Eyes aren't much use when you hunt at night and hole up in caves during the day: it's too dark to see. So many bat species have opted for echolocation to perceive their environment, squeaking and interpreting the returning echoes. While some bats produce narrow beams of sound that focus on individual obstacles, many produce a wide beam that could result in `cascades of echoes,arriving from different directions,' explains Cynthia Moss from the University of Maryland. Moss wondered whether bats emitting wide beams of sound interpret all of the returning echoes simultaneously to sense their surroundings, or focus the centre of the echolocating beam briefly on every feature in the environment, to build up the picture sequentially(p. 1011).

Moss, Annemarie Surlykke and Kaushik Ghose filmed and recorded the bats'echolocation calls as they flew through a gap in a mist net to capture a mealworm tethered on the other side. Analysing the position of each bat's beam of sound, it was clear that the approaching animals were investigating the lay of the land sequentially. First they focused the centre of their beam on one edge of the gap and then turned their attention to the second edge before focusing on the tasty treat beyond. The bats were scanning the scene sequentially, rather than `viewing' it all simultaneously, even though the beam of sound was wide enough to capture all of the features simultaneously. The team explain that this is similar to the way that we scan objects with our eyes and may give the bats a more accurate picture of their surroundings.

Surlykke, A., Ghose, K. and Moss, C. F. (
). Acoustic scanning of natural scenes by echolocation in the big brown bat, Eptesicus fuscus.
J. Exp. Biol.