Fly long, live longer

Natural History, Oct, 2005 by Stephan Reebs

In a laboratory in Atlanta, Catherine A. Bradley and Sonia M. Altizer, both ecologists at Emory University, put monarch butterflies through their paces on "flight mills." An insect is glued to the end of a horizontal rod that is free to rotate about a central axle. The monarch flutters round and round, while a computer registers the number and speed of the rod's rotations.

The purpose of the exercise is not to turn the butterflies into paragons of health, but rather to test their long-distance flight performance. Bradley and Altizer found that monarchs heavily infected with the common protozoan parasite Ophryocystis elektroscirrha are between 10 and 20 percent less proficient as fliers (measured by a combination of flight speed, endurance, and use of energy reserves) than their parasite-free kin.

The finding can help explain a curious phenomenon. Monarchs from the eastern United States and Canada undertake one of the longest insect migrations known (as far as 3,000 miles) to reach overwintering grounds in Mexico, and less than a tenth of the population carries parasites. In contrast, monarchs from the western U.S. migrate a shorter distance, to coastal California, and many more, about a third, are infected. The most extreme cases are the tropical monarchs, which aren't known to migrate at all; more than three-quarters of them are afflicted with O. elektroscirrha. Perhaps, Bradley and Altizer suggest, the rigors of travel cull butterflies infected with parasites. Hence, the longer the migration, the healthier the population. (Ecology Letters 8:290-300, 2005)