Adaptive and evolutionary aspects of call design in echolocating bats

Bat echolocation calls are often used as examples of ‘good design’ through evolution by natural selection. Theory developed from acoustics and sonar engineering permits a strong predictive basis for understanding echolocation performance. Call features such as frequency, bandwidth, duration, and pulse interval are all related to the ecological niche occupied by a given bat species. Call design can even be changed to a limited extent within species and even within individuals according to ecological challenges such as proximity to clutter. Recent technological breakthroughs have permitted advances in understanding adaptive aspects of call design in free-living bats. Stereo videogrammetry and acoustic flight path tracking have been used to reconstruct the flight paths of echolocating bats in the wild. These methods have permitted new insights into the call intensities used by echolocating bats, showing that echolocation calls can be among the most intense airborne vocalizations produced by animals. Acoustic tracking has clarified how bats vary the shape, or curvature of calls in relation to flight speed. During flight, ranging accuracy is perturbed first because bats call and receive echoes at different positions and second because echoes are modified by Doppler shifts. Certain call designs avoid both sources of ranging error, but only for a limited range of distances [the ‘distance of focus’]. Bats using broadband echolocation calls adjust call design in a range-dependent manner so that nearby obstacles are localised accurately. Finally, recent phylogenetic analyses based on genetic sequence data have shown that particular types of echolocation signals have evolved independently in several lineages of bats. Call design is often influenced more by perceptual challenges imposed by the environment than by phylogenetic constraints, and echolocation calls can serve as excellent examples of convergent evolution. Now that whole genome sequences of bats are imminent, understanding the functional genomics of echolocation will become a major challenge.