Internal Clock Leads Monarch Butterflies to Mexico

By John Roach
for National Geographic News
June 10, 2003
For every fourth or fifth generation of monarch butterflies that summer in the U.S. east of the Continental Divide, the pull of high-altitude Oyamel fir forests in central Mexico is irresistible.

By the millions each fall they point south and flutter up to 2,000 miles (3,200 kilometers) to reach the forests on a few small mountain peaks in an approximately 60-square-mile (155-square-kilometer) area in the volcanic highlands that serve as the butterflies' winter retreat.

For scientists, this annual migration is one of nature's greatest mysteries. Four to five generations separate the monarch populations that make the migration, so the butterflies that make the trek to Mexico are the great, great grandchildren of the previous generation to have made it.

"The ones that fly south have never been to Mexico before, they get there by pure instinct and then by pure instinct they come back, lay their eggs on milkweed and then die," said Lincoln Brower, a research professor of biology at Sweet Briar College in Virginia.

Brower and several other scientists have spent years trying to unravel this mystery. Now, a team of researchers with the University of Massachusetts Medical School in Worchester has added another piece to this puzzle by demonstrating that monarch butterflies depend on an internal clock to find their way.

Researchers have believed for a long time that the butterflies use the sun to navigate, but they were not certain as to how the butterflies adjust their direction throughout the course of the day as the sun moves across the sky.

"If you wanted to go southwest toward Mexico, you would set that angle with respect to the sun at sunrise," said Brower. "The trouble with that is the angular difference between the southward direction and the sun keeps changing as the sun goes across the sky."

The researchers at the University of Massachusetts Medical School had a theory that an internal clock serves to adjust the sun-compass heading that keeps the butterflies on their southerly course, as it does with long-distance migratory birds. Such internal clocks are known as circadian clocks, which are tuned to biological rhythms that recur on a daily basis.

"The circadian clock was felt to be important," said Steven Reppert, a neurobiologist at the University of Massachusetts Medical School who led the team that performed a series of genetic and behavioral experiments to prove the internal-clock hunch correct.

The results of their study were reported last month in the journal Science.

Orley Taylor, an entomologist at the University of Kansas in Lawrence, said he is not surprised that monarch butterflies have a circadian rhythm that plays a role in sun-compass orientation, but that more work is needed to show how the clock guides navigation.

"This is one step, among many, required to understand the monarch migration," he said.

Setting the Clock

To demonstrate that monarchs have an internal clock and that the clock is set by daylight, the researchers examined the time of day when adult monarchs emerge from their hard-shelled pupa, called the chrysalis.

The time of emergence from the chrysalis is a reliable marker of circadian function in other insects, according to the researchers. Monarch butterflies normally emerge in the morning hours.

To test if this timing is controlled by an internal clock, the researchers exposed the chrysalids to total darkness.

"If you put an animal in an environment where there are no time cues, which is a default for circadian clocks, circadian rhythms continue to tick and keep time," said Reppert.

When the butterflies were placed in total darkness, they maintained their emergence cycle. But when exposed to constant light the butterflies emerged throughout the day, an indication that the circadian cycle was disrupted. "It gummed up the circadian clock so it did not work properly," said Reppert.

To measure the circadian clock after the butterflies had emerged from the chrysalis, the researchers cloned the monarch's so-called period gene, known as per, which is known to set the internal clock in fruit flies.

They found that the gene's activity was high during dark hours and low during light hours, but when exposed to constant daylight for several days the rhythm was blunted.

"Constant light thus disrupts the underlying clockwork mechanism, leading to the disruption of output rhythms," Reppert and colleagues wrote in Science.

Flight Simulation

Next, the researchers studied how the manipulation of the daily light and dark cycles affects the orientation and navigation ability of the butterflies.

After being exposed to a light-dark cycle in the lab that resembled the outdoor lighting of the northeastern United States during autumn, the researchers placed the butterflies outdoors in a 40-gallon (176-liter) flight simulator with a directional recording device.

The simulator was developed in 2001 by biologists Henrik Mouritsen at the University of Oldenburg in Germany and Barrie Frost at Queen's University in Ontario, Canada and is considered the breakthrough that allowed this monarch migration research.

"I predicted that the machine would open a floodgate of research questions that could potentially be answered," said Brower. "This study is an example of that."

The butterflies are tethered to the recording device with a metal wire. They have complete freedom of movement on the horizontal axis, but cannot move up and down, explained Reppert. Air blown up from the floor simulates a lofting breeze.

When placed in the flight simulator after being housed in the autumn-like, light-dark cycle, the butterflies oriented themselves to the southwest, as if they were flying towards their Mexican wintering grounds.

Butterflies housed in a 1 a.m.-to-1 p.m., light-dark cycle flew towards the southeast, as expected. "By shifting the clock, you get a predicted shift in direction in where the animals orient," said Reppert.

However, when exposed to constant light before being placed in the flight simulator, the butterflies flew directly towards the sun, an indication that the direct light had disrupted the circadian rhythm, thus making it impossible for them get a sense of direction.

The team also found that while ultraviolet light is required for sun-compass navigation, some other wavelength of light was required for setting the butterflies' internal clocks. The difference may provide a means for untangling the two biological processes, they report.

"What the research is doing is allowing us to get into how navigation occurs from the circadian clock vantage point," said Reppert.

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