Echolocating bats have a legendary ability to find prey in the dark — so you'd think they would be able to tell the difference between water and a sheet of metal. Not so, report Greif and Siemers in Nature Communications. They have found that bats identify any extended, echo-acoustically smooth surface as water, and will try to drink from it (S. Greif and B. M. Siemers Nature Commun. doi:10.1038/ncomms1110; 2010).

The way in which bats locate point objects has been studied extensively, but how they recognize extended objects, such as pools of water, isn't known. As pictured here, bats drink while on the wing. Greif and Siemers hypothesized that, when searching for a drink, the animals look for the echo-reflection signature of water surfaces — the only extended, acoustically smooth surfaces in a bat's environment.

When a bat sends an echolocation beam at a glancing angle to a water surface, most of the beam bounces off the surface away from the animal, like light off a mirror. But a small part of the beam travels vertically down from its source, and is reflected right back to the bat. This reflection pattern could act as a flag for water.

Credit: D. NILL

To test this idea, the authors conducted experiments on 15 species of wild bat, placing them in a room that had two large plates on the floor. The plates were made of one of several materials: wood, metal or plastic. Each of the surfaces was either smooth or textured. The smooth surfaces reflect echolocation beams in the same way as water, and, sure enough, thirsty bats repeatedly tried to drink from these surfaces, but ignored the textured ones (see movie at http://go.nature.com/pnpal8). The authors thus concluded that bats use echolocation to recognize bodies of water.

When Greif and Siemers trialled juvenile bats that had had no previous contact with ponds, the animals also tried to drink from the smooth plates, thus revealing the water-location mechanism to be innate. What's more, the authors found that echolocation overrides conflicting sensory stimuli such as vision, chemoreception and touch. For example, if a smooth surface was placed on a table, the bats tried to drink from it even if they had already flown under the table. The authors suggest that innate water recognition in bats could be used to study the neural basis of habitat recognition.