Waddling
is a good way to make up for short legs - at least for penguins,
say UC Berkeley researchers
20
Dec 2000
By
Robert Sanders, Media Relations
Penguin
photos captured from a color video are available
for download
Berkeley
- 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 studying other animals in search of 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, San
Diego Sea World made these studies much easier. "The staff
at Sea World are experts in penguin care and handling and
are greatly responsible for the success of this project,"
Kram said.
###
Penguin
photos captured from a color video are available
for download
.
|
