Migratory bats are sensitive to magnetic inclination during compass calibration

The Earth’s magnetic field is used as a navigational cue by many animals. For mammals, however, there is little data to show that navigation ability relies on sensing the natural magnetic field. In migratory bats, however, the calibration of a magnetic compass became plausible following experiments demonstrating a role for the solar azimuth at sunset in their orientation system. Here, we investigated how an altered magnetic field at sunset changes the nocturnal orientation of the bat Pipistrellus pygmaeus . We exposed bats to either the natural magnetic field, a horizontally shifted field (120°), or the same shifted field combined with a reversal of the natural value of inclination (70° to −70°). We later released the bats and found that the take-off orientation differed between all treatments. Bats that were exposed to the 120° shift were unimodally oriented northwards, in contrast to controls which exhibited a North-South distribution. Surprisingly, the orientation of bats exposed to both a 120°-shift and reverse inclination was indistinguishable from a uniform distribution. These results provide the missing link that these migratory bats calibrate a magnetic compass at sunset, and for the first time, they show that bats are sensitive to the angle of magnetic inclination. ### Competing Interest Statement The authors have declared no competing interest.