monarch decline
Isotopes to the rescue
glider theory

Bitty brain. Big flight.
Imagine directing a flying machine with a brain the size of a pinhead to find a few mountaintops west of Mexico City. The prospects for success would be no greater than, say, a butterfly's chance of writing Hamlet. And yet year after year successive generations of monarch butterflies find the same tiny winter refuges west of Mexico City.

Gibo and glider
David Gibo and his glider. Courtesy of David Gibo.

A lot of people are trying to figure out how this happens -- by using radar, ground sightings, and analysis of tagged butterflies. David Gibo uses an intriguing approach based on another propeller-less craft -- the glider. Gibo, associate professor of biology at the University of Toronto, has managed to merge his love for two flying machines: gliders and monarchs.

At a gross vehicle weight of about half a gram, the monarch has a small gas tank (actually it stores energy in the form of fat), so it must let nature do the heavy lifting on its long migrations between Mexico and the United States and Canada. That involves maximizing time aloft with a tactic known to glider pilots -- finding thermals, or natural updrafts.

When Gibo took up gliding in the 1970s, he realized that monarchs too, needed free rides while flying hundreds of miles in advance of changing weather. As Gibo studied thermals from the ground, he noticed monarchs gaining altitude in them. Today, Gibo has more than 20 years experience at gliding. But he says monarchs are "still much better" at gaining altitude in thermals.

Male monarch
Photo by Karen Oberhauser.

© 1987, Monarchs in the Classroom, Monarch Butterfly Sanctuary Foundation.

you can call me king But how do monarchs know where they're going, particularly during the flight south? Remember that the individual monarchs that migrate south have never been at the Mexican reserves -- the butterflies that flew north the previous spring laid eggs, then died at the U.S. Gulf Coast. Indeed, it's the third or fourth generation that finally returns to Mexico.

Southward bound. Rule bound.
Like others, Gibo wondered how monarchs could navigate to a new place. Remembering the tiny size of the butterfly's computing equipment, he concluded "It's got to be rules that allow them to make their way across country with a reasonable degree of success."

Simple rules that could be stored in the pinhead brain of a butterfly. Gibo suspects that two rules rule when the butterfly is migrating south:

  1. Never allow yourself to lose latitude; make sure that each evening you're not further north than you started. This rule may be "relaxed" a bit in Georgia, Gibo says, when crosswinds come into play.

  2. Never lose ground and end up further from your overwintering site.

From there, Gibo, who says he's only seen two monarchs from a glider, says many possible navigation techniques may come into play. The Why Files covered animal migration.

Gibo says that when all rules are considered, the butterfly has a decision framework that will look something like this: "If this condition holds, and that condition holds, the direction to fly is such and such."

Want to navigate yourself over to some monarchical, isotopic reading?


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