Abstract
Desert ants are a model system for animal navigation, using visual memory to follow long routes across both sparse and cluttered environments. Most accounts of this behaviour assume retinotopic image matching, e.g. recovering heading direction by finding a minimum in the image difference function as the viewpoint rotates. But most models neglect the potential image distortion that could result from unstable head motion. We report that for ants running across a short section of natural substrate, the head pitch varies substantially: by over 20 degrees with no load; and 60 degrees when carrying a large food item. There is no evidence of head stabilisation. Using a realistic simulation of the ant’s visual world, we demonstrate that this range of head pitch significantly degrades image matching. The effect of pitch variation can be ameliorated by a memory bank of densely sampled along a route so that an image sufficiently similar in pitch and location is available for comparison. However, with large pitch disturbance, inappropriate memories sampled at distant locations are often recalled and navigation along a route can be adversely affected. Ignoring images obtained at extreme pitches, or averaging images over several pitches, does not significantly improve performance.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-015-1005-8) contains supplementary material, which is available to authorized users.
Highlights
The field of neuroethology owes a great debt to the pioneering work of Prof Rüdiger Wehner
We stress that this is a simple observational study to support the parameters of head pitch variation used in our modelling, and not an experimental study to determine the causes of that variation
If ants were actively stabilising, the head pitch should remain relatively constant, or at least fluctuate substantially less than the body pitch, but it is clear that for the most part it follows the body pitch
Summary
The field of neuroethology owes a great debt to the pioneering work of Prof Rüdiger Wehner. His work laid foundations for the emerging fields of computational biology and biorobotics by providing benchmark behavioural assays against which functional hypotheses, embodied as computer programmes, can be verified An example of his lasting legacy is found in the observation of visual homing behaviours in desert ants (Wehner and Räber 1979; reported in bees by Cartwright and Collett 1982, 1983), which inspired the long-standing hypothesis that insects store visual memories retinotopically, allowing the animal to return to a location by moving so as to increase the retinotopic match between what they currently see and their memory. The same retinotopic principle has been in used in the ‘visual compass’ or ‘alignment image matching’ hypothesis
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