for National Geographic News
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.
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