Temperate bats may alter calls to partially compensate for weather-induced changes in detection distance

Echolocation is the use of self-emitted calls to probe the surrounding environment. The atmosphere strongly absorbs sound energy, particularly high frequencies, thereby limiting the sensory range of echolocating animals. Atmospheric attenuation varies with temperature and humidity, which both vary widely in the temperate zone. Since echolocating insectivorous bats rely on ultrasound to capture insects, their foraging success might decrease with seasonal and daily variations in weather. To counteract weather-induced variations in prey detection, we hypothesised that European bats decrease call frequency and increase call energy when atmospheric attenuation increases, thereby maintaining their prey detection distance. Using acoustic localisation and automated call analysis, we measured call frequency and energy in free-flying bats of three common European insectivorous species. One species, Pipistrellus nathusii/kuhlii, increased call frequency, but simultaneously decreased call energy, while the two other species (P. pipistrellus and Myotis daubentonii) did not alter call parameters. We estimated the detection distance for prey based on the recorded call parameters and prey characteristics, using a custom-developed theoretical model. None of the three species maintained prey detection distance (it decreased by 1.7 to 3.4 m) when atmospheric attenuation increased. This study contributes to a better understanding of the sensory challenges faced by animals in fluctuating environments.