Experimental evidence for a magnetic sense in Neotropical migrating butterflies (Lepidoptera: Pieridae) ROBERT B. SRYGLEY * †, ROBERT DUDLEY ‡, EVANDRO G. OLIVEIRA § & ANDRE J. RIVEROS ** *Smithsonian Tropical Research Institute yDepartment of Zoology, University of Oxford, U.K. zDepartment of Integrative Biology, University of California-Berkeley xDepartamento de Biologia Geral, Universidade Federal de Minas Gerais **Departamento de Biologı ´a, Universidad Nacional de Colombia (Received 30 January 2004; initial acceptance 5 April 2004; final acceptance 12 April 2005; published online 9 December 2005; MS. number: 7987R) We tested whether migrating Aphrissa statira butterflies orient with a magnetic compass. We captured mi- grants flying over Lake Gatu ´ n, 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 adop- ted 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 to- wards the migratory direction. These differences in orientation support the hypothesis of a sense for mag- netic 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 unal- tered, 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. Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. For migrating butterflies, three not mutually exclusive hypotheses have been put forward to explain their abilities to orient and navigate: use of landmarks, orien- tation with a sun compass, and use of a geomagnetic compass (Brower 1996). To date, we have shown that Aphrissa statira butterflies (Pieridae: Coliadinae), which cross the isthmus of Panama in abundance during May– July, use local landmarks to correct for wind drift when crossing the Panama Canal (Srygley et al. 1996). We have also demonstrated use of a time-compensated sun compass by this species, which may be used for orienting over long distances (Oliveira et al. 1998). As with the sun, the earth’s magnetic field may also provide directional information for insects that navigate long distances. When approached with a strong magnet before release, headings of magnetized monarch, Danaus plexippus, butterflies were random whereas those of con- trol butterflies were directed to the southwest (Perez et al. 1999). However, these results appear to have been confounded by wind drift (Srygley & Oliveira 2001). Most recently, Mouritsen & Frost (2002) tethered Correspondence: R. B. Srygley, Smithsonian Tropical Research Institute, Apdo. 2072, Balboa, Republic of Panama (email: bob.srygley@zoo.ox. ac.uk). R. Dudley is at the Department of Integrative Biology, University of California-Berkeley, Berkeley, CA 94720-3140, U.S.A. E. G. Oliveira is at the Departamento de Biologia Geral, Instituto de Cie ˆncias Biolo ´g- icas, Universidade Federal de Minas Gerais, 30161-970 Belo Hori- zonte, MG, Brazil. A. J. Riveros is now at the Center for Insect Science, Arizona Research Laboratories, University of Arizona, 611 Gould-Simpson Science Building, 1040 E. Fourth, Tucson, AZ 35721-0101, U.S.A. 183 0003–3472/05/$30.00/0 Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 71, 183–191 doi:10.1016/j.anbehav.2005.04.013