Figure 1.

The different paths of the ball-rolling beetle S. satyrus rolling outwards from the centre of a circular arena (as seen from above) are reconstructed from video footage recorded under a sky lit by (a) full Moon (average length of the tracks, L = 132.6 ± 1.7 cm; rolling speed, V = 4.5 ± 0.3 cm⁻¹; n = 25), (b) quarter Moon (L = 135.1 ± 2.1 cm; V = 3.8 ± 0.3 cm⁻¹; n = 26), (c) crescent Moon (L = 134.5 ± 1.4 cm; V = 3.9 ± 0.2 cm⁻¹; n = 20) and (d) on a moonless night (L = 185.1 ± 12.2 cm; V = 3.8 ± 0.3 cm⁻¹; n = 25). The diameter of the arena was 3 m. A 1 m high wall around the arena prevented a direct view of the Moon or terrestrial landmarks. The tracking of the beetle began once it rolled out from the inner, 30 cm diameter circle. The tracks were analysed for speed and length. The orientation performance—as measured by these two parameters—remains consistent for as long as there is a Moon present in the sky (ANOVA, L: p = 0.33, V: p = 0.14). The tracks do, however, become significantly longer (more curved) if the beetles roll on a moonless night (ANOVA, p < 0.001). On these nights, there is no polarization pattern present in the sky to guide the beetles along a given route and their orientation along a straight line becomes significantly impaired. A moonless night is defined as a night when the Moon is lower than 18° below the horizon and the reflected light from this celestial body is no longer visible in the night sky.