Natural History, March 31, 2001
When Tim Griffin and Rodger Kram set out to study how penguins walk, they didn’t expect to be impressed. Compared with long-legged ostriches striding across a plain, waddling penguins come up short. Underwater they may be able to race like torpedoes in tuxedos, but on land they are more apt to evoke laughter than to inspire respect.
Previous research on penguins seemed to back up the laughter with hard numbers. Pound for pound, a penguin on land uses twice as much energy as other animals of its size to walk a given distance. Scientists laid the blame for this expense on waddling, the (presumably) energetically costly business of the bird’s throwing its body first to one side and then to the other as it walks.
Griffin and Kram, both at the University of California, Berkeley, decided to test the assumption by measuring the work involved in waddling. So they filmed emperor penguins walking over a force-sensitive plate. Their data enabled them to calculate not just the force of each step but also the direction in which the force was acting and how fast the penguins were moving.
Similar studies on humans and other land animals have shown that walking is a surprisingly efficient way to move. Planting a foot in front of your body as you walk forward, you rise up slightly. Once your body is positioned directly above the foot, you start to fall forward and downward. In this process, much of the kinetic energy of your forward movement is turned into gravitational energy, which is then transformed into moving forward again. The same process occurs when a pendulum converts the energy it derives from moving side to side into moving upward against gravity A walking person is like a pendulum turned upside down. Taking advantage of gravity this way saves lots of energy. Experiments have shown, for example, that a person’s muscles need to supply only 35 percent of the work they would have to perform if there were no inverted pendulum involved. As the walker “falls” with each step, the muscles manage to recover 65 percent of the energy they put into a stride.
Griffin and Kram were amazed to discover that in this respect the penguins were actually superior to humans, recovering up to 80 percent of the energy they put into each step among the highest rates ever recorded for any animal. How is this possible? Penguins not only rise and fall along the line in which they are walking (as we do); they also swing their bodies from side to side like pendulums. This side-to-side waddling provides additional energy for fighting gravity.
Energy from this sideways movement helps the penguin reach an upright position when only one leg is on the ground. As the bird swings back-or rather, falls-to the opposite side, it uses gravitational energy both to move sideways and to step forward.
Biologists have given waddling a bad rap, suggest Griffin and Kram. Penguins do pay a steep price to walk, but the researchers claim that waddling is not to blame. Instead, they propose, the trouble comes from having such short legs. Long-legged animals with longer strides maintain contact with the ground for more time during each step than do short-legged creatures. This allows a long-legged creature to use slower-working, more efficient muscle fibers. An emperor penguin is a hefty bird, weighing about forty pounds-in the same range as the flightless South American rhea, which is similar to an ostrich. But the emperor’s legs are only one-third the length of the rhea’s, or only about as long as those of the guinea fowl, a bird weighing only three pounds. Moving a rhea’s body around on a guinea fowl’s legs, a penguin has no choice but to use a lot of energy.
Like many animals, penguins are caught in a biomechanical bind. With their flipperlike wings, they are well adapted for swimming, and their short legs may help reduce drag underwater. But because they’re birds and not fish, penguins cannot completely give up life on land, where they find mates, lay their eggs, and raise their chicks. Emperors are, in fact, champion walkers, traversing up to 150 miles of frozen sea ice to reach their winter rookeries. Far from wasting energy, waddling may help keep a penguin alive.
Copyright 2001 American Museum of Natural History. Reprinted with permission.