When monarch butterflies head south for the winter, they depend on an internal clock to keep them on course. A circadian clock keeps time for most organisms and monarchs use it to judge the sun's position relative to the direction they need to head, says Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester.
Monarchs, like birds, use the sun to tell which way they are going. The sun helps these intrepid travelers migrate thousands of kilometers to a small winter retreat in Mexico. But exactly how the butterflies navigate there has been a mystery.
Reppert and his colleagues used behavioral and molecular studies to understand what makes the monarch's clock tick. In the lab they exposed some butterflies to a normal dawn to dusk schedule and others to a schedule where “dawn” was advanced 6 hours. Still another group experienced constant daylight. After the monarchs had adjusted their clocks to these new light regimens, Reppert's team tested their flight patterns outside.
Those on a regular day-night cycle flew southwest--the right direction for these migrants from Massachusetts. Those monarchs where first light was advanced were confused about the sun's position and headed southeast. And the constant light had totally disrupted the clock's input into navigation--all the butterflies did was head for the sun, Reppert and his colleagues report in the 23 May issue of Science.
The researchers also tracked the activity of a clock gene called period. In the insects placed on alternating light-dark cycles, the activity fluctuated with the changing light conditions. But the gene's activity did not change in constant light.
Taken together, the experiments demonstrate that the internal clock, perhaps timed by period activity, tells monarchs how to calibrate their movements against the sun. Without this clock, the sun would prove an unreliable landmark as it moves across the horizon. Thus the work “shows provocative links between the circadian machinery and the compass mechanism required for seasonal migration,” says Gene Robinson, a neurobiologist at the University of Illinois, Urbana-Champaign.
Related sites
Information on
monarch navigation
Discussion
and video of simulator used in Reppert's and others'
experiments
General information about
monarchs and details about a lay research program to track
them
)
)
)
)
)
)
)
)
)
)
)
