As the sun goes down outside of Sydney, Australia, bull ants march out of their nests and into the trees.
Once there, the ants attack other insects and arthropods, as well as collect tree sap and other resources, before returning to the nest with their haul before dawn.
However, unlike many other ant species, which follow the scent trails left by their sisters, bull ants of the species Myrmecia midas don’t appear to communicate by chemical signal. Instead, each forager forges its own path.
And that raises an interesting question: How do nocturnal bull ants find their way home in the dark?
“It’s always kind of been this mystery,” says Cody Freas, a neuroethologist at Macquarie University in Australia.
Fortunately, a new study appears to shine some light on this enigma: Bull ants use the moon’s polarized light like a nocturnal compass, the scientists conclude.
Polarized patterns
While human eyes are attuned to certain wavelengths of light emitted by the sun and stars or reflected from the moon, the top-most part of bull ants’ eyes can see a pattern in the sky that’s invisible to us—polarized light. This pattern shines through even when celestial bodies are obscured by clouds.
By manipulating the pattern with a filter placed over the ants as they returned to nests in the wild, the scientists were able to show that the insects were following that signal and making course-corrections based on it in real-time, as opposed to navigating based on a memory of its position.
The filter “basically changes the entire sky above them,” explains Freas, who was lead author of the study, recently published as a preprint in the journal eLife.
The ants showed themselves capable of navigating by polarized moonlight even during a crescent moon, when the light signal would have been just 20 percent as strong as a full moon—which itself provides light one million times weaker than direct sunlight.
While dung beetles are known to use polarized light from the moon and even stars to keep their balls rolling in a straight line, this is the first time an animal has been shown to navigate by polarized moonlight toward a fixed location—the bull ants’ home.
(Watch dung beetles take their dinner to go.)
Bulls on parade
While the study is the first to document the ants’ use of polarized light for nocturnal navigation, the scientists have had a hunch for awhile now that the moon was involved.
“A big hint that the moon was kind of at play is that you see a 20 percent increase in foragers when the moon is full, versus a new moon [when the sky is much darker],” says Freas, referring to an earlier study of closely-related M. pyriformis ants.
This suggested that bull ants could see better when moonlight was stronger.
Interestingly, despite traveling up to 78 feet along the ground in the dark, Freas says nearly every ant makes it back to the nest. In fact, in all his observations of bull ants, he’s only noticed a single ant go missing.
“They live for over a year, and they typically go to the same tree every night. So it’s not a new route every night,” says Freas. “So that suggests, at least to me, that they’re really good at getting home. There’s very little getting lost, and there’s also very little overnight predation.”
Bull ants are considered quite feisty, especially when disturbed by humans, which may also deter other predators. Freas says the largest bull ants can be more than an inch long, and capable of stinging curious scientists through their gloves. (He knows from experience, by the way.)
“If you’re moving around while you’re near the nest, they will home in on your movement and continue to go after you for multiple meters,” he says.
(Learn about an ant that rips itself apart to protect its own.)
‘Chasing ants around in the dark’
Since researchers previously discovered that dung beetles navigate by polarized light at night, finding the same ability in another insect isn’t too surprising. In fact, it’s likely still more insects are navigating by these cues, unbeknownst to us.
The catch is, you need a good behavior that allows you to test for it, says James Foster, a neuroethologist at the University of Konstanz in Germany who studies dung beetles.
For instance, when dung beetles collect their little ball of animal feces, they try to roll it away from competitors as quickly and efficiently as possible. Usually, that means a straight line.
Similarly, bull ants returning home from a night of foraging is a repeatable behavior that allowed the scientists to test their theories upon, he says. But there is a big difference between the two animals.
Dung beetles “don’t have to find any specific location,” says Foster who reviewed the study during the publication process. “Whereas for these ants, they have to find their way home quite precisely.” That makes the bull ants’ feat even more impressive.
The other big difference? Dung beetles don’t sting.
“I can’t imagine how difficult these experiments were to do, chasing ants around in the dark,” says Foster.
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