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Hightailing it out of the water: land-walking fish, robot show value of additional appendage.

Nothing conquers a slippery slope like a good twitch of the tail, say researchers exploring how vertebrates could have taken the first treacherous steps on land.

When early vertebrates invaded land 360 million years or more ago, their tails might have been crucial in helping them climb sloping sand or mud, suggests physicist Daniel Goldman of Georgia Tech in Atlanta. Such surfaces can suddenly shift from a solid heap to a flowing slide that sends climbers flailing. Using a tail the right way in a hop-swing kind of gait, however, lets little fish called mudskippers, as well as a dune-invading robot, get going on slippery slopes, Goldman and collaborators report in the July 8 Science.

With a well-timed tail push, "you can then get away with pretty crummy limb use and still get propulsion," Goldman says. A pioneering vertebrate didn't "have to be a ballet dancer."

Studying the function of tails among these early land vertebrates hasn't been simple, partly because of a poor fossil record. Paleontologists have found relatively few complete tail fossils from the transitional creatures, says Stephanie Pierce, curator of vertebrate paleontology at Harvard's Museum of Comparative Zoology. She and her colleagues have proposed that an early land invader called Ichthyostega moved right and left forelimbs forward together, similar to the way a person on crutches sweeps the supports forward in unison. So "crutching," as it's called, may have been a form of tetrapod movement.

Among modern species, little bulging-eyed, big-tailed fish called mudskippers crutch along somewhat like this on their front flippers when venturing onto dry land. Goldman's lab joined forces with mudskipper researchers to see how animals with a crutching gait could cope with changeable materials. On flat surfaces, mudskippers hardly ever do anything special with their tails. On sand tilted up 20 degrees, however, the fish added a tail push with almost every other step.

To analyze the contribution of that tail push, Goldman and colleagues sent a two-limbed robot with a movable tail up slopes of dry plastic particles or poppy seeds. (Sand is dangerous for robot parts.) Positioning the tail to one side and then pushing with it at just the right moment was "critical" on the 20-degree slope, Goldman says. Without that tail power, the robot often stranded itself.

For the research robot, a tail assist "sounds like a very simple maneuver, but to really explain why that works so well on sandy slopes is not trivial," Goldman says. The team was able to come up with a method for mathematically analyzing the first step of the climb. "The amount of physics on the second step is much more terrible to contemplate," Goldman says.

Caption: A small fish called a mudskipper (shown at the Georgia Aquarium In Atlanta) routinely ventures onto land with a hopping gait that inspired a robot study of the motion.


Please note: Illustration(s) are not available due to copyright restrictions.

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Title Annotation:LIFE & EVOLUTION
Author:Milius, Susan
Publication:Science News
Geographic Code:1USA
Date:Aug 6, 2016
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