Abstract
Increasing global light pollution1,2 threatens the night-time darkness to which most animals are adapted. Light pollution can have detrimental effects on behavior,3-5 including by disrupting the journeys of migratory birds,5,6 sand hoppers,7-9 and moths.10 This is particularly concerning, since many night-active species rely on compass information in the sky, including the moon,11,12 the skylight polarization pattern,13,14 and the stars,15 to hold their course. Even animals not directly exposed to streetlights and illuminated buildings may still experience indirect light pollution in the form of skyglow,3,4 which can extend far beyond urban areas.1,2 While some recent research used simulated light pollution to estimate how skyglow may affect orientation behavior,7-9 the consequences of authentic light pollution for celestial orientation have so far been neglected. Here, we present the results of behavioral experiments at light-polluted and dark-sky sites paired with photographic measurements of each environment. We find that light pollution obscures natural celestial cues and induces dramatic changes in dung beetle orientation behavior, forcing them to rely on bright earthbound beacons in place of their celestial compass. This change in behavior results in attraction toward artificial lights, thereby increasing inter-individual competition and reducing dispersal efficiency. For the many other species of insect, bird, and mammal that rely on the night sky for orientation and migration, these effects could dramatically hinder their vital night-time journeys.
Highlights
We investigated how light pollution, in combination with different celestial cues, affects orientation in the ballrolling African dung beetle Scarabaeus satyrus Boheman
Both dung beetles[22,33] and Drosophila[34] exhibit biases toward high-contrast cues under laboratory ll Despite this greater predicted reliability of the light-polluted starlit skies, beetles viewing them were worse oriented than those viewing an unpolluted starlit sky (W = 94, p = 0.038), performing to beetles with their dorsal eyes completely covered tested at the same site (Figure 4). This failure to orient when celestial cues were obscured by skyglow confirms that skyglow itself did not act as a compass cue, and that beetles exposed to both direct and indirect light pollution in earlier experiments relied on terrestrial cues. From this first simultaneous comparison of light pollution’s effects on both the visual environment and orientation performance of a nocturnal insect, we find that light pollution interfered with orientation behavior under most conditions
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
Summary
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. Available visual information in each scene was recorded using a calibrated camera system[25] and processed to quantify potential compass cues
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