How Bogong Moths Navigate by the Stars

Every year, the bogong moth undertakes an extraordinary journey across Australia.

As warmer spring weather arrives across the country, billions of bogong moths (Agrotis infusa) set off, flying unfailingly by night for up to 1,000 kilometres to somewhere they have never visited: the cool caves high in the Australian Alps.

There they enter a dormant state known as aestivation, waiting out the hot summer before dispersing to breed in autumn. Their offspring must then find their own way to the summer caves. Scientists have long been intrigued by how the moths manage this: a bogong lives for just one year, so the route must somehow be hardwired.

Researchers have now found one part of the answer: the moths follow the stars.

How bogong moths navigate at night

"In our study," neuroscientist Andrea Adden of the Francis Crick Institute in the UK told ScienceAlert, "we show that bogong moths can use the starry sky (without any additional cues) to fly in that migratory direction, which tells us that they can use it to navigate: fly in the correct direction stably over many kilometres to a specific migratory goal."

The bogong moth migration is a remarkable sight. The moths spend hours flying through the night, then rest during the day in whatever cracks and crevices they can find. Towns can even be blanketed in sleeping bogongs on their way to the Australian Alps, and the full migration may take many nights.

Animals use a range of signals and stimuli to navigate over long distances. Some have special adaptations that let them sense the magnetic field surrounding Earth. Others rely on visual clues, including the Moon, the Sun or landmarks.

Earlier research led by zoologist David Dreyer and senior author Eric Warrant of Lund University found that bogong moths combine magnetoreception with visual cues. Magnetism, however, may now prove less important than previously thought.

Building on that work, Dreyer, Adden, Warrant and their colleagues carried out a series of experiments to identify the visual cues involved. They used a Helmholtz coil system to cancel out Earth's magnetic field and projected different star-filled views onto a vacuum chamber. Even without the magnetic field, the moths flew in a direction appropriate to the season.

The researchers also presented the moths with different night-sky images while Adden used single-cell electrophysiology to record their brain activity.

"A very thin glass electrode (thinner than a human hair) is inserted into specific brain regions of a moth to penetrate the cell-membrane of certain navigation relevant neurons. The signal or electric activity of such a neuron is now amplified and recorded for subsequent analysis," Dreyer explained.

"While the cell was impaled, the moth was stimulated with rotations of a projected image of the starry sky and various controls. It turns out that about 28 of the recorded neurons responded to changes of the orientation of the starry sky, not the control image (image in which a randomised arrangement of the starry sky was presented)."

A longer journey than the dung beetle's

That rotation matters. To see why, consider another animal that takes its bearings from the stars: the dung beetle. Earlier research showed that dung beetles use a mental map of the stars to return home after rolling a ball away from a dung heap. Their journey, though, is very different from a bogong moth's.

"Dung beetles don't care where they end up with their dung ball, they roll their ball in a random direction away from competitors on the dung heap," Adden explained. "Also, dung beetles only need to get far enough from the dung heap to eat their meal in peace, a distance they travel in about 10 minutes."

A bogong moth's trip is far longer and can last several weeks, with hours of flight at a time. The stakes are much higher, too: a moth that fails to reach the cave before summer will not survive to the next breeding season.

"It needs to compensate for crosswinds and most importantly, if the bogongs predominantly use their sky compass, they would need to compensate for the celestial rotation over the course of a respective night," Dreyer said.

"This means that if bogong moths fly at an angle relative to a particular cue in the sky (for example, the Carina Nebula or the long axis of the Milky Way), this angle would need to change accordingly through steering to keep a straight line of flight."

The precise features of the stars that guide the moths remain unknown. Nevertheless, the team's research clearly demonstrates that, beneath a starry sky and without a magnetic field, the insects can still find their way.

Questions about the bogong moth's destination

"During our research, we've had two main questions. Firstly, how does the Bogong moth know the direction it needs to travel? And secondly, how does it know when to stop?" Warrant told ScienceAlert.

"We are starting to work on the second question now, to determine the sensory cues that might be associated with the destination – this is our next line of research. But another obvious area of future research is to try and understand how magnetic and stellar information is integrated in the brain."

Navigating by celestial cues is fairly common among animals: humans, some birds, and some seals and frogs do it. Other moths and butterflies navigate using the Sun. The bogong moth is therefore unlikely to be the only insect capable of navigating this way at night, although that makes its ability no less wondrous.

"That a tiny insect with a wingspan of 5 cm and a brain the tenth of the volume of a grain of rice manages to fly about 1000 km at nighttime, potentially just by using the stars to steer the course still amazes me," Dreyer said.

"Imagine someone gives you the task to walk such a distance without food or shelter, exclusively at nighttime without GPS or a compass. If one makes just a small, let's say five-degree, mistake while determining the walking direction on the first night, that means you are already 90 kilometres off target after 1000 kilometres, and if you have to walk on multiple nights, there is plenty of time for steering mistakes. The story doesn't get old."

The research was published in Nature.