Skylight polarization as perceived by desert ants and measured by video polarimetry

Abstract

North African desert ants belonging to different genera and inhabiting different areas (sand dunes, salt pans, inundation flats and gravel plains) exhibit different ways of skylight navigation: some rely especially on the polarized light in the sky, others depend more effectively on the position of the sun. Are these differences due to species- or genus-specific idiosyncrasies of the ant's skylight compass, or are they caused by differences in the overall degree of polarization prevailing in the celestial hemisphere that vaults the different kinds of habitat? Theoretically, such differences are to be expected, as various parameters known to influence the degree of polarization in the Earth's atmosphere – such as the albedo of the ground and the content of water vapour, dust and haze in the airlayers above the ground – do vary between the different types of habitat mentioned above. The first wide-field, video-polarimetric study of skylight polarization presented here clearly shows that at any particular locality the temporal (day-to-day) variations of the degree of skylight polarization are much more pronounced than the differences recorded at the same local time at different localities. In contrast, the angle of polarization is unaffected by atmospheric disturbances and accords well with the predictions of Rayleigh scattering. Consequently, differences in behavioural performances of navigating North African desert ants are due to interspecific and intergeneric differences in the ants' navigational systems rather than to general differences in the skylight stimuli experienced by the ants during navigation.