Here, we report that even the extremely dim celestial polarization pattern formed around a crescent Moon is sufficient to guide a ball-rolling beetle along a straight route. Moreover, the straight-line orientation on these dark nights is performed with the same precision and speed as beetles orienting under the much brighter sky lit by the Sun or full Moon. Even when challenged with major disturbances along their chosen route, such as a 90° rotation or a 5 cm fall, the compass systems of nocturnal and diurnal beetles show no difference in performance. These results strongly indicate that the light-sensitive optical compass sacrifices neither spatial nor temporal resolution when supporting the straight-line orientation in dim light.
In the laboratory, crickets have been shown to detect the direction of highly polarized light at light intensities that are even lower than that of a clear, moonless night sky [24
]. Dung beetles may do the same, but at such light levels the Moon, and its pattern of polarized light, are no longer present to guide beetles and their balls along a straight path. A polarization detection system with greater sensitivity than that demonstrated here would have no biological relevance for straight-line orientation in beetles, unless they can use polarized starlight [25
]. It is, however, hard to see how the combined polarized light patterns formed around thousands of stars would provide a navigator with directional guidance. On the other hand, an orientation to stars themselves could possibly explain why some of the beetles are able to orient along straight paths even when the Moon is well below the horizon (d
). It is interesting to note that the Milky Way was clearly visible during the experimental nights without a Moon, and ran like a bright streak across the sky, providing a potential orientation cue. In earlier investigations into the orientation performance of S. zambesianus
, the beetles rolled their balls of dung along spiralling tracks after the Moon and its pattern of polarized light had vanished from the sky [11
]. However, in these experiments, the Milky Way was positioned much closer to the horizon and was not visible from the arena. It remains to be tested whether dung beetles can use the Milky Way as an orientation cue for straight-line orientation.
With a precision equal to that found in the diurnal celestial compass, the nocturnal celestial compass displays a robustness throughout the lunar month that suggests it might be widely used by animals that need to find their way at night. Nocturnal bees [26
], crickets [24
], spiders [28
], tenebrionid beetles [29
] and possibly also birds [30
] are possible candidates that might benefit from the ability to orient using the dim polarization pattern formed around the ever-changing disc of the Moon.