Posted: 29 Mar 2010 02:17 PM PDT
A biomechanical analysis of running elephants has revealed that Earth's largest land animals do some strange things at high speed.
Unlike every other quadruped, they use all four legs for braking and propulsion, rather than rather dividing those tasks between hind and front legs.
Elephants also prove to be extremely inefficient while running. Compared to animals like horses, they perform quite poorly. Then again, given their size, running itself is quite an achievement.
"It's pretty cool that they can run at all. And they do it in such a weird way," said John Hutchinson, an evolutionary biologist at the University of London.
In a study published March 29 in the Proceedings of the National Academy of Sciences, Hutchinson's team videotaped six Asian elephants as they ran across mechanical plates that measured the force of each stride. By combining gait models distilled from the video with force measurements, they could quantify the elephants' biomechanics.
Surprisingly, they learned that braking and propulsion is performed equally by each leg. In other quadrupeds, rear legs are mostly used to push off, and front legs to slow down. The elephants' arrangement likely makes them more stable and reduces physical stress placed on each leg, said Huchinson.
Stress reduction could also explain the pronounced knee bend of running elephants. A relatively straight-legged gait, which elephants possess while walking, is better at converting muscular effort into physical force. However, it does little to absorb stresses generated when an elephant's leg hits the ground. Bent knees are a natural shock absorber.
To compensate for the loss of leverage, running elephants must work extra hard. The scientific term for limb leverage is "effective mechanical advantage," or EMA. While walking, elephants have an EMA of one, but it drops to 0.5 in running elephants, calculated Hutchinson.
Even as EMA declines, the force of an elephant's mass striking the ground rises with speed. "If EMA goes down by a factor of two, and the forces that EMA resists go up by a factor of three, muscles have to work six times as hard," said Hutchinson.
That inefficiency could explain why elephants are not known as long-distance runners, and have speeds of just 15 miles per hour. Then again, when you're as big as a small house, there's not much you need to run from.
Video and image: An Asian elephant and trainer in the study; the white patches are infrared reflectors used as reference points in motion models/John Hutchinson.
Citation: "The extraordinary integration of biomechanical compliance, leverage, and power in elephant limbs." By Lei Ren, Charlotte Miller, Richard Lair, and John Hutchinson. Proceedings of the National Academy of Sciences, Vol. 107 No. 13, March 30, 2010.
Posted: 29 Mar 2010 12:00 PM PDT
Bats can subtly adjust the frequency of the sounds they use to do echolocation to adjust to particularly cluttered terrain.
In a laboratory testing room filled with dangling plastic chains, bats wearing tiny, half-gram microphones were recorded flying through the obstacle course. When confronted with the forest of chains, the bats tended to reduce or increase the sounds they emitted by a few kilohertz. On their return flights, the path is clear and they stop tweaking their frequencies.
The researchers hypothesize that using multiple frequencies helps the bats resolve their environments faster than using single sound could allow.
"It's all a matter of matching the broadcast to the echo," said Mary Bates, a biology graduate student at Brown University and a co-author of the new study March 29 in the Proceedings of the National Academy of Sciences. "It's a matter of ignoring or not processing the things that are going to interfere and doing this careful matching of sound to echo."
The bats need data about the placement of the chains faster than the first round of echoes can provide, so they send out a second and third and a fourth batch. By changing the pitch of the noises slightly, the bats can differentiate between their different noises. Their brains then integrate those streams of sounds into a high-resolution 3-D map of the terrain.
Researchers have often marveled at bats ability to use echolocation even in very crowded environments or among many similar bats using their own sounds to resolve their environments.
"The navy is really interested in what we do because manmade sonar has come nowhere close to what bats and dolphins can do," Bates said.
The frequency-shifting trick is one way that bats are able to echolocate while moving quickly, Bates and her lab leader, Jerry Simmons hypothesize. They also use the trick when dealing with interference from other members of their species, they've discovered.
Images: J. Simmons, J. Barchi, J. Gaudette and J. Knowles.
Citation: "Echolocating bats shift frequencies to avoid broadcast-echo ambiguity in clutter" by ShizukoHiryua, Mary E. Bates, James A. Simmons, and Hiroshi Riquimarouxa. doi: 10.1073/pnas.1000429107
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