Credit: J. SIMMONS, J. BARCHI, J. GAUDETTE, J. KNOWLES

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1000429107 (2010)

In a cluttered environment, how does a bat emitting a rapid train of sonar pulses not mistake the echo of an initial pulse from a distant object for that of a later pulse from a closer object?

To find out, Shizuko Hiryu at Doshisha University in Japan, James Simmons at Brown University in Providence, Rhode Island, and their colleagues monitored big brown bats (Eptesicus fuscus) as they flew through rows of hanging chains (pictured).

The researchers recorded the bats' sonar signals using small radio microphones carried by the animals. They found that the bats produced pairs of sounds, with the intervals between sounds alternating between short and long. When the shorter pairs created interfering echoes, the bats shifted the first of the two sounds upwards in frequency by 3–6 kilohertz, and the second downwards by the same amount. The authors suggest that this allows the bats to avoid ambiguous echoes in crowded environments, such as dense vegetation.