MIGRANTS

Pressing Pause

Juvenile hormone (JH) is one of the defining differences between summer butterflies and fall migrants: migrants are deficient in JH. Many migratory behaviors are regulated by JH. Migrants need to live longer to reach their destinations, they can’t be distracted by reproduction, and they need extra energy stores—continued JH deficiency accomplishes all those things.

But what about time-compensated sun compass orientation, the behavioral hallmark of monarch migration? Does that, too, require a persistent lack of JH?

To find out, we treated fall migrants with methoprene, a potent JH analog, inducing them into a reproductively active state (Zhu et al., 2009). We tested these treated butterflies for oriented flight in a flight simulator, and compared them to normal summer butterflies and normal fall butterflies. Although their reproductive state matched the summer butterflies, their flight behavior continued to match the migrants—they exhibited directional flight, orienting significantly to the southwest.

To test for time-compensation in methoprene-treated fall monarchs, we compared butterflies kept on a normal light-dark cycle with butterflies kept on a 6-hour delayed light-dark cycle. Just like normal fall monarchs, the methoprene-treated migrants adjusted their bearing based on the 6-hour delay, demonstrating true time-compensated sun compass orientation.

So while JH deficiency regulates many migratory behaviors, it is not responsible for the maintenance of directional flight and time- compensated sun compass orientation. Increasing JH in migrants does not disrupt directed, time-compensated flight.