Penguins: Eye Structure
Myopic little men in tuxedos, or highly efficient
land/water animals? Recent research indicates there's more
to penguins than meets the eye. If you've every wondered
what it would be like to be able to see as clearly under
water as you can on land, just ask the nearest penguin.
Most aquatic animals are short-sighted on land. Most
terrestrial animals (and that includes us) are far-sighted
under water. But researchers have discovered that penguins
can apparently see equally well in both environments
because of the unique structure of their eyes.
Penguins have to be able to see well under water because
their diet consists mainly of plankton, molluscs,
crustaceans, and the inevitable fish. Through a special
slowing-down of their heart rate they are able, like many
other diving animals, to stay submerged long enough to
search out and chase whatever catches their fancy. On dry
land, it's a different story-or has been up to now.
Waddling along on their flat little feet, eyes fixed
intently on the ground, penguins appear myopic, inefficient
and generally out of place. In fact the reverse is true.
During a recent stay on the Falkland Islands, a Canadian
researcher discovered that penguins are able to recognize
individuals and navigate the rocky terrain on which they
live quite well. Long of body and short of leg, they
probably poke their heads forward as an aid to balance. As
for looking at the ground, they're merely-like us-keeping
an eye on where they're going. The human eye is adapted for
aerial vision, which is why scuba divers must wear goggles
or a face mask to re-introduce air in front of their eyes
in order to see clearly.
Among vertebrates, the bird eye is frequently described as
the most efficient. Its superior quality, combined with the
fact that a large number of birds-cormorants, pelicans,
seagulls, even ducks, as well as penguins-get their food
from water, obviously deserved research beyond that
possible in a controlled environment such as an aquarium or
zoo. Professor Jacob Sivak of the University of Waterloo
and his associate, Professor Howard Howland of Cornell
University, had a chance to do that research recently.
Their trip had but one purpose-to study the structure of
penguins' eyes while observing their natural habitat.
The Falkland Islands, off the coast of Argentina, offered
this opportunity, being one of the few areas outside
Antarctica where penguins can be found in large numbers.
Three of the sixteen known species were located there: the
Gentoo, which live on flat areas right off the beach; the
Magellan (also called Jackass), which live in burrows; and
the Rock-hoppers, which live among the rocks along the
cliffs. The Rock-hoppers were by far the most common,
having a population of well over 100,000. The general rule
is, the smaller the penguin, the meaner the temperament,
and the researchers did witness the odd fight. Their
flippers may look pretty useless out of water, but it's not
smart to play around with a penguin. He'll stand his ground
in a face-off and if you're foolish enough to get too
close, those flippers can knock you flat.
Dr. Sivak and his associate, however, had little trouble.
Rock-hoppers always congregate in fairly tight groups, as a
defense against predatory birds such as the skua (a large
seagull that thinks it's a hawk), and two more upright
figures in their midst didn't seem to bother them. Standing
as close to their subjects as 0.3m, the scientists used two
devices: one, developed by Professor Howland, to take
photographs of the penguins' eyes; the other, developed by
Dr. Sivak, to shine a series of concentric circles on the
cornea and give a measurement of how reflections of objects
are altered by curvature of the eye.
Despite the fact that all the work had to be done at
night-the only time the penguins' pupils were dilated
enough-the results were worth it. Comparison of the
photographs with similar photos of human eyes, and study of
the internal structure of the eyes of creatures discovered
killed by seal lions, proved the scientists' theory that
the penguin's eyes are the secret of its survival.
In general terms, a penguin eye and a human eye are almost
identical. Both have the same components necessary for
vision-a cornea through which light can enter; an iris
which controls the amount of light that enters; and a
crystalline lens that focuses the light onto the back of
the eye where a specialized membrane, the retina, receives
it and passes the message along the optic nerve to the
brain for interpretation. In the penguin eye, however,
there are many subtle differences. The cornea, for example,
is markedly flattened compared to ours -- so much so that
it almost resembles a window-pane. This greatly alters the
angle at which light can enter the eye and is very
important for underwater swimming, when light enters the
eye obliquely through a medium (water) whose density is
quite different to the density of air.
The penguin iris is controlled by a very powerful muscle
which is able to drastically alter the shape of the lens
attached to it, depending on whether the penguin is in or
out of the water. The lens, comparatively larger than ours
and differently shaped, focuses the light coming through
the flattened cornea onto the retinal body at the back of
the eye. In this way, the penguin eye adapts to whatever
medium it happens to be in at the time.
Interestingly, there was no evidence of eye problems (apart
from one incident of blindness due to injury) in the group
of penguins studied. Of course penguins don't read, watch
TV or encounter any of the numerous irritants we land-bound
animals subject ourselves-or are subjected-to during our
lifetime. Both the testing devices and methods used in this
study are easily adaptable for use with human eyes, paving
the way for fast, easy identification of eye problems.
Also, the researchers hope that the insights they've gained
into how animals deal with two environments may lead to
knowledge of how humans, in the future, might do likewise.