Internal Clock Leads Monarch Butterflies to Mexico

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.