The amazing story of how Monarch butterflies navigate from Canada to Mexico is beautifully recounted in Metamorphosis, the Beauty and Design of Butterflies. “Here’s a butterfly species that does something truly spectacular,” Thomas Emmel says in the film. Flying over two thousand miles without an experienced leader who has made the trip before, millions of these artistically-colored flyers arrive on schedule in the same trees their grandparents or great-grandparents had wintered in the previous year.
Somehow, with a brain the size of a pinhead and a body weighing less than an ounce, the butterfly contains hardware and software for long-range precision navigation, efficient energy utilization and genetic alterations that enable the “Methuselah generation” to survive ten times as long as their parents. Emmel comments that this phenomenon will likely occupy biologists’ attention for centuries to come. Researchers at the University of Washington announced this month that they have “cracked the secrets” of one small aspect of this wonder: the monarch’s internal time-compensated sun compass.
We saw from the film that the angle of the sun and its traverse through the daylight hours plays a role in the choice of flight direction. Using sunlight to navigate requires processing dynamic inputs:
“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. [Emphasis added.]
But how is this information processed in the butterfly’s tiny brain? The compass needs to be integrated with the clock.
Monarchs use their large, complex eyes to monitor the sun’s position in the sky. But the sun’s position is not sufficient to determine direction. Each butterfly must also combine that information with the time of day to know where to go. Fortunately, like most animals including humans, monarchs possess an internal clock based on the rhythmic expression of key genes. This clock maintains a daily pattern of physiology and behavior. In the monarch butterfly, the clock is centered in the antennae, and its information travels via neurons to the brain.
We saw electron micrographs of the antennae in the film. Covered with scales and tiny hairs, these organs are the sites for odor detection, enabling the monarchs to find mates and the females to find their host plants from miles away. Now we learn they are clocks, too:
In their model, 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 discern the sun’s position based on signals from the eyes. The balance between these control mechanisms would help the monarch brain decipher which direction was southwest.
Using models and live butterflies in a flight simulator, the researchers determined how they get back on track if blown off course. There’s a “separation point” that determines whether the individual will turn left or right. That point changes throughout the day. The insect’s onboard computer won’t let it cross the separation point, even if it requires a longer path to get back on course. The scientists also shed light on how the butterflies readjust their compasses for the return trip.
Their model also suggests a simple explanation why monarch butterflies are able to reverse course in the spring and head northeast back to the United States and Canada. The four neural mechanisms that transmit information about the clock and the sun’s position would simply need to reverse direction.
“And when that happens, their compass points northeast instead of southwest,” said Shlizerman. “It’s a simple, robust system to explain how these butterflies — generation after generation — make this remarkable migration.”
Simple, simply — while appreciating the insights our neighbors at UW have gained, such talk is premature. There’s a lot more going on. For one thing, not all individual butterflies head southwest and return northeast. The flight paths from Canada begin on different bearings and converge into a narrow flight path in southern Texas. Also, as Metamorphosis shows, a few individuals take a direct route across the Gulf of Mexico. Then, they all switch direction at a certain point, heading west through a pass toward the Trans-Volcanic Range. Then, within those mountains, they converge on specific trees in 12 known overwintering spots.
Time of day and sun position are necessary, but not sufficient, to explain these precise behaviors. If the researchers feel they have “cracked the secret” of maintaining a particular bearing, they have left all these other secrets unexplained. Their model only shows the bare outlines of a sophisticated, integrated system. The paper in Cell Reports says:
A long-standing, fundamental question about monarch migration has been how the circadian clock interacts with the changing position of the sun to form a time-compensated sun compass that directs flight. Here, we propose a neuronal model for both encoding of the sun’s azimuthal position, and molecular timekeeping signals, and how they can be compared to form a time-compensated sun compass. Our results propose a simple neural mechanism capable of producing a robust time-compensated sun compass navigation system through which monarch butterflies could maintain a constant heading during their migratory flight.
But as we just said, the butterflies don’t keep a constant heading. They make turns. And what tells individuals in Minnesota to head south and individuals in Pennsylvania to head southwest? Do their offspring for the three generations on the return trip keep the target information embedded in their memories? A compass is no good without a map. Both are no good without a brain to read them. Furthermore, as we reported about Bogong moths, some lepidopterans can reach their long-range destinations in the dark!
The researchers identified four genes that keep time by oscillating their expression levels. They identified how the compound eye determines azimuth and bearing even when the sun’s position is changing throughout the day. A butterfly, however, needs much more. It needs controls for altitude, pitch, and yaw. It needs a map of where it has to go. And it needs to monitor resources to ensure it has the energy to get there.
Nevertheless, we’re glad for any and all research that sheds light on the details of monarch butterfly navigation, especially when it can lead to intelligently designed applications that benefit everyone:
The model that we have built closes the loop between the time and azimuth stimuli and orientation control. As such, it provides an important framework for future studies of the monarch sun compass. Our framework can be used to design electrophysiological and flight recordings experiments to compare responses in monarchs’ neurons and model units and to determine the detailed architecture of neural circuits that implement the integration mechanism postulated by our model. It also provides a simple mechanism for navigation that can be used in devices that do not have the benefit of a global positioning system.
A complex, functional system that, when partially understood, can be “used to design” something else must have been designed itself.
Photo credit: Illustra Media.