Monarch butterfly in East Texas.
Every Fall, monarch butterflies begin their long-distance migration from the northern United States and southern Canada to central Mexico. Monarchs navigate this 2000 mile journey using a combination of inputs from the sun and from their own biological clocks. Every good Boy Scout knows that in order to navigate by the sun, you must account for the time of day because the cardinal directions’ relation to the sun changes as the sun moves across the sky. But without a wristwatch, the monarchs depend on their own internal sense of time and surprisingly, this sense of time doesn’t come from the brain, but instead comes from their antennae.
To determine which way is south (or south-west), monarch butterflies use an internal circadian clock to calibrate the information they get from the position of the sun. Circadian clocks regulate circadian rhythms, the natural ~24-hour cycle of many organisms. These biological clocks are composed of a molecular network of genes whose expression levels cycle up and down over the 24-hour day. Circadian clocks can be reset by external light and researchers can use artificial lighting to make changes in the internal clocks of butterflies kept indoors. A monarch that has been kept indoors with an artificial light-day cycle that is delayed from the natural light cycle will fly in the wrong direction when put outside. Their circadian clock is off-kilter and so their sun compass will be calibrated incorrectly.
It was always assumed that the circadian clock that modulates the navigation of the monarch was in the brain, since other circadian clocks are found there. However, a recent paper in Science showed that the circadian clock that helps the monarch
A pitstop on the way to Mexico.
navigate is actually found in the antennae. The study’s authors tested groups of monarchs whose antennae had removed as well as monarchs with their antennae painted black. Both groups were able to fly as vigorously as normal butterflies, but they flew in the wrong direction, illustrating that monarchs need their antennae to properly calibrate their sun compass.
As you might expect, the authors also found that circadian-clock genes were cycling up and down in the antennae as they do in the brain. Removing the antennae did not effect the rhythm of circadian clock genes in the brain. Furthermore, input from the brain was not necessary for the oscillations of the circadian clocks of the antennae– the circadian clock genes cycled even in detached antennae cultured in a petri dish and the cycling of clock genes in the cultured antennae could be reset by artificial light conditions.
The monarch’s sun compass is found in the central brain, so it was surprising to see that the circadian clock that calibrates it is in a structure outside of the brain. It is interesting that a neural circuit that regulates a complex behavior like migration has a major component outside of the brain.
