The Circadian Clock
Monarchs, like almost all living beings, have an internal timepiece called a circadian clock that keeps track of the time of day. Circadian clocks are an evolutionary consequence of living on a planet that rotates once every 24 hours, but their rhythms continue even without the daily cycle of light and dark. These endogenous rhythms allow organisms to anticipate, rather than merely react to, the predictable events in their daily lives.
In monarchs, the circadian clock determines when an adult butterfly will emerge from its chrysalis (termed adult eclosion). Normally this happens in the morning, after the sun comes up, but even in constant darkness adult butterflies emerge whenever morning would have been. The clock may also dictate when the fall migration begins, by keeping track of day length.
We also know that the monarch’s circadian clock cooperates with the sun compass to form the time-compensated sun compass. Throwing a wrench into the circadian clockwork destroys the butterflies’ ability to calibrate their orientation based on time of day. No matter what time of day it is, monarchs with disabled clocks exhibit disoriented flight behavior.
How does the monarch’s circadian clock keep time? Where is it located, in the brain and elsewhere? What are its molecular gears? And how does it communicate with the sun compass?