Visual information streaming through our peripheral vision is essential for keeping us on the straight and narrow. ‘When you move past an object, you expect the object to move sideways and if it does not, you know you are going to crash into it’, says Lutz Wiegrebe from the Ludwig-Maximilians-Universität Munich, Germany. Animals that use vision to negotiate their surroundings rely on rapid eye movements, known as saccades, which briefly track individual background objects as they appear to fly past, before flicking to the next: ‘You need landmarks for orientation’, says Wiegrebe. However, imagine negotiating a complex forest in the dark without visual guidance: ‘It would be dangerous if you did this on your bike’, says Wiegrebe. Yet, this is the situation faced by echolocating bats whenever they venture out. Wondering whether leaf-nosed bats use the acoustic equivalent of visual saccades – briefly training their sonar beam and ears on objects as they flit past – Wiegrebe and Kathrin Kugler investigated the possibility that one member of the family, the pale spear-nosed bat (Phyllostomus discolour), coordinates its ear movements and the movements of the noseleaf through which it emits sonar to scan its environment while moving.
As filming each agile aeronaut's facial movements while recording their echolocation cries during flight would be impossible, Wiegrebe and Kugler designed a custom-built cart in which a bat could sit as it went for a joyride past a wooden wall covered in vertical ridges that would produce a train of acoustic reflections. Having accustomed a bat to its fairground ride and ensured that it was content to echolocate naturally as it swooped past the wall in its Batmobile, Wiegrebe and Kugler filmed the animal's facial movements and recorded their echolocation cries with seven minute microphones mounted in a semi-circle around the animal's head. ‘The video analysis was the hardest thing to do’, says Wiegrebe, admitting that the painstaking job of reconstructing the bats’ facial movements in 3D was extremely intensive.
Despite the challenge of handling the massive data set, it eventually became clear that the bats flexed and twitched their noseleafs every time they emitted a sonar beam. ‘We assume that they do that to adjust the elevation of the emitted call, like, “do I point my call further up or down?”’, says Wiegrebe. However, instead of moving and training their ears on the returning acoustic reflection, the bats always swivelled their ears forward: ‘It is really a very short transient movement’, says Wiegrebe. And when the duo moved the vertically ridged wall further away from the bat's cart-ride, the animals delayed moving their ears forwards by a fraction of a second (3 ms): ‘This very fast behaviour is not stereotyped’, says Wiegrebe, ‘it is adjusted according to what the animals hear; namely, to the echo delay’. In contrast, between echolocation calls the bats twisted their ears to the side. Wiegrebe admits that he was surprised to see the movement, as it is unnecessary for the bats to move their ears to determine their distance from objects. He suspects that, instead, ‘They are listening everywhere like a cat does’.
So, pale spear-nosed bats transiently train their echolocation sonar beams on passing static obstacles and Wiegrebe is now keen to learn more about how the bats track moving objects as they manoeuvre through their nocturnal world.
