Incredible secret of monarch butterfly migration finally solved

Monarch butterflies are incredible navigators. Every year they fly thousands of miles back and forth between their wintering grounds in Mexico and their summer habitat in the northern U.S. and southern Canada, and until now no one has understood how they do it. The mystery is solved. A new study shows how they get the information to determine which way to go.

The study, which was published Thursday in Cell Reports, describes how researchers used a computer model of the compound eye of monarchs to figure out the puzzle, beginning with earlier studies that determined that the butterflies use the sun’s position in the sky, along with an internal clock, to figure out which way is south. This is called a time-compensated sun compass. However, the scientists had not figured out how that information translated from the butterfly’s brain into action.

As a monarch butterfly moves, its eyes constantly register where the sun is in relationship to the horizon. The butterfly always needs to know where the sun is in its east-to-west path, which is where the internal clock comes in.

Embedded in the monarch’s antennae are biological clocks that help the butterfly know what time it is. These clocks enable them to know whether the sun is rising or setting when it appears close to the horizon, thus knowing if it is east or west. The butterfly then knows whether to fly with the sun to its right or to its left in order to go south.

The connection between those two sets of data have been unknown. However, this new research developed what they think is the neural mechanism behind it by feeding the data they already knew about the monarch’s navigational system into a computer model.

What they found is that an oscillating neuronal signal goes to the butterfly’s brain as the receptive fields in its eyes detect the sun’s position. At the same time, the circadian clock is sending more oscillations. The combination of these signals tells the monarch’s brain how to adjust its course.

One of the most interesting findings was what happens if a butterfly is blown far off of its southerly course. If this happens, they actually rotate their bodies in a full circle in order to “reset” them to the proper direction. This occurrence did not happen only in the computer model, according to Eli Shlizerman, lead author on the study. The scientists observed that type of adjustment rotation in actual butterflies.

Then there’s the question of what happens when the insects need to return north, since they are able to find the correct direction right away without any aimless wandering. Apparently both north and south are somehow wired into the neurobiology of the monarchs.

The model used looked only at how the mechanism works when the sun is clearly visible. The butterflies are known to navigate on partly cloudy days by using polarized light. And they still go in the correct direction on overcast days. Steven Reppert, another of the researchers, says this is likely due to a sort of magnetic compass also found in the insect’s brain.

The next plan is for research on the biology of butterflies, to determine whether the model actually matches the monarch’s brain structure.

Marcus Kronforst, an ecologist at the University of Chicago who was not involved in the study, says it is important that people try and understand the incredible feat the butterflies accomplish in their “amazing long-distance migration.” Not only will the research help scientists understand the migration of monarchs, but it might also provide information on navigational tools of other migratory animals.