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First published online June 6, 2005
Journal of Experimental Biology 208, 2399-2408 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01613

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Do monarch butterflies use polarized skylight for migratory orientation?

Julia Stalleicken¹, Maya Mukhida², Thomas Labhart³, Rüdiger Wehner³, Barrie Frost² and Henrik Mouritsen^1,*

¹ VW Nachwuchsgruppe `Animal Navigation', IBU, University of Oldenburg, D-26111 Oldenburg, Germany
² Department of Psychology, Queen's University, Kingston, ON, Canada, K7L 3N6
³ Zoological Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

View larger version (30K): [in a new window]   Fig. 1. Detailed drawing of the polarizer holder used in the 2004 experiments for testing the monarchs' reactions to a 85° polarized light stimulus. The upper broken line indicates the level of the lid, which limited the butterflies' view of the sky. For detailed description, see Materials and methods.

View larger version (34K): [in a new window]   Fig. 2. Orientation of monarch butterflies exposed to different celestial cues. Each dot at the circle periphery (A,D,E,H) indicates the mean orientation of one butterfly flight. The arrows indicate group mean vectors. The broken circles indicate the radius of the mean vector required for significance at the P<0.05 and P<0.01 levels according to the Rayleigh test (Batschelet, 1981). B,C and F,G show the virtual flight paths flown by individual butterflies assuming a constant flight speed. They start in the center of the diagram and travel towards the periphery. Distances have been normalized. (A,B) With a 120° view of the clear blue sky including the sun, monarchs orient in their south–southwesterly migratory direction. (C,D) Under simulated overcast with only the geomagnetic field available, but no sun or polarized light cues, the butterflies were randomly oriented. (E–G) A 44° visual field of clear blue sky in the zenith but no direct view of the sun also led to random orientation (E) both for monarchs tested in the morning (grey dots, tracks in F) and in the afternoon (open dots, tracks in G). Since a few flights are bimodal and only the prominent peak of each bimodal distribution is indicated on the circular diagrams, a few points in the circular diagrams do not seem to coincide with the corresponding tracks. (H) The monarchs also did not show any preference to line up with the axis of skylight polarization in the zenith (indicated by four parallel lines).

View larger version (33K): [in a new window]   Fig. 3. Orientation of monarch butterflies tested under different artificial polarized stimuli. (A,B) 44° UVA-containing stimulus; (C,D) 85° UVA-containing stimulus without sun shades; (E,F) 85° UVA-containing stimulus with sun shades. None of the polarized light stimuli led to time-compensated compass orientation in monarchs based on the orientation of the polarizer (A,C,E; a point in NE/SW indicates orientation towards northeast or southwest) and the butterflies did not line up in any specific direction relative to the axis of polarization (B,D,F; a point in 0°/180° indicates orientation along the polarizer's axis whereas a point in 90°/270° indicates orientation perpendicular to the polarizer axis). All angles are doubled because of the bimodal ambiguity of all linear polarizers (see Materials and methods). The lack of orientation towards the large polarizer was observed at all times of day (grey dots and open dots indicate flights before and after 12:00 h EST, respectively; total time range 08:15 h to 16:30 h EST). For further explanation of symbols, see legend to Fig. 2.

View larger version (17K): [in a new window]   Fig. 4. Responses of monarch butterflies to 90° turns of polarizers or lids. Even though some monarchs changed their mean heading following a 90° clockwise turn of the 44° polarizer (A) or the 85° polarizer (without sun shades; B), their mean change was 0°. Furthermore, the animals' reactions in response to a turn of the polarizers were no different from their reactions when a lid with an opening exposing the blue sky was turned 90° (C). 0° indicates the mean direction chosen by each individual butterfly prior to the turn of the polarizer. Thus, a data point at 0° means that the orientation before and after the turn was identical. For explanation of symbols, see legend to Fig. 2.

View larger version (42K): [in a new window]   Fig. 5. Time-compensated orientation of monarch butterflies having a 120° view of the sky including the sun with and without occluded dorsal rim area (DRA). (A) In 2003, control monarchs oriented in their typical south–southwesterly autumn migratory direction. (B) Overpainting of the DRA did not affect their ability to orient in the migratory direction. (C) SEM image of the dorsal-most part of a monarch eye showing the extent of the paint mask covering the DRA. Such images revealed that the 14±4 (mean ± S.D.; range 4–37) most marginal rows of ommatidia were occluded in the dorsal half of the eye in the DRA-animals. Thus, the DRA, which has a maximal width of three rows of ommatidia (Labhart and Baumann, 2003), was amply covered in all overpainted eyes. (D) In 2004, non-clockshifted control monarchs showed an unusual mean migratory direction just east of south. (E) Clockshifted monarchs shifted their orientation +96°. (F) Clockshifted monarchs with their DRA occluded also shifted their orientation clockwise (+86°) as predicted when they use a time-compensated sun compass. For explanation of symbols, see legend to Fig. 2.

View larger version (11K): [in a new window]   Fig. 6. Abrupt orientation changes following a turn of the polarizer were observed in a few butterflies, although this behavior also occurred in other situations. (A,B) Tracks of two of the three individual butterflies (out of a total of 50 monarchs tested) that changed their orientation by 90° after the polarizer was turned by 90°. (A) 44° polarizer. (B) 85° polarizer. (C,D) Examples of what might be transient orientation responses towards the polarizer being turned by 90°. (E) However, one out of 10 monarchs showed a distinct 90° shift in orientation when the lid with an opening exposing the blue sky instead of a polarizer was turned. (F) A 90° turn observed in the same experimental condition (see E) but occurring before the turn of the lid. (G,H) Spontaneous 90° changes in orientation of monarchs flying under the artificial polarizer. Note that the animal in G altered its mean flight direction before the polarizer was turned and not in response to the turn. In H, the butterfly abruptly changed its orientation by 90° although the polarizer was never turned. In two-colored tracks, the orange parts indicate the virtual flight paths of the monarchs before the polarizer and/or the lid was turned. The blue parts indicate the tracks flown after the turn.

View larger version (16K): [in a new window]   Fig. 7. The geographical orientation of all butterflies that did not see the sun directly was random even with 140 tested individuals. The undisturbed geomagnetic field was available during all flights. For explanation of symbols, see legend to Fig. 2.