Each fall, monarch butterflies across Canada and the United States migrate over 2,000 miles to the relative warmth of central Mexico. Scientists have always been puzzled about how the butterflies complete their journey in such a predetermined way that, even after two months of flying, they always end up in a particular place in Mexico.
Now, the scientists say they have finally solved the mystery behind one of nature’s longest and most famous journeys. They published their findings in the journal Cell Reports.
Lead researcher Prof Eli Shlizerman, a mathematician at the University of Washington, and colleagues set out to study the monarch butterfly’s internal compass that they use to navigate through vast distances.
Previous research revealed that the insects use the position of the sun combined with an internal clock to determine which way is south – in what’s known as a time-compensated sun compass.
The scientists wanted to know how this information was integrated and turned into action inside the butterfly’s brain. So they created a model circuit that mimics the internal compass of monarch butterflies.
They found that the monarch’s input signals rely entirely on the Sun.
“One is the horizontal position of the Sun and the other is keeping the time of the day,” said Shlizerman.
As a Monarch butterfly flies along on a sunny day, its eyes are constantly registering where the sun is in relation to the horizon. Although the sun travels east to west, the butterfly needs to be able to determine which is which at a given moment. That’s where the circadian clock comes in.
These biological clocks are embedded within their antennae. It helps them determine the time. 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.