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15 January, 2000

Adaptive behavior and the Leopard seal.

One of the funniest and most interesting things to watch with Adelies is their aversion for water. One might think that penguins would love to enter the water to swim. After all, their bodies are supremely adapted to the marine world where they hunt for food and spend much of their time. This is in contrast to land, where the penguin is ungainly and awkward. Their legs are only three inches long, so they must waddle along the ground with baby steps, tripping on any small obstacle in their path. As they waddle, their tiny wings are held out helplessly in the air, as if only useful to balance their precarious shuffle.

Once in the water, the torpedo shape of the penguins body quickly makes up for its terrestrial limitations. The flipper like wings are quite sufficient to propel their perfectly aerodynamic form through the water at rapid speeds with amazing agility.

Despite all this, penguins are genuinely afraid of the water. When chased, they will never go into the water if given a choice. They are hesitant to even wade across a small stream. When they do enter the water, it is only if there are several birds to enter at the same time. Even then, some will chicken out at the last minute.

At first, I didnšt understand the penguinšs reluctance to enter the water. Then one sunny day we sat in the sun on the warm black sand of the beach and contemplated the thought of entering the ice filled water for a quick swim. At the time, showers were a rare event, delayed until group consensus deemed one absolutely necessary. Besides, our fresh water was running low and taking a shower involved a long, uphill carry of water from a glacier fed stream. Just as we began discussing the possibility of taking a dip in the ocean, the Adelies in front of us scattered and the serpentine form of a Leopard seal broke the surface of the water just off the beach. Leopard seals are the top predator in the area, feeding on penguins and Weddell seals as well as fish and krill. Leopard seals weigh upwards of 800 pounds, and are known to eat Weddell seals much larger than any human. The Leopard seals are also very stealthy when hunting. They often lay in wait for penguins (and beach bathers) just off the beach. Later, I witnessed a Leopard seal attack on an Adelie. Leopards kill by violently shaking their prey while tightly clinched in their jaws. The shaking is so strong that body parts are broken off and flung into the water for the skuas.

After witnessing this, I gained a new appreciation for the fear which the adelies show towards entering the water. A single penguin in the water has much less chance of escape from a Leopard seal than a large group. This is the same strategy employed by all schooling fish and flocking birds. In a large group the seal has difficulty singling out any one individual for attack. Thus the Adelies wait on the beach until there is a group of ten to forty birds ready to go. The group synchronizes their entry into the water by exchanging a series of grunt like noises. These grunts increase in intensity like the cheers at a sporting event, until the penguin in the front of the line finally takes the plunge or in some cases chickens out and lets the next guy have a go at it. This behavior gives the penguins a distinct advantage in terms of surviving a Leopard seal attack. Those penguins that do not exhibit this behavior-the lone swimmers-no longer exist. They were eaten a long time ago and did not pass on their genes to many offspring. Because this fear of water is instinctive and passed on genetically, the penguins do not discriminate between bodies of water that are potentially inhabited by Leopard seals and water that is safe. To an Adelie, all water is to be entered carefully and with a lot of other penguins. Though this may not seem logical to us, the penguins appear to be thriving; antarctic beach bathers wouldn't do so well.

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