Getting It Just Right 
Sacha Vignieri  Volume 339, Number 6125, Issue of 15 March 2013
North American monarch butterflies are known for the massive southern migrations they undertake each year. During these migrations, a single butterfly may fly over 2500 miles south to the overwintering site, where it enters diapause, a hibernation-like state. This same butterfly will then fly northward in the spring, where it will become reproductive before dying and leaving the remainder of the return trip to its offspring. Monarchs coordinate their southward flight on the basis of a Sun compass mediated through circadian clocks located in the antennae. Guerra and Reppert now show that northbound flight is mediated by the same Sun compass mechanism. This finding begs the question, however, of how individuals are able to "switch" their compasses to fly south in the fall and north in the spring. Exposure of fall remigrants to variations of the day length and temperature conditions encountered at overwintering sites revealed that although day length has no effect on flight direction, temperature was able to facilitate the directional switch. Specifically, butterflies exposed to temperatures that were too warm did not switch their flight directions, whereas those exposed to appropriately cold temperatures flew north as expected. Thus, cold temperatures during overwintering are necessary for the completion of the enigmatic migration. Furthermore, warmer winters at these sites, due to climate change, may interfere with the monarch's migration home.

Curr. Biol. 10.1016/j.cub.2013.01.052 (2013).