Moving on—or not: UChicago biologist works to decipher the mystery behind the monarch butterfly migration

September 29, 2020

If you spotted some of the distinctive orange and black monarch butterflies making their way across Illinois this fall, you may not have realized that you were witnessing an annual pilgrimage of millions of North American insects that spans several months and consists of a journey of up to 3,000 miles. 

September is prime monarch-sighting season across the state as the butterflies make their annual pilgrimage to the south, heading down to Mexico's warmer climate for the winter months. Beginning in mid-March, the butterflies return to the north to lay their eggs, and the cycle continues with a new generation. But what factors control this precise and consistent migration pattern? Marcus R. Kronforst, Professor of Ecology and Evolution, is determined to find out.

"Our lab is looking at what environmental factors make the monarchs know that it’s time to migrate," Kronforst explained. "We’re also studying the butterflies genetically to determine what it is that makes them able to respond to those cues."

The annual monarch migration is an endangered phenomenon. The population size of the eastern monarch has decreased by more than 80% in the past 30 years, and the western monarch’s decline has been even more severe. The cause of this reduction is unknown, though researchers speculate that loss of milkweed and flowering plant habitats due to climate change and shifting agricultural practices could be related factors. Kronforst's work incorporates the biology of the insects in addition to the environmental issues at play: he wants to learn what makes certain monarchs migratory and others not.

Large populations of monarch butterflies exist outside of North America, and in many of those locations, the insects do not migrate. However, the monarch butterfly is known to have originated in North America, which means that all other populations of monarchs dispersed to various locations at some point—and may have lost the ability to migrate over time. To analyze the different traits of these butterflies, Kronforst secured a large batch of monarch caterpillars from Puerto Rico to compare their behavior with Chicago-based insects.

Kronforst, along with postdoctoral researcher Micah Freedman, has taken extreme care to ensure that the Puerto Rican and the Chicago insects are exposed to identical outdoor summer and fall environmental conditions. All of the monarchs are attached to a tether on the roof of the Biological Sciences Learning Center to see what direction they want to fly in. In previous versions of the experiment that only included native North American monarchs, the butterflies instinctively knew to fly south. Once the Puerto Rican caterpillars emerge as adults this fall, Kronforst is eager to see what they'll do.

"If the Puerto Rican monarchs don't fly south, we’re also raising some Puerto Rican and Chicago crosses, and we will test many of these to see if they want to fly south," Kronforst said. "That will really help us understand the genetics involved in migration."

Monarchs are widely available from commercial breeders. Together with PhD student Ayse Tenger-Trolander, Kronforst previously studied the migration patterns of butterflies bred in captivity. In those experiments, when it came time to test the monarchs in the fall, they did not show a desire to fly south. The genetic makeup of the insects changed enough in captivity that they no longer behaved like North American monarchs living in the wild. 

"The behavior of the bred monarchs suggests that something in their genetic makeup deteriorated over time," Kronforst explained.

It is yet to be determined whether the Puerto Rican monarchs will demonstrate similar genetic changes that inhibit migratory behavior.

Along with genetic traits, environmental factors also influence the monarchs' behaviors. The butterflies need to receive the correct environmental inputs that alert them when it becomes late summer or fall, when it’s time to develop into migratory monarchs and fly south. Kronforst's previous experiments with North American monarchs within the laboratory's environmental chambers resulted in the insects failing to migrate, which is why all of his current monarchs are being raised outdoors.

"If we tinker with the natural environment in any way, we mess it up," Kronforst said. "Something related to the environment is a factor, and it's telling the monarchs how to behave."

Kronforst hopes that this research will yield new information about specific environmental and genetic factors that drive the iconic annual migration of monarch butterflies. Furthermore, he hopes the results of this work may one day provide insights that help stem the decline of monarch butterflies and ensure a future for their migration.