As our Canadian summer has once again turned to autumn, an amazing animal is migrating south. No offense to the iconic Canada goose, but the mighty monarch butterfly travels up to 4,000 kilometres each fall to spend its winters down south — in either Mexico or California, depending on which coast it originated from!
How on earth do monarchs find their way so far from home, when many people can’t find their cars in a mall parking lot? Well, they fly south using the sun as a compass.
Fast fact: Monarchs in Eastern Canada fly to the same forest reserve in central Mexico every year, but how they find this specific forest is a mystery.
Monarchs have special cells in their eyes, called photoreceptors, which detect the angle of light coming from the sun. This information is sent to the brain for processing, so that monarchs know where they are relative to the sun.
There’s one problem, though — the sun is a moving target. Earth’s rotation makes the sun appear to move across the sky from east to west every day. To continually adjust their internal compass, monarchs use a circadian clock. This is a biological clock (which most animals have) that coordinates activities in rhythm with our 24-hour day. With this clock, monarchs “know” that it’s 9 a.m. and not 4 p.m., and that the sun should be on their left instead of their right to head south. This whole system is called a time-compensated sun compass.
Fast fact: In 2009, scientists discovered that the monarch circadian clock that calibrates the sun compass is in their antennae, and it sends time information to the compass area of the brain.
Once monarchs arrive in the warm south, they spend the winter there living off their fat reserves. In the spring, with just a few weeks left of their life cycle, they begin the trek north to Canada. Between March and September, three or four generations of monarchs make their way north by following the emergence of milkweed — the only plant that monarch caterpillars eat. The last summer generation uses environmental cues (decreased light, lower temperature) to trigger migration. Since several generations of monarchs are born throughout the summer months, the monarchs that eventually migrate south the following fall are actually the great-great- (and sometimes great-) grandchildren of those that made the trip the previous year. Incredibly, these butterflies are able to find their way back to the same location based on instinct and sunlight compass alone!
Fast fact: Milkweed contains a toxin that stays in the bodies of monarch butterflies. It doesn’t hurt them, but it makes birds vomit, so birds learn to eat something else for lunch!
For a good overview of monarch butterflies, check out:
For detailed information about the time-compensated sun compass, check out:
For a video of monarch butterflies at their wintering ground in Mexico, check out:
For more information about the wintering ground in Mexico, check out: