A couple of months ago, in reading a book called The Natural History of Seeing, I came across an unusual story about the Moken, nomads who live off the sea in the Mergui Archipelago of Myanmar and Thailand, and dive in search of clams, sea cucumbers, and other sea creatures. Strangely, it turns out that the Moken can see more clearly underwater than the rest of us, about twice as well according to a study by Swedish vision scientists. This struck me as pretty odd—not the sort of variation you’d expect within our species. It’s as if the Moken, at least in this one trait, are on their way to becoming marine mammals.
One trick the Moken use to see more clearly in water is to make their pupils as small as possible. Constricting the pupils is a simple way to sharpen an image on the retina; basically, funneling light through a small hole reduces the scatter of light entering the eye, making for a relatively clear image even without focusing. This is the principle behind a pinhole camera (which has no lens) or the increased depth of field in a photo when you make a camera’s aperture smaller.
Moken child under water with constricted pupils. Modified from a photo by Anna Gislen.
The fact that the Moken constrict their pupils under water especially caught my eye. It immediately made me think of gartersnakes, and not just because, having studied these reptiles for years, they sometimes randomly pop into my brain. What I was remembering was a study I worked on many years ago, with a German vision scientist named Frank Schaeffel, aimed at figuring out how gartersnakes see clearly under water.
One of the main findings of our study was this:
A gartersnake (Thamnophis atratus) with large pupil in air (above) and constricted pupil in water (below). Photos by Frank Schaeffel.
I remember sitting in a darkened lab at Cornell, watching the blown-up image of a submerged gartersnake on a video monitor, and asking Frank if the constricted pupil was significant. He was surprised to see this too and said, in a way that struck me as both excited and understated, something like, “Oh yeah that could make its vision sharper.” It wasn’t a major “Eureka!” moment, obviously, but it was definitely a satisfying little discovery.
Since my knowledge of optics was pretty limited, I ended up doing a silly experiment involving a pie tin and a filled bathtub to satisfy myself that this pupil constriction mechanism really works. Using the tip of a pen, I punched a tiny hole in the tin, then immersed my head in the tub and, looking through the hole, tried to make out some print on a plastic bag. Sure enough, the print, although fuzzy, was definitely clearer than it had been without the pinhole apparatus. (Anyone can try this at home—just be careful not to cut yourself on the jagged edges of the hole, as I almost did. Best to pound the metal edges flat, in fact.)
Anyway, the Moken are doing the same thing that these gartersnakes do and for the same basic purpose, to find things under water (although the gartersnakes are looking for fish and tadpoles rather than clams and sea cucumbers). This doesn’t give me any great new insight about synchronicity or the unity of life (although it does make me think of something called the Baldwin Effect). It’s just one of those quirky little things that makes an old snake biologist smile.
As a postscript, Swedish children who trained for a month to try to see an underwater pattern more clearly, ended up constricting their pupils and seeing just as well in water as the Moken. Presumably, similar “training” in the course of their daily lives is what gives the Moken their good aquatic vision. They’re probably no more genetically adapted to see under water than anybody else.
This story also brings back another memory, of being forced by a swimming instructor to retrieve an object—I think it was a set of swim-locker keys—from the bottom of a pool. Who knows, maybe when I did that I made my pupils into pinholes too.
The Monkey's Voyage 