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Colorful compass clocks

North America has two migratory populations of monarch butterflies. One breeds west of the Rocky Mountains and overwinters in forested areas along the coast of California. A much larger population breeds east of the Rockies and migrates up to 2,500 miles (4,000 km) from southern Canada to overwintering sites in mountain forests near Mexico City.

As autumn approaches, decreasing daylight triggers hormonal changes in monarchs that lead to reproductive diapause, the cessation of mating behavior. In addition to their overwhelming urge to fly south, migrants have greater fat stores, enhanced cold tolerance, and increased longevity.

Migrating monarchs appear to use tail winds to conserve energy and reduce wing wear. At their overwintering sites, monarchs are mostly quiescent but occasionally leave their roosts to drink water from dewy fields and streams nearby.

Around spring equinox, monarchs begin the journey northward from overwintering sites. They breed and lay eggs on newly emerged milkweed in the southern part of the United States, and then they die. As milkweed returns to the northernmost portions of the breeding range, adults of the new generation finish the journey their parents started. During the summer, two or more short-lived generations are produced.

Because the populations that migrate in the autumn are three to five generations removed from those that occupied the overwintering sites the previous year, the fidelity with which monarchs return to the sites is remarkable. It also indicates that migration is genetic instead of learned.

Monarchs have a time-compensated sun compass—they use the sun to determine their flight orientation and have an internal clock that allows them to maintain their flight orientation in a south or southwest direction as the sun moves across the sky during the day. Researchers have demonstrated the time compensation of the sun compass by shifting migratory monarchs' day and night cycle in the laboratory and then exposing them to sunlight. Jet-lagged butterflies flew in the wrong direction.

The day and night cycle sets the monarchs' internal clocks. The clocks themselves have "molecular gears"—several genes that interact to switch each other on and off in a cyclic manner. Scientists initially identified four cells in monarchs' central brain as the clock, but a recent study found that monarchs also have clocks in their antennae.

The compass portion of the time-compensated sun compass consists of cells in the eye that respond to ultraviolet light. Scientists are still researching how the monarchs' compass and clocks interact, as well as how monarchs may use Earth's magnetic field to guide them on their incredible journey.

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