The migration of the Monarch butterfly is one of the most renowned seasonal movements of any animal, made even more impressive by the fact that the 8,000-kilometer (5,000 mile) round trip is not always completed by a single generation. But how the insects manage this epic journey has eluded researchers for a long time. Now, scientists have cracked how this internal compass allows the butterflies to travel such vast distances, and have built a computer model to show how it works. 

Bringing together both biologists and mathematicians, the team has worked out how the different sense organs of the insects come together to calculate what direction they need to fly in – southwest – during their migration from the northern United States and Canada, to central Mexico. They found that the antennae of the butterflies keep count as a clock, while the eyes provide the information on the exact position of the Sun, which allows them to calculate the correct direction.

“We created a model that incorporated this information – how the antennae and eyes send this information to the brain,” explained Eli Shlizerman, a mathematics and engineering professor at the University of Washington, and co-author of the study published in Cell Reports. “Our goal was to model what type of control mechanism would be at work within the brain, and then asked whether our model could guarantee sustained navigation in the southwest direction.”

The fact that the butterflies use their antennae as an internal clock has been known for a while, and they do it in the same way that most other animals, such as humans, manage it. These are based on the rhythmic expression of key genes in the butterfly’s tissue, and help to maintain the insect’s daily physiological and behavioral processes. The information from the “clock” in the antennae is then passed down to the brain via neurons, and is combined with the information on the location of the Sun coming in from the azimuth neurons in the large, complex eyes.

Using a combination of these two inputs, the butterflies then make adjustments to their course in order to continue heading south. So far, the computer model the researchers created seems consistent with the observed behavior of the insects, but there are some aspects for which their model cannot account for, including when they take “long, slow, or meandering” corrections when blown off course.

And what, then, happens when the butterflies reach their end point, and want to return back north? The researchers think the solution is quite elegantly simple – that the neural mechanisms that relay the Sun and time information simply reverse direction. “And when that happens, their compass points northeast instead of southwest,” says Shlizerman. “It's a simple, robust system to explain how these butterflies – generation after generation – make this remarkable migration.”

Now that they know how the butterflies process this information, the team is working to build a robotic butterfly that can use its own internal clock and combine it with the position of the Sun, to follow the insects as they make their epic flight across North America. 

^{Image in text: The butterflies make the migration in vast numbers, often landing in the same small patch of forest, and coating all the branches. Noradoa/Shutterstock}