in rays, which you cited).
Here is the citation for that note: 1996 "Why are Toads Right-Handed?" Nature, T. Naitoh and R.J. Wassersug, vol. 380, 1996, pp. 30-1. The slightly longer version, in Japanese, is: Naitoh, T. and R.J. Wassersug 1996 "Is the Anuran Right Handed?" T. Naitoh and R.J. Wassersug, Iden (The Heredity), vol. 50, 1996, pp. 8-9.
But I Can't Stop Asking Questions
I find this information comforting. But like most good answers to scientific questions, it spurs the asking of a new question.
In humans, occasionally a specimen (or, if you prefer, a person) is born with all the organs left-right reversed from the usual arrangement. The condition is called "situs invertus," with the heart being on the right side of the body, etc.
I asked Wassersug whether the condition is ever found in frogs. Here is his reply:
I don't know if it has ever been reported in frogs. It may be less likely in their hearts, though, since frog hearts are less asymmetric than mammalian heart.
And so there is a new mystery, albeit probably a small one. If anyone has the answer to it, I would love to hear from you.
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