A little mystery ...
It was the "sneaky snapper" (November 1, 2009) that first got me thinking about this. On that particular turtle, with its snout pointing straight up out of the water, the dark line across the eye seemed WAY off from the way I usually see it on a turtle face. Look how the black eye-line is completely perpendicular to the line of the mouth:
Then I went back and looked at my other turtle pictures, and realized that there was something interesting going on.
I found a good example in Sparky's baby pictures. (Sparky is a painted turtle hatchling I captured a few years ago
from the pond – more about Sparky is coming soon.) On the left, looking straight at me, the eye-line is parallel
to the mouth... but a few minutes later on the right, with her head turned up toward me, the eye-line is at an obvious
angle to the mouth:
How do they do it?
A bit of Googling, plus a connection with a very helpful neuroscience professor, began to unravel the mystery of the
rotating turtle eyes. That black line across the eye stays parallel to the water surface, no matter what, by virtue of a sophisticated sensing-and-control system – not unlike what keeps human eyes steady as we move around. This effect
is particularly easy to observe in turtles because of that black line across the eye.
Here's how it works:
The vestibular system works by a complex interaction of sensors that use fluid to sense both inertia (start,
stop, which way) and gravity (which way is up). When the turtle's head moves, the eye moves to compensate.
And it works! You'll always see that dark eye-line (the "iris line") parallel to the pond surface:
WHY do they do it?
Here's a quote from an article written by a researcher who did groundbreaking work
on this interesting topic:
[This linear eye structure], which is stabilized to the horizon, appears to be a highly functional specialization for an animal living close to the earth's surface, for which the bulk of relevant stimuli
must originate at or near the horizon.
Kenneth T. Brown, "A Linear Area Centralis Extending Across the Turtle Retina
and Stabilized to the Horizon by Non-Visual Cues". Vision Research, Vol 9,
pp 1053-1062. Pergamon Press 1969, Great Britain.
In other words, the turtle spends its life close to the ground and the pond surface, and therefore
needs to have its best vision aligned horizontally in order to find food, seek mates, and avoid danger.
Look carefully at the "iris line" of these two turtles, holding their heads at different angles. Always horizonal!
Of course, this begs the question... what about FROG eyes ?
Frog eyes have a pupil that is almond-shaped when it constricts in bright light (the pupil is huge and round in the dark) – so there appears to be some sort of horizontal axis in a frog eye. Does a frog eye align to horizontal like a turtle eye?
Frogs don't ordinarily angle their heads up and down like turtles, so it's hard to tell if there is VOR stabilization going on. But the frog below was screeching in distress while being grabbed by a snake, and tilted its head at an unnatural angle. You can see that its eye is NOT aligned with horizontal. (Maybe adrenaline interferes with the VOR system?) Any experts out there who know something about a frog's VOR ?
(For the rest of this surprising snake-and-frog encounter, see the food chain page.)