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Do leaf-cutter ants Atta colombica obtain their magnetic sensors from soil?

Robert B Srygley
Behavioral Ecology and Sociobiology 2014 v.68 pp. 55-62
Atta colombica, Pachycondyla, insects, invertebrates, soil
How animals sense, process and use magnetic information has remained largely elusive. In insects, ferromagnetic particles are candidates for a magnetic sensor. Recent studies suggest that ferromagnetic minerals from soil can be incorporated into the antennae of the migratory ant Pachycondola marginata (Oliveira et al. 2010). We used Neotropical leaf-cutter ants Atta colombica to test whether soil contact is necessary for the acquisition and use of a magnetic compass. Atta colombica typically relies on pheromonal trails, but is the only invertebrate known to use a magnetic compass to update a calculated path integrated home vector (Riveros and Srygley 2008). Here we show that contact with soil is necessary for A. colombica to incorporate magnetic particles that can be used as a magnetic compass, and 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 magnetic field, yet significantly oriented homeward despite the experimental occlusion of any geocentric cues. In contrast, workers from a soil-exposed colony oriented homeward, shifted with the magnetic field or oriented in an intermediate direction. Homeward orientations under shifted magnetic fields suggest that leaf-cutter ants are able to determine a path integrated home vector in conditions where only proprioceptive information is available. Strikingly, ants from the soil-free colony also had ferromagnetic particles, yet, as observed by Ferromagnetic Resonance (FMR), these particles differed from those in soil-exposed ants and evidently were not associated with a magnetic compass sensitive to the horizontal shift in the magnetic field.