Do leaf-cutter ants Atta colombica obtain their magnetic sensors from soil?
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How animals sense, process, and use magnetic information remains elusive. In insects, magnetic particles are candidates for a magnetic sensor. Recent studies suggest that the ant Pachycondyla marginata incorporates iron-containing particles from soil. We used leaf-cutter ants Atta colombica to test whether soil contact is necessary for developing a functional magnetic compass. A. colombica is the only invertebrate known to calculate a path-integrated home vector using a magnetic compass. Here, we show that A. colombica requires contact with soil to incorporate magnetic particles that can be used as a magnetic compass; yet, we also show that ants can biosynthesize magnetic particles. Workers from a soil-free colony ignored a 90° shift in the horizontal component of the geomagnetic field, yet oriented homeward despite the occlusion of any geocentric cues. In contrast, workers from a soil-exposed colony oriented to an intermediate direction between their true and subjective home in the shifted field. Homeward orientations under shifted fields suggest that ants calculated a path-integrated vector using proprioceptive information. Strikingly, ants from the soil-free colony also had magnetic particles; yet, as observed by ferromagnetic resonance, these particles differed from those in soil-exposed ants and were not associated with a magnetic compass sensitive to this experimental manipulation.
KeywordsHoming Navigation Orientation Path integration Proprioception
We thank Hubert Herz for allowing the use of the laboratory-maintained, soil-free colony. We thank L. Senior (USDA-ARS) for measuring the ant orientations from the video tapes. We thank J. Gaskin, S. Adamo, and two anonymous reviewers for their comments on an earlier version of this manuscript. The Autoridad Nacional del Ambiente (ANAM) granted permission to conduct the research in Panama and export ants to Brazil for physical analysis. This project was supported in part by Centro Brasileiro de Pesquisas Fisicas (CBPF) of the Ministério da Ciência, Tecnologia e Inovação (MCTI). AJR received support from National Science Foundation grant IOB-0519483 (to Wulfila Gronenberg) and from the United States Department of Agriculture (USDA).
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