Submitted to: The Royal Institute of Navigation
Publication Type: Abstract Only
Publication Acceptance Date: 11/16/2007
Publication Date: N/A
Technical Abstract: Leaf-cutter ants Atta colombica forage over 250 m in structurally-complex, Neotropical rainforests that occlude sun or polarized light cues. Night foraging makes the use of celestial cues and landmarks all the more difficult. Typically leaf-cutter ants follow architecturally-modified, pheromonally-marked trails between foraging sites and nest entrances. However, this combination of chemical and visual cues is likely to be complemented by a compass, used when foraging, where foraging trails degrade, or when ants become displaced. We investigated the directional cues used by leaf-cutter ants to orient homeward by experimentally reversing the polarity of the local magnetic field and transferring homeward-bound ants from a foraging trail to a table in a chamber that occluded landmark and celestial cues. For one colony, the experiment was performed in daylight, whereas for the second colony the experiment was performed at night in absolute darkness. In both experiments, Control ants demonstrated path-integration and walked directly towards the nest. When the sun’s disc was unobstructed by clouds, the ants’ orientations did not differ from the Control ants, indicating that magnetic cues were not used. When the sun’s disc was obstructed by clouds, then the ants shifted their orientation relative to Control ants and those in reversed-polarity under sun. In darkness one-half of the Experimental ants oriented according to the reversed field (geographically 180° opposite to the nest’s direction), indicating that they used a magnetic compass to update their positional reference derived from path-integration. The other half walked towards the nest suggesting that they may have used an egocentric reference to measure their rotation when displaced, although other explanations have not been entirely excluded. In a third experiment, we experimentally subjected the ants to a strong magnetic pulse to disrupt a magnetic compass. In this third colony, Control ants demonstrated path-integration and walked directly towards the nest, whereas Experimental ants oriented randomly. We conclude that the leaf-cutter ants use the Earth’s magnetic field as a reference by which to orient when path integrating towards home. To characterize the magnetic sensor, we conducted room temperature SQUID magnetometry and Ferromagnetic resonance measurements. The head and antennae combined had three times more magnetic material than the abdomen plus thorax (antennae: 8 ± 4%; head: 67 ± 10%; thorax: 37 ± 8%; and abdomen: 9 ± 6%). Ant heads had organized magnetic material that was not observed in antennae. The coercive force was 38 ± 2 Oe and the magnetization ratio was 0.05 ± 0.01. These values correspond to particulate dimensions of 17 ± 4 nm with respect to Goya’s curve of spherical magnetite nanoparticles and 305 ± 15 nm with respect to Ozdemir’s curve. Nanoparticles in the ants’ heads are in the Taux diagram region characteristic of multi-domain magnetite.