Monarch migration appears to occur without internal maps

Nature : Monarch butterflies are renowned for their 4000-km migration from Canada to a small area in Mexico. How they navigate during that migration is an open question. To test whether the butterflies had map-like knowledge, Henrik Mouritsen of the University of Oldenburg in Germany and his colleagues displaced some butterflies about 2500 km to the west of their normal range. Instead of adjusting to a south-southeasterly heading, they maintained their normal southwesterly direction. Studying data collected over decades, Mouritsen noticed that a distribution of the butterflies’ migration matched a migration model of them simply following a general compass direction. He believes his test with the displaced butterflies supports compass-only models over those that propose the butterflies also have map knowledge that allows them to orient themselves to their target. He says the butterflies probably use other cues, such as smells, and behavioral traits, such as not flying over mountains or open water, to help them reach their destination. However, the study has received some criticism for not comparing the distributions with compass-and-map models and for removing the butterflies so far from their native environment. Further studies will be necessary to determine which distribution models better fit the butterflies’ actual behavior.