The Northern Leopard Frog loads its tendons before leaping. “Muscles alone couldn’t produce jumps that good,” said graduate student Henry Astley. Image: Mike Cohea/Brown University |
Some
species of frogs and many other animals are able to jump far beyond
what appear to be their capabilities. The trained contestants in the
frog-jumping competition in Calaveras County, Calif., come to mind, but
even ordinary frogs can leap several times farther than their physiology
would seem to allow.
“Muscles
alone couldn’t produce jumps that good,” said Henry Astley, who studies
the biomechanics of frog jumping at Brown University.
In a paper published in Biology Letters,
Astley and Thomas Roberts, associate professor of biology, show that
the key to frogs’ leaping lies in their stretchy tendons: Before
jumping, the leg muscle shortens, loading energy into the tendon, which
then recoils like a spring to propel the frog up, up and away. Even
though as much as a quarter of a frog’s body mass is in its legs, it
would be physically incapable of jumping as far without the tendon’s
services.
“In
order to get truly exceptional jumping performance, you need some sort
of elastic structure,” said Astley, a fourth-year graduate student in
Roberts’s lab in the Department of Ecology and Evolutionary Biology.
Astley
and Roberts examined jumps by the northern leopard frog (Rana pipiens),
a pond frog common in the northeast United States. The pair implanted
metal beads into the shin bone, ankle bone and leg muscle of four frogs
and then recorded their leaps with 3-D X-ray video technology developed
at Brown. The video, filmed at 500 frames per second and displaying the
jump about 17 times slower than normal, tracks the changes in the leg
muscle’s length and joint movement before, during and after a jump.
As
the frog readies itself to leap, its calf muscle shortens. After about
100 milliseconds, the calf muscle stops moving, and the energy has been
fully loaded into the stretched tendon. At the moment the frog jumps,
the tendon, which wraps around the ankle bone, releases its energy, much
like a catapult or archer’s bow, causing a very rapid extension of the
ankle joint that propels the frog forward. The entire jump—from
preparation to leap—lasts about a fifth of a second, the experiments
showed. Other frog species jump much faster.
“It’s
the first time we’ve really gotten the inner workings, that we’ve put
all the pieces (to frog jumping) together,” Astley said. “We now have a
clearer idea what’s going on.”
How
the tendons, muscles and joints work in frog jumping may help explain
how other animals are such head-scratching leapers—invertebrates like
the humble flea or the grasshopper or vertebrates like guinea fowl and
bush babies.
“Frogs
are interesting in their own right, but we are also confident that this
study gives us insight into how muscles and tendons work together in
animal movement,” said Roberts. “Other studies have presented evidence
for an elastic mechanism, but Henry’s gives us the first glimpse of how
it actually works.”
The National Science Foundation funded the research.
Untitled from Brown PAUR on Vimeo.
