Migrating butterflies may use temperature to choose their flight direction.

Each autumn, monarch butterflies (Danaus plexippus) migrate more than 3,200 kilometres down eastern North America to overwinter in Mexico. The insects navigate by internal clocks that are calibrated by sunlight. But what halts the journey south? The answer is the cold.

Steven Reppert and Patrick Guerra of the University of Massachusetts Medical School in Worcester caught butterflies at the beginning of the autumn migration and kept them in the chilly temperatures that they would experience at sites in the mountains of central Mexico. When released after 24 days in the cold, the butterflies flew off northward, the direction they would normally take in spring. Simulating changes in day length did not have this effect.

Worryingly, if climate change obscures the cold signal, butterflies may keep flying south and not know when to return.

Curr. Biol. http://dx.doi.org/10.1016/j.cub.2013.01.052 (2013)