In a laboratory at the University of Massachusetts School of Medicine, researchers manipulated the light exposure on developing Monarchs and found that the small brains of the insects developed a precise sense of time, a biological clock that helps the animals calculate the time of day.
This time sense along with the observed angle of the sun enables one generation of Monarchs each year to migrate from as far north as Massachusetts to a pine forest in the foothills of the Sierra Madre Mountains near Mexico City. It is the longest known migration of any butterfly, and scientists have long wondered how the fragile, colorful animals do it.
Dr. Steven M. Reppert, senior author of a study appearing this week in the journal Science, said that experiments show the length of daylight helps set the internal clock in the developing Monarch. When the season is right, a special generation of Monarch migrators arises among the butterflies, and these animals begin the long journey to Mexico.
In the experiments, Monarchs were subjected to different patterns of light and darkness during the time they were developing from caterpillar to butterfly.
The mature Monarchs were then placed in a laboratory device resembling a barrel that was rigged to a machine that detected flight patterns. The butterflies were tethered to a fine wire that allowed them to fly horizontally in any direction they chose. The machine recorded the direction of flight.
Reppert said the experiments showed that Monarchs exposed to normal sunlight during development tended to correctly fly southwest, the direction of their overwintering destination in Mexico.
But butterflies that developed in the presence of constant light lacked this navigation skill. Instead, these Monarchs always flew toward the sun, east in the morning and west in the evening.
Light exposure during development of some of the butterflies was shifted to mimic the effects of daylight starting six hours early. This apparently set the insects' internal sense of time, called the circadian clock, to a point about six hours off the actual position of the sun. Researchers found that these insects flew toward the southeast, an error of more than 100 degrees off the correct migratory path.
"Our work gives clear, direct evidence of the important role of the circadian clock for time compensated navigation of the Monarch," said Reppert.
The Monarch has long fascinated researchers because of its uncanny ability to migrate. The butterflies live in spring, summer and early fall across a range spanning some 6 million square miles in North America. Yet, one generation each year migrates to a single overwintering area in Mexico.
"A very precise navigation mechanism is needed" to do that, said Reppert.
At least four generations of Monarchs live each year. The first generation emerges from eggs laid in Texas and Louisiana in the early spring. The larvae feed on the leaves of milkweeds and then develop through a chrysalis into a butterfly. These adult Monarchs then fly farther north, following the emerging milkweeds, and breed again.
This continues until the fall, when a generation of Monarchs, apparently responding to the changed angle of the sun, emerges from the chrysalises and immediately begins flying toward Mexico, a journey that may take two months.
Monarchs overwinter by the millions in Mexico, and then that same generation flies to the southern United States to mate and die, starting the cycle over. The migrating Monarchs live about nine months. The other generations live for only weeks.
Because of the annual migration timing, Reppert said, it is clear that the ability to migrate is an instinct passed genetically from generation to generation.