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John Toon
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published
October 16, 2018
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Atlanta, GA
An adult elephant weighs about five tons. His biggest threat is a fraction of that. But for a pachyderm to hit a tiny mosquito when dropped on its back, an elephant must generate the same torque as that needed to speed up a car.
This is one of the conclusions of a new Georgia Institute of Technology study that showed how animals use their tails to keep mosquitoes away. The researchers also discovered that mammals shake the tip of their tails at a speed of one meter per second, almost the same speed as that of a mosquito.
The study and its results could help engineers discover new methods of building robots and energy-saving machines that protect humans and animals from mosquitoes.
"Most people assume that animals use their tails to get rid of insects, but we wanted to know how they did it," said David Hu, a Georgia Tech professor who oversaw the study. "They basically have two methods of attack: swish and swat."
Blowing at one meter per second, an animal creates enough wind to prevent nearly 50% of mosquitoes from landing on its back.
The Georgia Tech team determined this success rate by building its own mammalian tail simulator. They placed a fan on top of an acrylic cylinder filled with 10 mosquitoes, and then turned the machine at different speeds to see how many insects had reached the top.
"Making the fan run faster than an animal's tail keeps mosquitoes away, but it takes a lot more energy to turn that fast," said Marguerite Matherne, PhD in mechanical engineering. student who led the study. "It's more efficient to swing the tail at the right speed."
The swish is not perfect: about 15% of people who nibble are found in the skin of the animal. That's why they also rely on the swat, second layer of defense.
Matherne went to the Atlanta Zoo and directed a video camera on elephants, zebras and giraffes. She also went to an equestrian farm. After hours of footage on the backs of the animals, she noticed that their tails had two swaying parts: the upper part was made of bone and skin and the lower part was mostly hair. She found that researchers could accurately model the tail as a double pendulum. This is what mammals use to hunt mosquitoes accurately.
"Our model shows that the swatting movement of the two segments of the tail can be reproduced by controlling only the hinge located at the top. Robotics have trouble accurately controlling double clocks, "said Matherne. "By adjusting the torque during our simulations, we were able to control both movements."
The tail of an elephant weighs about 25 pounds. To lift and break it laterally in 1.3 seconds, the huge animal must generate the same torque as the engine of a sedan, or 350 Newton meters to be exact.
Humans have been resorting to a kind of fly deterrent for centuries. Matherne and Hu's paper also looked at one of the newest devices – the ShooAway – which uses two swivel arms to counter the theft of mosquitoes. The Georgia Tech team replaced its fan with a ShooAway and found that the product was as effective as an animal's tail, although it runs faster than necessary.
Hu previously studied how dogs quivered to stay dry, how frogs used their sticky tongue to catch prey, and how mosquitoes flew in the rain. He chose the animal tails after hearing Matherne relate that he had been hit in the face while riding like a child.
"She has been mated enough times to know that horses can deliver a very good sting," said Hu. "We wanted to know why the swat should be so powerful. It turns out that they swing their tails at a top speed that generates a small stream of air, then sweep those who manage to land by activating only the muscles at the base of the tail. "
The document "Mammals Repel Mosquitoes with Tails" is published in the Journal of Experimental Biology. The research was funded by the National Science Foundation by the PHY-1255127 award.
QUOTE: Marguerite E. Matherne, Kasey Cockerill, Yiyang Zhou, Mihir Bellamkonda and David L. Hu, "Mammals Repel Mosquitoes with their Tails" (Journal of Experimental Biology 2018) http://jeb.biologists.org/content/221 / 20 / jeb178905
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Writer: Jason Maderer
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