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How Do Monarch Butterflies Migrate?

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In fall monarch butterflies from across North America migrate to the relatively warmer climes of Mexico.

The journey is one of more than 2,000 miles to central Mexico which generations of monarch butterflies repeat instinctively. It continues to this day despite the fact that numbers of the brightly-colored butterflies are declining due to the loss of milkweed, their sole larval food source.

Monarch Butterflies – Researchers crack genetic code behind compass

Scientists now think that they have found out how the monarch butterflies know where to fly. A team of researchers thinks that it has found the secret behind the genetically encoded compass that they use to find their way.

“Their compass integrates two pieces of information—the time of day and the sun’s position on the horizon—to find the southerly direction,” says Eli Shlizerman, assistant professor at the University of Washington, who has joint appointments in the applied mathematics and the electrical engineering departments.

Previous research showed that monarch butterflies could combine the time of day and the sun’s location, but until now scientists did not understand how the receive and process the information.

“We wanted to understand how the monarch is processing these different types of information to yield this constant behavior—flying southwest each fall,” Shlizerman says.

Information on the sun’s position is gathered by the eyes, while the time of day is known due to the monarch butterflies’ internal clock. This clock is based on the expression of key genes.

The clock is centered in the antennae and sends information to the brain via neurons. The researchers recorded signals from the antennae nerves as they sent clock information and light information to the brain.

“We created a model that incorporated this information—how the antennae and eyes send this information to the brain,” Shlizerman says. “Our goal was to model what type of control mechanism would be at work within the brain, and then asked whether our model could guarantee sustained navigation in the southwest direction.”

According to the model, signals from clock genes in the antennae are controlled by two neural mechanisms. The model also relied on the existence of a similar system that would discern the position of the sun based on signals from the eyes, and these two mechanisms would enable the monarch butterflies to find which way was southwest.

The scientists also believe that the shortest route is not always the best. The model builds in a separation point that controls whether the monarch butterflies turn left or right to get back to southwest.

“The location of this point in the monarch butterfly’s visual field changes throughout the day,” Shlizerman says. “And our model predicts that the monarch will not cross this point when it makes a course correction to head back southwest.”

The scientists believe that the butterflies turn whichever way will not make them cross the separation point. This would need additional study in order to be confirmed, although current observations suggest that it is true.

“In experiments with monarchs at different times of the day, you do see occasions where their turns in course corrections are unusually long, slow, or meandering,” Shlizerman says. “These could be cases where they can’t do a shorter turn because it would require crossing the separation point.”

The model would also allow an explanation for how monarchs know how to get back northeast. The neural mechanisms would simply have to reverse direction to guide the monarch butterflies back to the U.S. and Canada.

“And when that happens, their compass points northeast instead of southwest,” says Shlizerman. “It’s a simple, robust system to explain how these butterflies—generation after generation—make this remarkable migration.”

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