An Australian bull ant is the first animal known to use the patterns produced by polarised moonlight to navigate its environment.
They may have a brain the size of a pinhead and terrible eyesight, but bull ants can navigate at night using even the faintest polarised light emitted by a crescent moon.
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| Nocturnal bull ants use polarised moonlight to find their way back to their nests Cody Freas |
Light from the sun and the moon consists of waves that vibrate in different directions, but on entering the atmosphere it becomes polarised, meaning the waves are aligned in the same direction across the sky. Many animals can detect this polarity in sunlight and use it to get a sense of direction, even if they can’t see the sun directly.
Moonlight, however, can be as dim as a millionth of the strength of the sunlight that still glows after sunset, in the twilight before full darkness.
The bull ant Myrmecia midas, which is native to Australia, is the first animal known to use the polarity of moonlight for navigation, says Cody Freas at Macquarie University in Sydney.
These ants emerge from their nests after sunset to forage for food in trees and then return home in the dawn twilight.
To test how the ants navigate, Freas and his colleagues placed a polarising filter above them as they returned to their nests and rotated it. The filter, which altered the polarisation patterns of the moonlight, made the ants change direction in predictable ways.
Once the filter was removed, the ants immediately changed their course back to the correct route.
“They’re moving on the ground, under shrubs and trees, so a direct view of the sunlight and moonlight is often blocked,” says Freas. “But they have still got patches in the canopy where the sky is clear and this polarising pattern makes its way through to the ants. That is enough for them to keep their celestial compass working through the night.”
Adding further weight to the finding, the researchers found that when a new moon means there is no moonlight, fewer bull ants attempt to reach trees to feed after twilight.
Kate Umbers at Western Sydney University says it is a “beautiful study”. “It provides a marvellous example of how sophisticated our little friends in the undergrowth really are,” she says. “I hope next time people look at the moon, they think of a bull ant peering at it, trying to find her way home.”
Until now, only dung beetles were known to use moonlight polarisation patterns to guide their movement, but they solely do this to move their dung balls away in a straight line, rather than to navigate to a specific location.
Reference:
bioRxiv DOI: 10.1101/2023.12.28.573574