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El milagro de las mariposas


Cada año a fines del mes de octubre, millones de mariposas monarca llegan a los bosques Michoacanos, después de un largo viaje de cerca de 5.000 Km, desde el norte de Estados Unidos y sur de Canadá para reproducirse en México; meses más tarde, a finales de marzo retonan siguiendo un ritual antiquísimo.

Since Pre-hispanic times, people in Mexico noted the monarch's arrival at precisely the same time every fall. In fact, the name for the monarch butterfly is the "harvester butterfly" in the native language of the Purépecha Indians. As the people observed, the monarchs appear each fall during the corn harvest. The Mexican holiday of Dia de los Muertos also occurs at this time. Many people in the region believe the monarchs are the souls of their ancestors, returning for their annual visit.

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.