If tiger feet were built the same as mongoose feet, they’d have to be about the size of hippopotamus feet to support the big cats’ weight.
So why aren’t they that big?
For decades, researchers have been
looking at how different-size legs
and feet are put together across the
four-legged animal kingdom, but
until now they overlooked the shoes,
those soft pads on the bottom of
the foot that bear
the brunt of the
animal’s walking and
running, said Kai-Jung
Chi, an assistant professor of physics at National
Chung Hsing University
She recently ran a
series of calibrated
compressive tests on
the foot pads of carnivores that have that extra
toe halfway up the foreleg,
including dogs, wolves,
domestic cats, leopards,
With help from her
former thesis advisor at Duke
University in Durham, N.C., Chi
found that the mechanical properties of the pads vary in a predictable
manner as animals get larger.
In short, bigger critters need stiffer
To arrive at her findings, Chi
measured the relative stiffness of
the pads across species—how much
they deformed under a given
amount of compression.
“People hadn’t looked at pads,”
said Louise Roth, a Duke University
associate professor of biology and
evolutionary anthropology who was
Chi’s thesis adviser at Duke. “They’ve
been looking at the bones and
muscles, but not that soft tissue.”
Whether running, walking or
standing still, the bulk of the animal’s
weight is borne on that pillowy
clover-shaped pad behind the four
toes, the metapodial-phalangeal
pad—or m-p pad, which is made
from pockets of fatty tissue hemmed
The softer pads on the rear of
the bigger animals may help them
recover some energy from each step,
and provide a bit more boost to their
propulsion, Roth said.
Why doesn’t a tiger need feet as big as a hippo’s? It all comes down
to the differing stiffness of the foot pads in a large number of four-legged carnivores, according to two researchers, one in Taiwan and
the other in North Carolina.
in by baffles
of collagen. Chi
put these pads in
the strain meter by
any surrounding struc-
tures. (She used the pads
from animals who had
Her analysis of 47
carnivore species shows
that the area of their
m-p pads doesn’t increase
at the same rate as the body sizes.
But the stiffness of pads does
increase with size, and that’s what
keeps the larger animals’ feet from
being unwieldy, she said.
The researchers also found that
larger animals have a pronounced
difference in stiffness between the
pads on the forelimbs and the pads
on the hind limbs. Bigger animals
have relatively softer pads on their
rear feet, whereas in smaller animals
the front and rear are about the
don’t scale up the same way.
“A mouse and an elephant are
made with the same ingredients,”
Roth said. “So how do you do that?”
Earlier research had found that
the stresses on the long bones of
the limbs stay fairly consistent over
the range of sizes, in part because
of changes in posture that distribute
the stresses of walking differently,
Roth said. But that clearly wasn’t
enough by itself.
Think of the way a large predator
folds up its forelimbs and launches
itself with its hind legs, Roth added.
“It’s as if the foot pads’ stiffness is
tuned to enhance how the animal
moves and how strength is maintained in its bones,” Roth said.
Chi is now researching the construction of the human heel, using
many of the techniques—and much
of what she learned from her foot-pad research.
The research appeared in Febru-
ary in the Journal of the Royal Society,
Interface. JEAN THILMAN Y