Butterfly hide-and-seek.

Butterfly hide-and-seek

We've all been mightily impressed by monarch butterfiles, those fragile insects that migrate thousands of miles each year. But what if they've just been running around the corner and acting inconspicuous? We'd all feel pretty silly.

One researcher has proposed something like that. According to Adrian Wenner of the University of California in Santa Barbara, the evidence is against true monarch migration, at least in the western United States.

For years, the idea has been that monarchs in the West winter on the central California coast, leaving in February for the cooler regions of the Pacific Northwest and central Rockies. But after a study of patches of milkweed (the host plant for monarch larvae) in the Santa Barbara area, Wenner says, "We can find [monarch] caterpillars any time we want. We can follow the generations right through the summer."

Wenner proposes that, rather than a true migration, the butterflies undergo a large-scale but less dramatic yearly expansion and contraction of their range. Butterfields at the extremes of the range don't migrate, he says; they die off.

According to Patrick Wells of Occidental College in Los Angeles, results of his metabolic studies of the butterflies are also inconsistent with long migrations. Monarchs are loaded with fat stores when they reach their winter sites, he says, and lean when they leave. "If they had just flown two or three thousand miles, they should not be fat," he says. "If they left lean in the spring, they wouldn't have the fat stores necessary for migration."

Most of the other researchers at the conference were unimpressed by the arguments, according to Christopher Nagano of the Los Angeles County Natural History Museum. "It was an idea that was proposed, ... investigated and discarded," he says. "A lot of the pieces [Wenner and Wells] were using to support their hypothesis just weren't correct."

Stephen Malcolm, who studies monarchs in the eastern United States, proposes an explanation for year-round monarchs in the Santa Barbara region. The same thing happens in souther Florida, he says, where milkweed is present throughout the year. The milkweed plants, and the mild environmental conditions associated with them, appear to dampen the migratory cues of the monarchs. Butterflies that stumble onto these areas as they head southwest toward the winter sites in central Mexico sometimes settle down; like the monarchs Wenner found in California, they reproduce throughout the winter. "You can't extrapolate from one county in California, and suggest that monarchs in all of the western United States don't migrate," Malcolm says.

Different species of milkweed leave characteristic traces in the butterfields, and Malcolm and his collagues, at the University of Florida in Gainesville, have traced the routes of the butterflies by analyzing these dietary "fingerprints." Their studies indicate that monarchs that, as caterpillars, feed on milkweed in the northeastern United states spend the winter in Mexico and then fly back north. Other researchers, using tags, have reported monarchs in Mexico that were tagged in Canada. And studies in Costa Rica, Nagano says, show that monarchs there migrate over the mountains from the Caribbean to the Pacific side of that country. (This suggests that consrvation effrots should be expanded from the Caribbean side of Costa Rica to the Pacific, Nagano adds.)

Other research indicates it may not be so difficult for the butterfly to balance its metabolic budget. According to Nagano, David Gibo at the University of Toronto has reported that the butterflies ride thermal air currents, which could carry them long distances with minimal energy expenditure. Gibo, a glider-plane pilot as well as a biologist, has observed monarchs at altitudes of about 5,000 feet.