Magnetic compass orientation of monarch butterflies (Lepidoptera : Danaidae : Danaus plexippus L.): Did observer effects lead to false positive results?

Magnetic compass orientation in migratory monarch butterflies (Dannus plexi ppus L.) was recently reported by Etheredge et al. (1999). Our attempts to replicate and extend their experiments failed, leading us to test alternati ve hypotheses, specifically the effect of an observer adjacent to the exper imental arena. With the observer at 60 degrees, monarchs oriented toward th e observer (P < 0.005, V-test); with the observer at random locations, mona rchs oriented toward the observer (P < 0.05, V-test), and with no observer visible to the butterflies, orientation was not significantly different fro m random (P = 0.14, Rayleigh test). These findings suggest that observer ef fects, possibly due to positive phototaxis, could explain the results inter preted as magnetic compass orientation by Etheredge et al. (1999).