Waddling is a good way to make up for short legs - at least for penguins, say UC Berkeley researchers

December 19, 2000

Penguins are notoriously inefficient walkers, expending twice as much energy in walking a given distance as any other animal of the same weight.

But don't blame it on their funny, waddling gait.

A new study by researchers at the University of California, Berkeley, shows that waddling actually helps penguins conserve energy. The real problem is their short legs.

"Our findings indicate that walking is expensive for penguins not because of their waddling, but because they have such short legs that require their leg muscles to generate force very quickly when they walk," said Timothy Griffin, an integrative biology graduate student in UC Berkeley's College of Letters & Science. "When we compare penguins to animals with similar leg lengths, they burn about the same amount of calories per unit mass."

Griffin and Rodger Kram, a former assistant professor of integrative biology at UC Berkeley, report their findings in the Dec. 21/28 issue of the journal Nature. Kram is now in the Department of Kinesiology and Applied Physiology at the University of Colorado, Boulder.

The results have implications not only for the ecology and evolution of penguins, but for waddling behavior in other animals, and maybe even pregnant women.

"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," Griffin said. "This information may lead to improved understanding, evaluation and treatment of individuals with gait disabilities."

Griffin and Kram performed their experiments on Emperor penguins at San Diego Sea World's "Penguin Encounter" exhibit, a large refrigerated penguin city housing some half dozen penguin species. With the help of Sea World staff, the UC Berkeley scientists and their student team nudged the naturally curious penguins across a force platform to measure the side-to-side and fore-and-aft forces they exert while walking, plus the vertical forces supporting their weight.

Emperors are the largest penguin, weighing around 40 pounds and standing more than 3 1/2 feet tall. The team studied five Emperors, whose normal walking speed was about 1.5 feet per second.

Based on their measurements, Griffin and Kram discovered that waddling helps the penguin walk more efficiently.

"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," said Kram. "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."

The waddling motion is analogous to an inverted pendulum swinging rhythmically back and forth, Griffin explained. At the end of each swing, when the penguin is momentarily still, the energy of side-to-side motion is stored as potential energy. On the return swing, this is converted efficiently into energy of motion that peaks as the penguin rocks back through the vertical. The energy again is stored as potential energy as the penguin comes to a stop at the opposite extreme. The penguins also rock fore and aft as they move forward, employing the same inverted pendulum motion used by all animals, even four-legged animals. The percentage of energy retained during one stride - two step cycles - is called the recovery rate.

"We were actually very surprised when we found a high recovery rate, up to 80 percent in some penguins," Griffin said. This is among the highest of any terrestrial animal. In humans the recovery rate during walking is about 65 percent, meaning 35 percent of the energy is lost.

The UC Berkeley team calculated that, without the side to side motion, penguins would be less efficient. "The penguin's rocking motion helps raise their center of mass," said Griffin. "Without it, their muscles would have to make up that work."

The idea that the penguin's waddling gait was inefficient arose after measurements of the metabolic costs of walking made in Antarctica in 1977 by a team from Duke University. The researchers calculated energy consumption by having the penguins walk on a treadmill and breathe through a mask that measured oxygen consumption. Penguins turned out to burn twice as many calories when walking compared to a other animals of similar mass, such as a dog.

These costs are of great importance to penguins because some, such as the Emperor, may walk over 100 miles from their rookeries to the open sea after fasting for four months during the harsh Antarctic winter. An inefficient walk requires more energy and more food.

Evidently, Griffin said, penguins have made an evolutionary tradeoff. Their short legs make them more streamlined swimmers and divers, even if their walking is less efficient. Short legs may also help reduce heat loss, especially while incubating the eggs in winter.

The team's findings also have implications for a general theory of walking. The study provides strong evidence that the cost of generating muscular force to support body weight is an important determinant of the metabolic cost of walking. This supports a general theory that relates the mechanics and energetics of locomotion and which, until now, applied only to running gaits.

Kram and his late Harvard colleague C. Richard Taylor proposed this theory in 1990, before Kram moved to UC Berkeley. They suggested that running animals use energy at a rate inversely proportional to the time the foot applies force to the ground during each stride. Short-legged animals are in contact with the ground for a shorter time than long-legged animals running at the same speed, so their muscles must exert their force more quickly. Because faster muscles are less economical, and because the muscles of smaller animals have to exert larger muscle forces relative to body weight, smaller animals are less efficient runners.

"This study provides some of the first evidence that the cost of generating force hypothesis may also apply to walking," Griffin said. "This has important implications for what sets the costs of walking in both humans and other animals."

Kram agrees and is using other animals to look for more direct ways to study the energetics of walking.

"Running is different from walking, in that energy is stored in the tendons like in a spring, which makes running a very efficient, bouncing motion," Kram said. "When you walk, though, the muscles have to do more work than in running and they also have to generate force to keep the leg rigid so your legs don't buckle under the pull of gravity. So, it's quite a surprise that our ideas about running extend to walking also."

Griffin is analyzing data on other penguin species studied at Sea World, including Adelie, Gentoo, Macaroni and King penguins, and is studying four-legged animals as well to gain further insight into the mechanics and energetics of walking. Although he and Kram would like to study penguins in Antarctica, it is a lot easier to go to San Diego. "The staff at Sea World are experts in penguin care and handling and are greatly responsible for the success of this project," Kram noted.
NOTE: Tim Griffin can be reached at UC Berkeley's Locomotion Lab: (510) 642-8662 or tmgriff@uclink4.berkeley.edu. Rodger Kram can be reached at Rodger.Kram@colorado.edu or (303) 492-7984.

Penguin photos captured from a color video are available on the web at http://www.berkeley.edu/news/media/download/. To obtain video footage, contact Robert Sanders at UC Berkeley, (510) 643-6998, or Hal Parham at Sea World, (619) 226-3982.

University of California - Berkeley

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