How does the monarch butterfly find its way thousands of miles?
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The monarch butterfly orients itself using a real solar compass compensated for time: its antennae house an internal clock which adjusts its trajectory according to the time of day, supplemented by sensitivity to the earth's magnetic field. Remarkably, the migration takes place over several successive generations, with no individual making the complete round trip.
A double-compensated compass
The monarch's compound eyes perceive the position of the sun, but this information alone would cause its course to deviate over the hours as the star moves across the sky. This is where the antennae come in: they contain a molecular circadian clock that, independently of the brain, calculates the time of day and corrects the flight angle accordingly. Experiments have shown that monarchs whose antennae are painted black (blocking light without amputating them) lose this capacity for temporal compensation, while their brain clock continues to function normally.
In overcast weather, a second system takes over: sensitivity to the inclination of the Earth's magnetic field, probably based on cryptochromes present in the antennas, would make it possible to maintain a general course towards the southwest during the autumn migration towards the oyamel forests of central Mexico, a journey which can exceed 4000 kilometers.
A memory that does not last in a single lifetime
No monarch completes the complete round trip cycle. Individuals that migrate to Mexico in the fall live for several months through a reproductive diapause, but their descendants the following spring, who move north, live only a few weeks and take turns over two to five generations before reaching Canada. The return journey to the wintering sites is therefore accomplished by great-grandchildren who have never seen these places, which implies an orientation program that is largely innate rather than learned through individual experience.
