Every year, the bogong moth makes an epic journey across Australia.
When the warmer days of spring spread across the country, billions of bogong moths (Agrotis infusa) up stakes and fly, unerringly and only at night, up to 1,000 kilometers (620 miles) to a place they have never been before: the cool caves high in the Australian Alps.
There, they will enter a state of dormancy – called aestivation – to wait out the hot summer before dispersing again to breed in autumn, creating the next generation of moths to find their way to the summer caves. Exactly how they accomplish this feat has long fascinated scientists: the lifespan of the bogong is just one year, so the route must be hardwired in somehow.
Now, a piece of the puzzle has been found. They follow the stars.
“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 kilometers to a specific migratory goal.”
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The flight of the bogong moths is an amazing thing to experience. They fly for hours through the night, stopping to rest during the day in any crannies and crevices they can access. It’s not unheard of for a town to be blanketed with napping bogongs on their way to the Australian Alps; the entire migration can take many nights.
To navigate long distances, animals rely on a variety of signs and stimuli. Some may use special adaptations to sense the magnetic field that encompasses the planet. Others may use visual cues, such as following the Moon, the Sun, or landmarks.
Previous research led by zoologist David Dreyer and senior author Eric Warrant of Lund University showed that bogong moths use a combination of both magnetoreception and visual cues. It now appears magnetism might not play as big a role as thought.
To build on these earlier findings, Dreyer, Adden, Warrant and their colleagues have now conducted a series of experiments to find out what the visual cues in question might be. Using a Helmholtz coil system, which nullifies Earth’s magnetic field, they projected different starry vistas onto the vacuum chamber, and observed that the moths still flew in a seasonally appropriate direction.
They also showed moths different images of the night sky while Adden recorded their brain activity using single-cell electrophysiology.
“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 randomized arrangement of the starry sky was presented).”
That rotation is important, and to understand why, we have to consider another animal that uses the stars as a guide: the dung beetle. Previous research has shown that dung beetles use a mental stellar map to return home after rolling a ball away from the dung heap. But their journey is quite different from the one bogong moths undertake.
“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.”
The journey of a bogong moth is much longer, taking up to several weeks, for hours at a time, with much higher stakes: if the moth doesn’t make it to that cave in time for summer, it’s not going to survive into 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.”
We don’t know exactly what stellar properties the moths are basing their navigation on, but the team’s research clearly shows that, in the absence of a magnetic field, and under a starry sky, they are still able to find their way.
“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.”
Celestial navigation is pretty common in the animal kingdom. Humans do it, some birds can do it, and some seals and frogs. Other moths and butterflies use the Sun to navigate. So it’s unlikely that the bogong moth is the only insect that can navigate at night in this way. That, however, does not make it any less of a wonder.
“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 kilometers off target after 1000 kilometers, 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 has been published in Nature.
