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

Abstract

A degree-of-polarization threshold for orientation behaviour is 15 reported for nocturnal dung beetle Escarabaeus satyrus in the context 16 of measurements showing changes in the degree of polarization of 17 skylight with lunar phase. 18 Abstract 19For polarized light to inform behaviour, the typical range of degrees of 20 polarization observable in the animal's natural environment must be 21 above the threshold for detection and interpretation. Here we present 22 the first investigation of the degree of linear polarization threshold for 23 orientation behaviour in a nocturnal species, with specific reference to 24 the range of degrees of polarization measured in the night sky. An 25 effect of lunar phase on the degree of polarization of skylight was 26 found, with smaller illuminated fractions of the moon's surface 27 corresponding to lower degrees of polarization in the night sky. We 28 found that South African dung beetle Escarabaeus satyrus 29 (Boheman, 1860) can orient to polarized light for a range of 30 degrees of polarization similar to that observed in diurnal insects, 31 reaching a lower threshold between 0.04 and 0.32, possibly as low as 32 2 0.11. For degrees of polarization lower than 0.23, as measured on a 33 crescent moon night, orientation performance was considerably 34 weaker than that observed for completely linearly-polarized stimuli, 35 but was nonetheless stronger than in the absence of polarized light. 36

Light pollution forces a change in dung beetle orientation behavior

“…This skyglow obscured the Milky Way, leaving only bright stars visible (Figures 1A and 1B). As reported in previous studies, 24,26 on the moonlit night, skyglow reduced the degree of polarization of the lunar skylight polarization pattern, from 60%-70% (the typical range around the full moon 24 ) to just 19% (Figure S1; more typical for a crescent moon 24 ). Direct light pollution from nearby buildings was also present under all celestial conditions, either projected onto the observer or reflected from other buildings.…”

“…These effects may extend to the plethora of other night-active species, including arthropods, seals, and migratory birds, that rely on stars, 15 the moon, 11,12 and patterns of polarized skylight 13,14 to orient. The sky measurements presented here (Figures 1 and 3) and in previous studies 35,36 show how artificial skyglow can obscure all but the brightest stars and reduce the polarization of lunar skylight, 24,26 decreasing the availability of natural celestial compass cues.…”

“…In this study, we investigated how light pollution, in combination with different celestial cues, affects orientation in the ballrolling African dung beetle Scarabaeus satyrus Boheman. This nocturnal species performs a well-described orientation behavior, [19][20][21][22][23][24] typically relying on the Milky Way 19,20,22 and the lunar polarization pattern 19,21,22 to hold its course. We recorded orientation behavior at a light-polluted urban site and a dark-sky rural site under moonlit, starlit, and overcast skies.…”

“…The starlit sky experiments were carried out on nights when the moon was more than 18 below the horizon, and lunar skylight was not present, 24…”

“…It should also be noted that, despite the decrease due to skyglow (Figure S1), at 19%, the degree of polarization of lunar skylight at the light-polluted site remained above the 11% threshold reported for this species. 24 As a consequence, these beetles may have been able to utilize compass information from skylight polarization, in combination with the moon's position, 21 to achieve natural orientation behavior. Less-sensitive species may therefore be more strongly affected by the degradation of polarized skylight.…”