Researchers have cracked the secret of the internal, genetically encoded compass that millions of monarch butterflies utilize to determine the direction -- southwest -- they should fly each fall to reach central Mexico.

Eli Shlizerman, University of Washington assistant professor, said "Their compass integrates two pieces of information -- the time of day and the sun's position on the horizon -- to find the southerly direction."

Shlizerman, who has joint appointments in the Department of Applied Mathematics and the Department of Electrical Engineering, collaborated with the University of Massachusetts and University of Michigan researchers to model how the monarch's compass is organized within its brain.

Monarchs use their big, complex eyes to monitor the sun's position in the sky. But the sun's position is not enough to determine direction. Each butterfly should also combine that information with the time of day to know where to go.

The monarchs possess an internal clock based on the rhythmic expression of key genes. This clock maintains a daily pattern of behavior and physiology. In the monarch butterfly, the clock is centered in the antennae, and its information travels through neurons to the brain.

Shlizerman's collaborators, including Steven Reppert at the University of Massachusetts, recorded signals from antennae nerves in monarchs as they transmitted clock information to the brain and light information from the eyes.

In the study, two neural mechanisms -- one inhibitory and one excitatory -- controlled signals from clock genes in the antennae. Their model had a similar system in place to recognize the sun's position based on signals from the eyes. The balance between these control mechanisms will assist the monarch brain decipher which direction was southwest.

Based on their model, it seems that during course corrections monarchs do not make the shortest turn to get back on route. Their model includes a distinctive feature -- a separation point that would control whether the monarch turned left or right to head in the southwest direction.

The findings have been published in the journal Cell Reports.