Polarized skylight does not calibrate the compass system of a migratory bat

Abstract

In a recent study, Greif et al. (Greif et al. Nat Commun 5, 4488. (doi:10.1038/ncomms5488)) demonstrated a functional role of polarized light for a bat species confronted with a homing task. These non-migratory bats appeared to calibrate their magnetic compass by using polarized skylight at dusk, yet it is unknown if migratory bats also use these cues for calibration. During autumn migration, we equipped Nathusius' bats, Pipistrellus nathusii, with radio transmitters and tested if experimental animals exposed during dusk to a 90° rotated band of polarized light would head in a different direction compared with control animals. After release, bats of both groups continued their journey in the same direction. This observation argues against the use of a polarization-calibrated magnetic compass by this migratory bat and questions that the ability of using polarized light for navigation is a consistent feature in bats. This finding matches with observations in some passerine birds that used polarized light for calibration of their magnetic compass before but not during migration.

Keywords: Chiroptera; mammalian navigation; migration; orientation; sensory ecology.

Figure 1.

Vanishing bearings of bats translocated to the release site (asterisk) in a presumed unfamiliar area 11 km away east from the coastal migration corridor. The natural coastline, where bats were caught and treated, follows the line of longitude. North (0°) is the top of the circular plots. Arrows depict the mean and vector length of all individual migratory flights after departure of the control group tested for natural polarization direction (PN) and the experimental group (PS) treated with a 90° shifted polarization direction (n_PN = 20, n_PS = 20). p-values from the Rayleigh tests are shown. (Online version in colour.)