Eastern China’s horseshoe bat can pluck insects out of the air while ducking and weaving between tangled forest branches, courtesy of the animal world’s most effective sonar system.
Its echolocation is so sophisticated acoustic engineers are baffled by it. So mechanical engineer Rolf Mueller and his team at Virginia Tech built a bat-inspired robot to help them work out how the bat’s sonar sense works. The robot was unveiled at the annual meeting of the American Acoustical Society in May.
“This is the first prototype where we have been able to recreate all the dynamics that we know a bat has,” Mueller says.
The bat rapidly wiggles its ears and nose – and so does the robot. While most bat species emit sonar chirps through their mouths, the horseshoe bat uses its nose instead. The nose is enlarged with extra “noseleaf” structures around the nostrils giving it a shape like a megaphone.
After filming horseshoe bats with high-speed cameras, Mueller’s team discovered the bats twitch their noseleaves just as they emit a sonar chirp. As the echo from the sonar chirp returns, the bat flaps its ears. These changes, both to the nose and ears, occur “three times faster than a person can blink their eyes”, says Philip Caspers, a graduate student in Mueller’s lab. The team suspects these tricks must be improving the quality of the bats’ sonar “picture”, but how it works is a mystery.
Mueller’s dream is to see the bat-inspired sonar system fly
Last summer Mueller witnessed the bats’ extraordinary skills firsthand. One moonless night on Mount Liantai in Eastern China he stood waiting at the mouth of a cave for the bats to emerge. He expected them to fly up and over the trees of the dense forest before them. But when the bats did leave the cave they dived straight past him into the forest, whizzing through the canopy and weaving between the tangled branches. Despite the darkness the bats could dodge every twig and leaf using sonar alone. “That’s my career dream, to have a little flying device that can do that,” Mueller says.
But so far even our best sonar systems, which can be seven metres in diameter and composed of a thousand individual emitters and receivers, would be completely overwhelmed by the complex environment of a forest thicket. So how do the bats do it?
Having now built his robotic pair of bat ears and a bat nose, Mueller has a few theories he plans to put to the test. One way bats hone their view of the world is by emitting very short, sharp chirps which give precise echoes that accurately tell the bat the location of obstructions and prey. Mueller and his team suspect the horseshoe bats’ nose twitching encodes additional information into each chirp – perhaps a pattern of peaks and troughs in the click – a little like quickly changing the shape of your mouth from an “O” sound to an “E”. As the pattern echoes back off their surroundings, perhaps the bats are able to discern extra information, such as the texture of objects around them.
The team also suspect the ear flapping might help filter a noisy signal by catching only the soundwave echoes – especially important for bats living in a dense forest.
Mueller’s team hopes to create smaller, more efficient sonar systems with much higher resolution than can currently be achieved. These would be a boon for autonomous underwater vehicles used to explore the seafloor. But Mueller’s dream is to see the bat-inspired sonar system fly – and his team has already demonstrated that their small, light robotic sonar can be fitted on to a quadrocopter drone.
Paul Pounds, a robotics researcher at the University of Queensland, says ultrasonic sensors are already common in robotics, although their use is almost entirely limited to basic range-finding, such as monitoring a drone’s height above the ground. Obtaining a high-precision three-dimensional picture using sonar would be a breakthrough. “If they achieve that, then it could be a game-changer, but there is still a long way to go,” he says.
