Bats' Acoustic Blindness: An internet audiovisual illustration
We present here three video and sound sequences of captive mouse-eared bats (Myotis myotis) approaching a live cricket fixed on the surface of a doormat. Video and sound sequences match each other. Video sequences can be played back simply by selecting the links below. Clips are played back using Quicktime. Selecting a sound sequence will initiate playback.
The videos and sound sequences presented here accompany the paper Arlettaz, R., Jones, G. & Racey, P.A. 2001. Effect of acoustic clutter on prey detection by bats. Nature, Lond. 414: 742-745.
The ultrasonic microphones used to record the bats are visible on both sides of the feeding arena. The synchronisation between image and sound was made possible by the brief duration (ca. 2 ms) of the lightening of a flash, covered with an infrared filter (the sudden flash lightening is visible on the pictures below the doormat on the right if played back slowly) and the change of voltage elicited when the flash was triggered, which was recorded on sound channel two. The noise of the flash reloading is also easily recognizable. Note the reduction in flight speed of the bats when approaching prey as well as the silent/low intensity echolocation calls preceding prey approach. In the sound sequence Bat-3, the hard clicks correspond to the noise provoked by bat's ring (see Norman, A. P., G. Jones & R. Arlettaz (1999). Noctuid moths show neural and behavioural responses to sounds made by some bat-marking rings. Anim. Behav. 57: 829-835).
Video sequences 1-3 show a mouse-eared bat approaching a live cricket placed on a doormat (mimicking a lawn situation where the bats forage in the wild). The bat circles around the cricket, and lands on the doormat where it takes the cricket in its mouth before flying off. The accompanying audio sequences show how the bat reduces the intensity of its calls prior to prey capture. At this time the bat probably listens to prey-generated sounds to localize the cricket, which is difficult to localize by echolocation because prey echoes are masked by echoes from the doormat.
We present here additional video and sound sequences of mouse-eared bats capturing prey in other experimental situations: open air (prey suspended from the ceiling), flat surface of a perspex plate, and among leaf litter.
In open air the bat captures a cricket hanging from fishing line in its tail membrane, and emits a characteristic series of echolocation calls at a high repetition rate (a terminal buzz)to localize the prey. The echolocation performance of the bat in this situation is not compromised by clutter (echoes from objects other than the target of interest). The situation in leaf litter resembles that with the doormat described above. Here echolocation calls are fainter than those emitted in open space as the bat listens for prey-generated sounds to locate the prey item in clutter. When a cricket is presented on a perspex sheet (resembling the water surface), the sheet acts as an acoustic mirror, and echoes from the bat's calls are reflected away from the bat. Echoes from the target are very conspicuous to the bat, which emits a terminal buzz similar to those produced when foraging for aerial prey.
See ARLETTAZ, R., JONES, G. & RACEY, P.A. 2001. Effect of acoustic clutter on prey detection by bats. Nature, London 414: 742-745 for details of this research.