Though they glide through water with the greatest of ease, penguins on land are ungainly creatures. Indeed, when it comes to walking, penguins expend twice as much energy as any other terrestrial animal of the same size. Their distinctive waddle, however, is not to blame. According to research published today in the journal Nature, this graceless gait is the most efficient way for penguins to move around on terra firma.

In order to study the biomechanics of penguin walking, graduate student Timothy M. Griffin of the University of California at Berkeley and Rodger Kram of the University of Colorado coaxed emperor penguins at San Diego Sea World to waddle across a force platform. This enabled the researchers to measure side-to-side and back-and-forth forces, as well as the vertical forces supporting the animal's weight. "Our hunch was that if penguins are trying to move forward, but expend energy rocking side to side with this awkward, roly-poly, back-and-forth movement, then it's got to be wasted energy," Kram recalls. "But what we found is that they are inefficient because of their short legs and big feet, and waddling is a means to cut their losses."

In fact, with regard to the percentage of energy retained during one stridea measure known as the recovery ratepenguins scored surprisingly well: whereas the human recovery rate, for example, is about 65 percent, that for emperor penguins is up to 80 percentamong the highest of any terrestrial animal. Without that side-to-side motion, the team determined, penguins would be less efficient. "The penguin's rocking motion helps raise their center of mass," Griffin explains. "Without it, their muscles would have to make up that work." Still, penguins wouldn't have to waddle if they didn't have such diminutive legs in the first place. But these proportions facilitate diving and swimming. They may also reduce heat loss, which would help the penguins to incubate their eggs during the bitterly cold Antarctic winter. Apparently, he observes, penguins have made an evolutionary trade-off.

Griffin is now studying other penguin species and four-legged animals to further explore the mechanics and energetics of walking. "Our knowledge gained from penguins provides novel insight into the gait mechanics of humans with increased lateral movements, such as in pregnant women or obese individuals," he says. "This information may lead to improved understanding, evaluation and treatment of individuals with gait disabilities."