SCIENCE

The uncanny mechanisms that monarch butterflies use to navigate thousands of kilometers has long baffled scientists. A new study suggests how they determine which way to go. (Christian Science Monitor)

Read our profile of the “lepidopterist neurobiologist” who helped author the new study.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

It takes monarchs about two months to migrate to Mexico. It’s important to remember that monarch migration consists of several generations—the butterflies that finish a migration are not the butterflies that began it. Photograph by Joel Sartore, National Geographic

Monarchs are so plentiful in Mexico’s Sierra Chincua sanctuary they coat tree trunks in what appears to be a thick, soft, black. Photograph by Joel Sartore, National Geographic

The shape and colors of millions of insects are only visible on closer inspection. Photograph by Joel Sartore, National Geographic

On closer inspection still, the monarchs look like stained glass—but are much more delicate. Photograph by Joel Sartore, National Geographic

Scientists have only recently understood how monarchs’ brains are able to navigate such large distances—they rely on internal and external signals communicated by the butterfly’s eyes and antennae. Photograph by Bianca Lavies, National Geographic

Discussion Ideas

• According to the fascinating new study, monarch butterflies make their great migrations using what’s called a time-compensated sun compass. What two pieces of data contribute to a time-compensated sun compass?
  • solar azimuth, or the horizontal position of the sun in the sky. The sun travels east to west, and a southern-migrating butterfly needs to be able to determine which is which at any given moment.
  • internal clock. An animal’s circadian rhythms recur despite any changes in the environment.

• What body parts do monarchs use to determine the position of the sun in the sky?
  • Just the ones you’d think—their eyes. “As the receptive fields in the eyes detect the sun’s position, an oscillating neuronal signal is sent through to the brain. Meanwhile, the circadian clock is also sending oscillations.”

• What body part do monarchs use to access their circadian clock?
  • “Embedded within the insect’s antennae are these biological clocks. They help the butterfly determine what time it is. So if the sun is close to the horizon, these clocks indicate whether it is rising or setting and therefore if it is in the east or west. Then, if the sun is in the east, for example, the butterfly flies with the sun on its left to go south.”

• How do signals from the butterfly’s eyes and antennae work together to help monarchs navigate?
  • “The rate and combination of these neuronal signals tell the brain what signal to send to the body to adjust course. It dictates how much of an angle to change and whether it should turn left or right.”
    • FYI: If monarchs are pushed wildly off-course, “they will actually rotate their bodies around in a full circle as a sort of resetting method.”

• If monarch migration depends on the visible position of the sun, how do the butterflies navigate on overcast days?
  • Still a mystery! Researchers, including our friend Dr. Reppert, think the insects might have a sort of magnetic compass in their brains.

TEACHERS’ TOOLKIT

Christian Science Monitor: Scientists crack mystery of migrating monarch navigation

Nat Geo: Meet Dr. Steven Reppert, who’s picking the brains of butterflies!

Reppert Lab: Migration

Nat Geo: Monarch Butterfly Life Cycle and Migration activity

(extra credit!) Cell Reports: Neural Integration Underlying a Time-Compensated Sun Compass in the Migratory Monarch Butterfly