Brian Handwerk
National Geographic Ultimate Explorer
For centuries the seminomadic Moken people have lived as hunter-gatherers,
dwelling on boats or stilted dwellings along the coasts of Myanmar (Burma)
and Thailand.
They harvest the sea's bounty by traditional methods. Without masks or scuba
gear, they are able to gather tiny shellfish and other food on the ocean
floor at depths as low as 23 m.
It's a difficult way to survive, but scientists have learned that these sea
gypsies have an important edge. Studies of Moken children have shown them to
have incredible underwater vision‹twice as good as that of European children
of the same age.
Anna Gislén, of Sweden's Lund University, did the study after hearing of the
Moken's talents from a colleague.
"Another scientist, Erika Schagatay, was in the south of China working with
sea nomads and their diving response," Gislén recalled. "She noticed that
the children were picking out small brown clams from among brown stones. To
her, this was incomprehensible, as she could hardly see them with goggles,
and the children used no such thing. It was not her area of science, so
eventually it ended up on my desk."
Gislén ventured to Thailand's Surin islands where she conducted underwater
tests on Moken children and compared their scores with those of European
kids vacationing in the area. Her results were first published in the May
13, 2003 issue of Current Biology.
Gislén found no differences in the children's respective eye structures or
in their vision on land. Underwater, however, it was a different story. The
Moken children displayed underwater vision twice as sharp as their European
counterparts.
Their secret lies in the way their eyes adapt to the underwater environment.
Underwater Adaptations
The refractive power of the eye's corneal surface, a key to clear vision, is
greatly reduced underwater. The different densities of air and water cause
the problem. Water has similar density to fluids inside the eye, so
refraction is limited as light passes into the eye.
But the Moken are able to accommodate, or muscularly change the shape of the
eye's lense, in order to increase light refraction.
"It seems they have learned to control their accommodative response, such
that they can voluntarily accommodate even in the blurry underwater
environment," Gislén explained. "Normally, severe blur does not elicit
accommodation, and no accommodative response can be found in untrained
European children." The Moken's pupils also adapt, constricting to a mere
1.96 mm. The European children's pupils constricted to only 2.5 mm.
"Their constricting pupils improve vision further," Gislén said. "It's the
same process that improves focal depth if using a camera with a smaller
aperture."
The Moken children use these adaptations to forage for small clams and sea
cucumbers at depths of 3 to 4 m. It's a key to survival, but is it learned,
or might there be a genetic component?
"I think that in general this is very hard to know," Gislén said. "Genes and
environment are so intertwined it's hard to separate them. What I do know is
that we have [more recently] trained European children to become as good at
underwater tasks as the Moken children. So training seems to do the trick."
"However," she continued, "I cannot rule out that genes may influence the
speed of learning, or that the Moken children may be better at things we did
not test underwater, due to some genetic component."
Gislén hopes to continue her research further afield, testing and comparing
the underwater vision of other sea nomads who dive even more than the Moken.
Research is expensive, however, and further limited by the shy and reclusive
nature of many sea peoples.
Other subjects may be similarly elusive.
"I have also heard about monkeys that forage in the waters around Sri
Lanka," Gislén said. "It would be interesting to see whether they use the
same strategies as humans apparently do to see food items on the seafloor."
As fascinating as her study has been, Gislén stresses that her research is
just a single example of the incredible adaptive powers of the human body.
"I think that the human body is extremely flexible, much more than we may be
aware of," she said.
"The diving response is another good example of adaptation," she continued.
"Some tribes of sea nomads in the Philippines can dive down 60 to 70 m, pick
some pearls and then go up again, holding their breath for about 6 to 7
minutes," Gislén added, "Europeans told to do the same thing would just
shake their heads and say it was impossible. But clearly it's not."
Neither is clear underwater vision, apparently, if you have the right
training ‹ or if you happen to be a Moken.
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