The Monarch butterfly is a species of butterfly that occurs around the world, but 
predominately in North America where it is famous for its long range migration. In these migrations the butterflies move during the autumn from regions in Canada and the northern United States to regions in Texas and Mexico to escape the winter. They return during next spring. What is even more impressive about this migration is that the lifespan of the butterflies is too short for each individual to complete the whole trip. Adults die in breeding grounds in the north and the next generation returns to the wintering grounds in Mexico, where they have never been. This raises the obvious question: How do they know where to go?
Biologists have long studied migrating animals and distinguish between true navigators and compass navigators. True navigators – like migratory birds and sea turtles – have the ability to assess their latitude and longitude and can thus move using an internal map. This amazing ability allows them to correct their course if e.g. a storms pushes them of course. With the help of these corrections the migrating species will end up in a very narrow geographic region at the end of their journey.
Compass navigators on the other hand can only assess the direction in which they want to go (e.g. to the south east) but do not know where exactly they are. They can accordingly not perform course corrections if they should be brought off course and have a wider geographic distribution at their destination.
In a current study [1] Mouritsen et al. make the case that the Monarch butterfly is a compass navigator rather than a true navigator. They base this on an experiment in which they moved Monarch butterflies across Canada and determined the direction in which they took of. This direction was the same at the initial location and the location on the other side of Canada, indicating that the Monarchs did not perform a course correction. They also analyzed data from capture-release studies of the past 50 years that looked at the movement of butterflies during migration periods. This analysis showed that the data is more compatible with the hypothesis of a compass navigator than a true navigator. Support for this hypothesis comes from another recent study that shows that the butterflies use the sun and an internal clock to find out the vector along which they move. The direction along this vector depends on whether they have experienced the sufficiently low temperatures they would experience during the winter in Mexico. This temperature dependence might explain why Monarch populations on e.g. Hawaii show no migratory behavior.
However the experiment by Mouritsen et al. has been criticized for its methodology [3] and it does not explain the narrow geographic range in which the monarch butterflies end up at the end of their migration. This seems more in line with true navigation and the sun compass may only be one instrument in the toolkit of the butterflies. On the other hand geographical features and prevailing wind directions might also be able to assist the butterflies in reaching their destination without true navigation.
Stephan Koehler
[1] Mouritsen et al, “An experimental displacement and over 50 years of tag-recoveries show that monarch butterflies are not true navigators”, PNAS 110(18), 7348 (2013)
[2] Guerra and Reppert, “Coldness triggers northward flight in remigrant Monarch Butterflies”, Current Biology 23(5), 419 (2013)
[3] Oberhauser et al, PNAS 110(39), E3680 (2013)