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.