Abstract:
We tested whether migrating Aphrissa statira butterflies orient with a magnetic compass. We captured migrants flying over Lake Gatun, Panama, and exposed experimental butterflies to a strong magnetic field. These and unmanipulated control butterflies were released back over the lake. Experimental butterflies had a more dispersed pattern of orientation than control butterflies. The average direction adopted was northeast, 160 anticlockwise to the natural migratory direction. Unmanipulated control butterflies adopted two diametrically opposed orientations: one shifted 33 clockwise, and another 147 anticlockwise, to the migratory direction. Control and experimental butterflies differed in that some controls oriented towards the migratory direction. These differences in orientation support the hypothesis of a sense for magnetic orientation cues. Unmanipulated butterflies released over the lake when the sky was completely overcast were significantly oriented towards their direction before capture (187 and 203, respectively), further supporting the magnetic compass hypothesis. In a third experiment, we obstructed sun compass cues and reversed the horizontal component of the local geomagnetic field to position magnetic north towards the geographical south pole within a flight arena into which we released individual butterflies. Experimental butterflies experiencing the reversed magnetic field oriented on average 180 opposite to their natural migratory direction. Control butterflies, for which the position of magnetic north was unaltered, were oriented both towards and 180 opposite to the natural migratory direction. This difference between orientations of control and experimental butterflies also supports the hypothesis of a sense for magnetic orientation cues.
