See It to Believe It: Moths Use Stars to Navigate During Migration

Imagine traveling 600 miles to an unfamiliar place at night, without GPS or any maps. Now, consider that you only have one chance in your life to make this journey. Fortunately, humans don’t have to go through this experience, but moths do.

An annual pilgrimage. Every year, billions of Bogong moths (Agrotis infusa) in Australia undertake this annual pilgrimage. Notably, a group of scientists recently discovered that these moths navigate by observing stars.

According to a study published in Nature, the Bogong moth is the first known insect to use a stellar compass for long-distance navigation. This skill was previously believed to be exclusive to some nocturnal migratory birds.

A lifetime journey. Each spring in Australia, Bogong moths undertake a journey of up to 600 miles. They hatch in the warm plains of Southeastern Australia. Then, they fly to a few cold caves in the Australian Alps, where they spend the summer (a process called “estivation”). When autumn arrives, Bogong moths return to their breeding grounds to reproduce before ultimately dying.

A central question that has intrigued biologists for years is how they manage to make this journey. No one shows them the way. Each individual moth makes this round trip only once. The same research team had already demonstrated in 2018 that moths are sensitive to the Earth’s magnetic field, similar to other animals. Still, researchers suspected there was more to the moths’ navigation strategy, so their focus shifted to the sky.

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A planetarium for moths. To confirm their hypothesis, researchers designed an ingenious experiment. They captured moths during their migration and placed them in a flight simulator. This device, similar to a small planetarium, held the moth in place while allowing it to flap its wings and turn freely, with a sensor recording its flight direction.

The team conducted several tests:

1. Canceling out the magnetic field: Using a coil system, researchers created an environment devoid of magnetic cues, ensuring that any correct orientation was due to visual inputs.
2. Projecting a natural starry sky: Under an artificial night sky that mimicked the stars visible during migration, moths consistently oriented themselves in the correct migratory direction: south in spring and north in autumn.
3. Rotating the sky 180 degrees: In the final test, researchers rotated the starry sky projection 180 degrees. The moths’ reaction was immediate and astonishing: They reversed their flight direction almost exactly 180 degrees.
4. Projecting random stars: To confirm that moths weren’t simply responding to light, the team projected an image with the same number of stars and brightness, but with a random distribution that lacked recognizable patterns. Under these conditions, moths flew disoriented and without a clear direction.

Unexpected findings. These results conclusively showed that moths not only see stars but also use them as a true compass. Moths use stars to maintain a specific geographical course for hundreds of miles.

It was already known that other insects, such as dung beetles, use the Milky Way for orientation. They primarily do so to move in a straight line away from a pile of manure, which is a short-term navigation method without a fixed destination.

However, the Bogong moth’s method is much more complex. It uses stars to “distinguish between specific geographical directions” and navigate toward distant targets. This is a radically different and much more sophisticated form of navigation.

A brain connected to stars. Researchers didn’t stop at observing behavior. They also analyzed the moths’ brains and discovered visual neurons that specifically responded to the rotation of the starry sky. Interestingly, these neurons showed maximum activity when the moth was heading south, regardless of the season. This suggests that moths have a neural system wired to detect fundamental celestial orientations.

Magnetic compass. In another experiment, scientists observed that on completely cloudy nights, when the stars were invisible, moths still oriented themselves correctly. As such, the Bogong moth has an incredibly robust and redundant navigation system. It utilizes both the Earth’s magnetic field and a stellar compass, likely using one system to calibrate the other or to take over when one fails.

There are still mysteries to unravel, such as which exact stars or constellations Bogong moths rely on or how they compensate for Earth’s rotation during the night. However, it clearly wasn’t just ancient sailors who looked to the stars to find their way. A tiny insect, on its unique journey, does so as well.

Image | Ojaswi Pratap Singh

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