一项研究发现，依靠回声定位的蝙蝠就像微调收音机一样，可能通过在导航和发现猎物的时候调整它们的频率从而让声波干扰最小化。利用固定在大棕蝙蝠头上的微型麦克风，James Simmons及其同事记录下了这些蝙蝠在穿过悬挂在一个飞行舱室的天花板上的塑料链障碍物的时候发出的声音。在训练这些蝙蝠沿特定路线飞行之后，这组科学家重新排列了这些链子，从而观察这些蝙蝠如何通过堆满了障碍物的未知环境。这个舱室被设计成能够再现潜在模糊，如果最初来自一个物体的反射和后来来自其他物体的反射同时到达蝙蝠，这种模糊就可能发生。这组作者报告说，在这种实验情境下，这些蝙蝠通过发出频率稍高或者稍低的额外声音从而对这种经过修改的环境做出响应，这可能解决模糊并帮助识别一个不同的飞行路径。这些发现可能有助于解释使用回声定位的动物如何区分杂乱的环境导致的重叠的声波。

参考文献：Published online before print March 29, 2010, doi: 10.1073/pnas.1000429107

FM echolocating bats shift frequencies to avoid broadcast–echo ambiguity in clutter
Shizuko Hiryua,b,1, Mary E. Batesc, James A. Simmonsb,d,1, and Hiroshi Riquimarouxa,b

Abstract
Sonar broadcasts are followed by echoes at different delays from objects at different distances. When broadcasts are emitted rapidly in cluttered surroundings, echo streams from successive broadcasts overlap and cause ambiguity in matching echoes to corresponding broadcasts. To identify reactions to ambiguity in clutter, echolocating bats that emit multiple-harmonic FM sounds were trained to fly into a dense, extended array of obstacles (multiple rows of vertically hanging chains) while the sonar sounds the bat emitted were recorded with a miniature radio microphone carried by the bat. Flight paths were reconstructed from thermal-infrared video recordings. Successive rows of chains extended more than 6 m in depth, so each broadcast was followed by a series of echoes from multiple rows of chains that lasted up to 40 ms. Bats emitted sounds in pairs (“strobe groups”) at short (20–40 ms) interpulse intervals (IPIs) alternating with longer IPIs (>50 ms). For many short IPIs, the stream of echoes from the first broadcast was still arriving when the second broadcast was emitted. This overlap caused ambiguity about matching echoes with broadcasts. Bats shifted frequencies of the first sound in each strobe group upward and the second sound downward by 3–6 kHz. When overlap and ambiguity ceased, frequency shifts ceased also. Frequency differences were small compared with the total broadcast band, which was 75–80 kHz wide, but the harmonic structure of echoes enhances the differences in spectrograms. Bats could use time–frequency comparisons of echoes with broadcasts to assign echoes to the corresponding broadcasts and thus avoid ambiguity.