Orienting to Polarized Light at Night—Matching Lunar Skylight to Performance in a Nocturnal Beetle

Abstract

AbstractFor polarized light to inform behaviour, the typical range of degrees of polarization observable in the animal’s natural environment must be above the threshold for detection and interpretation. Here we present the first investigation of the degree of linear polarization threshold for orientation behaviour in a nocturnal species, with specific reference to the range of degrees of polarization measured in the night sky. An effect of lunar phase on the degree of polarization of skylight was found, with smaller illuminated fractions of the moon’s surface corresponding to lower degrees of polarization in the night sky. We found that South African dung beetleEscarabaeus satyrus(Boheman, 1860) can orient to polarized light for a range of degrees of polarization similar to that observed in diurnal insects, reaching a lower threshold between 0.04 and 0.32, possibly as low as 0.11. For degrees of polarization lower than 0.23, as measured on a crescent moon night, orientation performance was considerably weaker than that observed for completely linearly-polarized stimuli, but was nonetheless stronger than in the absence of polarized light.Summary StatementA degree-of-polarization threshold for orientation behaviour is reported for nocturnal dung beetleEscarabaeus satyrusin the context of measurements showing changes in the degree of polarization of skylight with lunar phase.