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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.

 



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