Abstract
We present evidence that the faint polarized moonlight pattern of the sky can be used for navigation in a diurnal animal, the bull ant Myrmecia tarsata. This comes despite this species lacking the highly refined low-light visual specializations of nocturnal bull ants. Celestial bodies can provide animals with directional information, yet direct observation can often be occluded. Positional information of solar and lunar cues can be estimated via their polarized light pattern, present across the sky. The sun's polarization pattern is widely used in animals, and a similar, yet much fainter, pattern is produced by the moon. However, given the inherent variability of the moon, it is unknown how widespread moonlight use is in navigating animals. Here, M. tarsata, which forages throughout the day, returning home at sunset, uses both solar and lunar polarized light to navigate. We compare this to the closely related nocturnal sympatric Myrmecia midas navigating under identical conditions. Both species clearly use solar and lunar polarized light patterns to navigate, but M. tarsata showed degraded performance under polarized moonlight as a function of lunar phase, decreasing performance as illumination decreased. Myrmecia midas, in contrast, exhibited impressive attendance to lunar polarization patterns throughout the lunar month.