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No. 116: Mar-Apr 1998

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Monarch Compasses

Field experiments down the years suggest that migrating birds use a variety of strategies to chart their courses with high precision. The geomagnetic field, the sun, the stars, prominent landmarks, and even odors help guide them across the continents and open seas. But birds are considered highly evolved animals so their sophisticated navigational techniques are not especially surprising.

Monarch butterflies, however, are mere insects, with tiny brains (navigation-data processors) and not much in the way of the environment sensors and internal clocks required for long-distance migration. Yet, some of these colorful insects manage to flutter up to 4,000 kilometers from the eastern U.S. and Canada to their wintering grounds in Mexico. How do they do this?

S.M. Perez et al have now shown that monarch butterflies are equipped with a sun compass; that is, they chart their courses by noting the sun's changing azimuth. This feat requires not only the measurement of solar azimuth but also reference to an internal clock. Humans cannot do this without artificial instruments.

Furthermore, even on cloudy days, migrating monarchs fly in the proper direction (generally south-southwest). Apparently, they also have evolved a backup navigation system, perhaps a geomagnetic compass.

(Perez, Sandra M., et al' "A Sun Compass in Monarch Butterflies," Nature, 387:29, 1997.)

Comment. Somewhere in the tiny bodies of the monarchs are packed sun-azimuth sensors, internal clocks, magnetic-field sensors, and a nervous system that converts the incoming data into signals to the wings. Their genomes must also include map information to pass on to their progeny.

From Science Frontiers #116, MAR-APR 1998. 1998-2000 William R. Corliss