Other Environmental Themes

Navigation by Compass & Clock

How do monarch butterflies make their way from Canada and the northern United States, where they spend the summer dining on milkweeds, to their winter refuge in the mountains of central Mexico more than 2,000 miles away? Do the butterflies—like sea turtles, spiny lobsters, and other creatures—use a magnetic compass to guide their path?

Nature is built on diversity, and monarchs have apparently developed a different approach to navigation, relying on the sun as a beacon and on an internal timepiece that calibrates the sun's motion across the sky. These conclusions are based on experiments involving 59 wild butterflies that were captured during the autumn migration and briefly placed in an outdoor flight simulator at Queen's University in Ontario, Canada. Butterflies entrained to a normal light-dark cycle flew southwest, straight toward their Mexican wintering grounds. Other butterflies, whose daily biological cycles known as “circadian rhythms” were either six hours advanced or six hours delayed, misinterpreted the sun's position and flew in the wrong direction, heading either southeast or northwest. The butterflies became disoriented under simulated overcast conditions, when the sun was obscured, but did not modify their bearing when subjected to varying magnetic fields.

A team from the University of Massachussetts corroborated these results in a 2003 Science study, while probing the genetic machinery that might underlie migratory behavior. Monarchs exposed to constant light conditions for several days navigated atrociously, flying directly toward the sun without compensating for the time of day. These butterflies, whose circadian clocks were out of whack, showed abnormal activity in a gene called “per,” which had been identified in previous fruitfly experiments as critical for setting the internal timepiece. Butterflies accustomed to normal light-dark conditions flew southwest, the proper course, and their per gene showed expected swings in activity

This work, which illustrates the importance of a functioning circadian clock to successful migration, is just a start, the Massachusetts team admits. Tracing light-input pathways into the monarch brain, they say, "should aid our understanding of the clock-compass interface and further illuminate the mechanisms of monarch butterfly migration."