A swishy arrangement: Tails are all torque and plenty of action

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There are few sights more magnificent in the animal kingdom than a horse’s tail and mane flowing in the breeze at full gallop, but that rear appendage has a much more important role – dealing with insects.

In fresh research, scientists have delved deeply into the insect-swatting magnificence of mammalian tails, describing their findings in a paper just published in the Journal of Experimental Biology.

The swinging of a mammal’s tail has long been thought to deter biting insects, which, in cows, can drain up to 300 millilitres of blood per day. But just how effective is it?

Researchers from the Georgia Institute of Technology, in a combined experimental and theoretical study, filmed horses, zebras, elephants, giraffes and dogs swinging their tails. They found that their tails swing at triple the frequency of a gravity-driven pendulum, and requires 27 times more power input.

Tails can also be used like a whip to directly strike at insects.

This whip-like effect requires substantial torque from the base of the tail – in fact, the torque can be comparable to that of a car, but still within the physical limits of the mammal.

 

It was found that swishing their tails at about 1 meter a second, the mammals were able to produce enough breeze to deter many from landing.

Based on their findings, Marguerite Matherne, Kasey Cockerill, Yiyang Zhou, Mihir Bellamkonda and David Hu designed and built a mammal tail simulator to replicate the draft from a swishing tail, showing the air movement generated was indeed enough to deter many mosquitoes from landing.

A horse swats flies with its tail. Georgia Tech researchers have studied how animals such as horses use their tails to generate airflow that keeps mosquitoes away – and swats them if they land. (Credit: Marguerite Matherne, Kasey Cockerill, and Yiyang Zhou, Georgia Tech
A horse swats flies with its tail. Georgia Tech researchers have studied how animals such as horses use their tails to generate airflow that keeps mosquitoes away – and swats them if they land. Photo: Marguerite Matherne, Kasey Cockerill, and Yiyang Zhou, Georgia Tech

The researchers suggest their findings could help engineers discover new methods of building robots and energy-efficient machines that protect humans and animals from mosquitoes.

“Most people assume that animals use their tails to swat at bugs, but we wanted to know how they do it,” explains Hu, the Georgia Tech professor who supervised the study.

“They basically have two methods of attack: the swish and swat.”

Swishing at one meter per second, an animal creates enough wind to keep nearly 50 percent of mosquitoes from landing on its rear end, it was found.

The simulator comprised a fan placed atop an acrylic cylinder filled with 10 mosquitoes. The study team then spun the machine at different speeds to see how many insects reached the top.

“Running the fan faster than an animal’s tail kept even more mosquitoes away, but it takes a lot more energy to spin that quickly,” said Matherne, a mechanical engineering doctoral student who led the study.

“It’s more efficient to swing their tails at just the right speed.”

Marguerite Matherne, a mechanical engineering Ph.D. student, and Professor David Hu, pose with the mammal tail simulator used to study the airflow needed to keep mosquitoes from landing. (Credit: Rob Felt, Georgia Tech)
Marguerite Matherne, a mechanical engineering Ph.D. student, and Professor David Hu, pose with the mammal tail simulator used to study the airflow needed to keep mosquitoes from landing. Photo: Rob Felt, Georgia Tech

The swish isn’t perfect, with about 15 percent of the biters finding their way to the animal’s skin. That’s why they also rely on the swat – the second layer of defense.

The study began with Matherne going to the zoo in Atlanta and pointing a video camera at elephants, zebras and giraffes. She also went to a horse farm.

With hours of footage of animals’ backsides, the hard work began.

Tails have two fundamental parts that sway back and forth. The top part is bone and skin, and the bottom part is mostly hair. She found that the researchers could accurately model the tail as a double pendulum – and that’s what the mammals use to accurately swat mosquitoes.

“Our model shows that the swatting movement of both segments of the tail can be reproduced by only controlling the hinge at the top,” Matherne said.

“Roboticists have struggled to accurately control double pendulums. By adjusting the torque during our simulations, we could control both movements.”

An elephant’s tail weighs about 25 pounds. To lift it up and snap it sideways in 1.3 seconds, the huge animal must generate the same amount of torque as the engine of a sedan — 350 Newton meters to be exact.

Humans have used some kind of fly deterrent for centuries. The paper also looked at one of the more recent devices — the ShooAway — that uses two spinning arms to thwart flying mosquitos.

The Georgia Tech team replaced their fan with a ShooAway and found that the product is just as effective as an animal’s tail, although it spins faster than necessary.

Hu has previously studied how dogs shake to stay dry, how frogs use their sticky tongues to grab prey and how mosquitoes fly in the rain. He chose animal tails after hearing Matherne talk about being hit in the face while riding horses as a child.

Close-up image shows a mosquito with a horsetail. (Credit: Candler Hobbs, Georgia Tech)
Close-up image shows a mosquito with a horsetail. Photo: Candler Hobbs, Georgia Tech)

“She’s been swatted enough times to know that horses can deliver a pretty good sting,” said Hu. “We wanted to know why the swat had to be so powerful. It turns out they swish their tails at a tip speed that generates a small air flow, then swat away those that manage to land by activating only the muscles at the base of the tail.”

The findings may help in developing new mosquito-repelling strategies that do not depend on chemicals.

The research was funded by the Georgia Institute of Technology School of Biology, the Elizabeth Smithgall Watts endowment, the National Science Foundation, and a graduate research fellowship DGE-1650044.

Mammals repel mosquitoes with their tails
Marguerite E. Matherne, Kasey Cockerill, Yiyang Zhou, Mihir Bellamkonda, David L. Hu
Journal of Experimental Biology 2018 221: jeb178905 doi: 10.1242/jeb.178905

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