Monarch butterflies are starting their annual migration to various warmer areas of the country, as well as the mountains of central Mexico.
These amazing creatures migrate south in the fall, with some populations traveling as far as 4,580 miles. In the spring, it takes several generations of the butterflies to cover this distance. The butterflies mate and lay eggs along the way, with each new generation of adult butterflies continuing the journey north along the migratory route.
How do they keep flying in the right direction? It turns out that monarchs use two completely different navigation systems, depending on whether it is a sunny or cloudy day. When the sun is shining, monarchs navigate by the sun and use a time-compensated sun compass.
Until recently, scientists were stumped about how monarchs navigate on cloudy days with no sun to navigate by. It turns out that monarchs use geomagnetic clues—they use a sophisticated magnetic inclination compass system, using the angle of the Earth’s magnetic field to guide their movement. This is similar to that used by much larger-brained migratory vertebrates such as birds and sea turtles.
But the new research shows that the navigation system monarchs use on cloudy days uses light as well as geomagnetic cues. Monarchs have special cells in their antennae that can detect ultraviolet light, and since that light penetrates clouds, it is a cue that they can use even on cloudy days.
In effect, on cloudy days monarchs use a very sophisticated light dependent magnetic inclination compass. It turns out that these beautiful butterflies have a navigational system that is truly amazing!