Rewinding in bull ants

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Imagine walking home from your familiar neighborhood store and, just as you approach your home, you find yourself mysteriously back at your starting point, the store, once again. Would you feel more uncertain about the familiar route when walking home again? We imposed this kind of manipulation on night-active Australian bull ants, Myrmecia midas, known for their fierce stings and their use of views around their nest for homing. These ants forage on eucalypt trees near their nest, and we took foragers from their foraging tree and kept them overnight with some honey water. The next morning, we let the ants walk home from near the foraging tree, the bull-ant equivalent of a familiar neighborhood store. As the forager neared her home, we caught her and put her back at the starting point, a procedure we call rewinding. Over repeated rewinds, ants displayed what we consider as signs of ‘uncertainty’, although at the moment, we do not know how this uncertainty manifests in the ants’ brain. The ants meandered more to the left and right, they stopped more often to scan their environment, and after 5 or more rewinds, some ants even started off in the opposite direction. This is understandable as, by tallying the path that they had already walked, a process called path integration, they should have walked way past the nest by this point. But the ants did not continue in the opposite nest-to-foraging-tree direction for long. After less than a meter, they all turned around and followed their usual view-based route once again. Studying how ants, with their small brains, cope with navigational challenges such as those imposed by rewinding may give insights into the design of autonomously navigating agents, which also have to cope with unexpected changes in the environment in which they are navigating.

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