Image credit: Thomas Shellberg via Unsplash (CC0)
Light travels through space as waves, normally oscillating in various orientations. When light waves enter the earth’s atmosphere, they become polarised, meaning the waves oscillate along a single plane. Polarised light creates a predictable pattern that is imperceptible to humans but visible to many animals. This pattern provides animals a reliable ‘sky compass’ based on the sun’s position during the day, which is particularly useful because it is visible even when the sun is obscured.
However, many nocturnal animals can successfully navigate well after sunset when this solar sky compass is absent. The moon, reflecting sunlight, creates a similar polarisation pattern in the sky. However, moonlight is about a million times dimmer than sunlight, so nocturnal animals need highly specialised visual systems to detect these lunar cues.
One of these nocturnal animals is the large-eyed bull ant (_Mymecia midas_). These ants rely on solar polarised light to navigate during twilight, when they climb into the canopy of trees surrounding their nest. However, these ants often return home overnight, suggesting they can use other celestial cues when navigating, such as the moon or stars. However, it remains unknown which cues bull ants use at night.
To determine whether bull ants can detect polarised moonlight to orient themselves, Freas et al. used polarisation filters to rotate the polarisation pattern in the night sky above two bull ant nests. The researchers found that when the polarisation pattern was rotated, the forager bull ants altered their direction in line with the change in the polarised pattern, showing that they were using this pattern to navigate. Interestingly, the ants could detect the lunar polarisation pattern even when the moon was below the horizon, or when only a sliver of the moon was reflecting sunlight, indicating extremely high visual sensitivity in these animals.
Freas et al. show for the first time an instance of animals using polarised moonlight patterns to navigate. This type of investigation can be used to develop navigational systems for robotics. The results suggest that a polarisation sensor with sufficient sensitivity can be used to create a sky compass to navigate both at night and during the day.