Moonlight Polarisation Pattern Guides Nocturnal Bull Ants Home

Abstract

AbstractCelestial cues are used by many animals to orient and navigate their environment. The sun or moon can provide directional information via their position; however, they can often be obstructed by clouds, canopy or the horizon. Despite being hidden, these bodies can still provide compass information through the polarised light pattern they produce/reflect. Sunlight produces polarised light patterns across the overhead sky as it enters the atmosphere, and polarised light is a well-known compass cue for navigating animals. Moonlight produces a similar pattern, albeit a million times dimmer than sunlight. Polarised moonlight’s use has, until now, only been demonstrated for straight-line orientation in nocturnal dung beetles. Here we show the first evidence that polarised moonlight forms part of the celestial compass of navigating nocturnal ants. Nocturnal bull ants leave their nest at twilight and rely heavily on the overhead solar polarisation pattern to navigate. Yet many foragers return home overnight when the sun cannot guide them. We demonstrate that bull ants use polarised moonlight to navigate home during the night, by altering the ambient polarisation pattern above homing ants, who alter their headings in response. Furthermore, these ants can detect this cue throughout the lunar month, even under crescent moons, when polarised light levels are at their lowest. Finally, we present evidence that the polarised moonlight pattern is being incorporated throughout the night into these ants’ path integration system for homing, as polarised sunlight is incorporated throughout the day.