Flight performance, echolocation and foraging behaviour in pond bats, Myotis dasycneme (Chiroptera: Vespertilionidae)



Flight performance and echolocation behaviour of the pond bat, Myotis dasycneme (Boie, 1825), were investigated over canals and a lake in the Netherlands. Multi-flash stereophotogrammetry, linked with synchronous recording of echolocation calls, was used to reconstruct the three-dimensional flight paths of bats and to correlate flight behaviour with changes in pulse emission during echolocation. Echolocation calls during commuting flight at the canal were sigmoidal in structure, with an initial steep frequency modulated (FM) sweep followed by a more shallow sweep, ending with a second, rapid FM sweep. Similar calls were produced during the search phase of foraging, though longer duration quasi-CF (constant-frequency) calls, with longer interpulse intervals and much reduced initial and final FM sweeps, were produced by bats hunting over a large lake. Overall, calls were generally lower in frequency, with significantly longer pulse durations and interpulse intervals, compared with those of smaller trawling Myotis bats. Flight speeds were compared with predicted optima from aerodynamic models. Commuting bats exceeded predicted maximum range speed, which was considerably higher than that noted for other smaller species of trawling Myotis. Flight speed was significantly higher closer to the roost, at a narrower canal site. A negative correlation between wingbeat frequency and flight speed, and a facultative 1: 1 link between pulse emission and wingbeat during search phase were established. During commuting, the bats flew at a height above that which would normally confer substantial power savings through ground effect. It is argued that, by flying at a lower height, bats would compromise commuting speed. Echolocation calls produced during attempted prey capture were remarkably similar in design to those produced by those other Myotis species that forage in a similar fashion. Prey was detected at 2 m, and reaction distance was 1.8 m. Evidence of a physiological limit on pulse duration and interpulse interval during prey capture is presented. Overlap of emitted pulse and received echo may degrade the performance of echolocation in many bats, and overlap is generally avoided in M. dasycneme except perhaps when the bat is very close to the prey item.