Bats are the ultimate evolutionary opportunists. To cash in on the bonanza of insects that take to the air after dark, the mammals have developed agile flight and a unique echolocation system to locate fast-moving bugs in the dark. However, Benjamin Falk from Johns Hopkins University, USA, explains that although bat flight and echolocation had been studied extensively, they had never been investigated simultaneously: wind tunnels are too noisy to record echolocation calls and the animals' natural movements were too complex to analyse as they flew freely. That was until 3D motion capture technology became available. The sophisticated system allows scientists and Hollywood directors to reproduce complex natural movements for research and the movies. So, when Cynthia Moss acquired 10 motion-tracking cameras while Falk was in her lab at the University of Maryland, USA, the scene was set for him to find out more about how nimble bats coordinate their echolocation calls with flight.
But first Falk had to find bats that were keen to fly for him. Fortunately, big brown bats (Eptesicus fuscus) often make their homes in attics and their human landlords are perfectly happy for Falk to rid them of the uninvited guests. And, once the bats were settled in the lab, Falk recalls that they were very cooperative. ‘They are very food motivated’, he chuckles, explaining that they soon learned to negotiate a series of net obstacles in return for a mealworm reward suspended in the middle of the room. Designing a course where the bats had to make a sharp turn through a hole in one net before climbing steeply and turning through a hole in a second net, Falk and Joseph Kasnadi then gently attached 13 reflective dots at specific locations on the bats’ wings and bodies before filming their flights with the motion-capture system while recording their echolocation calls.
‘The first major hurdle was figuring out how to analyse all of the motion-tracked data from the reflective markers’, says Falk, who had to painstakingly reconstruct the bats’ movements from the trajectory of each reflective dot. ‘Then we could look at how the wings and body moved together and connect that to the echolocation calls’, Falk recalls. Having meticulously reconstructed the motions and echolocation calls from almost 250 flights, Falk could see that the animals flew fast towards the first net (4.15 m s−1) before dropping their speed and turning hard through the first hole. Then they climbed at 2.8 m s−1 up to the second hole before turning through it at 207.5 deg s−1 to intercept the mealworm treat. And when Falk analysed the timing of the echolocation calls relative to each wing beat, he saw that the echolocation calls tended to coincide with wing's upbeat.
Explaining that bats usually exhale during the up-stroke of a wing beat, Falk says, ‘The bats emit these sounds in time with their wing beats to take advantage of the respiration cycle…they are basically able to produce these sounds with no energetic costs’. However, the bats were able to shift the timing of their echolocation calls to later in the wing beat when turning. Falk also noticed that instead of clustering calls as they turned corners, the bats produced more clustered calls while flying straight, suggesting that the call pattern is not a by-product of the turning movement.
Having shown that bat echolocation calls are tightly coordinated with their flight pattern, although they can adapt their call patterns to suit a manoeuvre, Falk is keen to learn about how bats cope with gusts of wind.
