A BAT NEVER FORGETS: INTERNAL MAPS AID FLIGHT

Although driving home from work or walking around a supermarket without bumping into things may seem simple, these tasks require constant processing of sensory updates from our environment, as well as memory. If we become disorientated, we can just stop, have a look around and adjust our bearings. For bats faced with similar tasks, such as flying around foraging or flying home to roost, life is a little more complex. Many bats have poor eyesight and use echolocation to navigate, sending out calls and then interpreting the returning echoes as ‘views’. ‘A bat updates its view of the surrounding world five to 20 times per second during normal flight’, says Jonathan Barchi, a PhD student from Brown University, USA. ‘This

may not seem low, but when combined with the range of view echolocation provides [up to 5 m] and flight speed of these bats [up to 3 m s⁻¹], it means that they are only getting a few views of any given region of a space before leaving it behind.’ As bats don't have the luxury of hovering to re-orientate, Barchi wondered whether bats rely on an especially well-developed internal map to enable them to quickly navigate and manoeuvre. Working with his supervisor James Simmons and an undergraduate student, Jeffrey Knowles, he turned to the big brown bat, Eptesicus fuscus, to begin his investigation (p. 1053).

Over a period of 6 days six bats were repeatedly released from the same spot into a dark room cluttered with chains hanging from the ceiling in defined locations. The team mapped the movement of the bats as they flew about this obstacle course using highly sensitive microphones embedded in the room's walls; based on the timing with which these microphones picked up the echolocation calls, the researchers could then calculate exactly where in the room the bats were.

After just 2days most of the bats had already started to adopt stereotyped but individual and unique flight paths. ‘The bats seemed to learn how to fly a nice smooth path through the field of obstacles, without needing to make sharp turns or other abrupt changes in course’, remembers Barchi. Having remarkably quickly sussed out where all the obstacles lay in the room, they rarely deviated from their chosen aeronautical trajectory. Next, the team wondered how well the bats would cope if they were released from different points in the room. Unperturbed, the bats were soon back to their established paths. When tested again after a month's break, the bats still remembered the room, and flew in their precise characteristic flight paths as if no time had passed at all.

In short, these bats learnt very quickly how to efficiently avoid collisions, and they developed an internal map of the room that they remembered for future guidance. Barchi suggests that their maps might allow them to devote more attention to foraging for prey by making navigation a background process. The map may also help them adjust to changes in their flight environment; when the team changed the location of the chains to be a mirror image of the original setup, the bats had to deviate from their previous preferred flight paths or risk crashing into the chains. However, they devised a new flight path more quickly than when they first experienced the original layout, suggesting that their maps helped in some way.

So, it seems that these airborne mammals overcome the disadvantages speedy flight poses for navigation by developing precise and stable internal maps.