Scientists have finally cracked the mystery of the internal compass that monarch butterflies use to guide their migration toward the Rio Grande on their 2,000-mile journey to the warm region of Mexico.
"Their compass integrates two pieces of information - the time of day and the sun's position on the horizon - to find the southerly direction," said Eli Shlizerman, a University of Washington assistant professor and lead author of the study.
Previous research has revealed how the monarch butterfly integrates the time of day and location of the sun into their travels, but until now, scientists have never been able to model how the monarch's brain receives and processes this 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 said.
In order to determine direction, monarchs must combine information on the sun's position in the sky gathered from their compound eyes with the time of day using its internal clock, which hinges on the rhythmic expression of genes.
Previous research has shown that the monarch's clock is centered in the antennae, where the information it receives then makes its way to neurons in its brain.
"We created a model that incorporated this information - how the antennae and eyes send this information to the brain," Shlizerman said. "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."
The new model consists of two neural mechanisms - one inhibitory and one excitatory - that control signals from unique internal clock genes located in the antennae. The balance between these two mechanisms helps the monarch brain determine which location is southwest.
The team also revealed that when corrections are needed during migration, monarchs don't just take the shortest turn to get back on track. Instead, they utilize a separation point that determines whether the monarch turns left or right to head in the southwest direction.
"The location of this point in the monarch butterfly's visual field changes throughout the day," Shlizerman said. "And our model predicts that the monarch will not cross this point when it makes a course correction to head back southwest."
Using the new model, if a monarch gets lost or off track during their migration due to wind, they turn whichever direction that doesn't require crossing the separation point.
Further research will need to confirm if this new model of the monarch butterfly's internal compass fits with what is known about butterfly brain anatomy, physiology and behavior.
The findings were published in the April 14 issue the journal Cell Reports.
