Diurnal Bull Ants Navigating by Moonlight: Polarised Light Homing in Myrmecia tarsata

We present evidence that the faint polarised moonlight pattern of the sky can be detected and used for navigation in the diurnal bull ant Myrmecia tarsata, despite this species lacking the highly refined low-light visual specialisations of the nocturnal bull ant, Myrmecia midas. The position of celestial bodies such as the sun and moon can provide navigating animals directional information, yet direct observation can often be occluded. Animals can estimate positional information of solar and lunar cues via their polarised light pattern, present across the sky. The suns polarisation pattern is widely used in animals and a similar, yet much fainter pattern is produced by the moon, yet it is unknown how widespread moonlights use is in navigating animals. Here, we demonstrate that the bull ant Myrmecia tarsata, which forages throughout the day, returning home at sunset, can use both solar and lunar polarised light patterns to navigate. We compare these findings to the closely related M. midas navigating under identical light conditions, as this nocturnal bull ant is known to rely on these polarised light patterns as part of their celestial compass. While M. midas and M. tarsata can clearly use both the solar and lunar polarised light patterns to navigate, M. tarsata showed degraded performance under polarised moonlight as a function of lunar phase, decreasing performance as illumination decreased. M. midas in contrast, exhibited impressive attendance to the overhead lunar polarisation pattern throughout the lunar month, illustrating the highly specialised low-light-detection adaptations of bull ants.