pyriformis workers have evolved nocturnal visual adaptations, which include large lenses and wide photoreceptors to increase their optical sensitivity by 27-fold compared with their diurnal relatives. Ambient light intensity during sunset triggers the onset of foraging and it most likely controls the cessation of foraging before sunrise ( figure 1 c). A majority of these solitary foraging ants carry out only one foraging trip each day, restricting the visually demanding task of navigating between the nest and the foraging tree to the dim-lit conditions of the evening and morning twilight. Individual ants forage strictly on one tree over at least two months and each nest remains faithful to 1–3 trees for the entire lifetime of a colony. They are long lived, with individuals capable of surviving for more than a year in natural conditions (A. The nocturnal ant Myrmecia pyriformis ( figure 1) presents an unusual opportunity to address this question, since they have predictable foraging patterns and key aspects of their ecology and visual anatomy have been well documented. So, is visual navigation affected by subtle changes in the landmark panorama? Demonstrating the use of visual landmarks has often involved training animals to new landmarks, or making familiar landmarks unavailable or blocking parts of the panorama, which causes significant changes in the insect's visual field. For this, they rely on the entire landmark panorama and indeed when an artificial panorama is rotated, ants travel in the direction predicted by that panorama. During homing, individuals move and compare their current view to a previously memorized view and travel in the direction that provides the least image difference.
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Ants are thought to learn landmark information through a carefully constructed series of learning walks carried out in multiple orientations around a goal. This is perhaps because they rely most on visual landmark information. However, when the pattern of polarized skylight is experimentally rotated, the ants only partially compensate for this change. Nocturnal ants derive compass information from the pattern of polarized skylight. So, how then do nocturnal ants navigate?īoth diurnal and nocturnal ants navigate using celestial and terrestrial visual information.
At night, light intensity is about a 100 million times dimmer than during the day, decreasing the detectability of visual information, which makes navigating at night a challenge. However, a large number of ants are active at night and face navigational challenges similar to their diurnal counterparts. Most of our current knowledge about ant navigation has come from day-active ants. Among these, visual orientation has been extensively studied because the visual navigational information can be quantified, relevant sensory systems can be characterized and the visually driven behaviour can be monitored under natural conditions. This article is part of the themed issue ‘Vision in dim light’.Īnts derive compass information from a range of cues including the pattern of polarized skylight, the landmark panorama, odours and magnetic fields. We document how their behaviour changed over subsequent nights and discuss how the ants may detect and respond to a modified visual environment in the evening twilight period. Their foraging success decreased and they looked around more, including turning back to look towards the nest. We found that immediately after the trees were removed, ants walked slower and were less directed. We filmed the behaviour of ants close to the nest and tracked their entire paths, both before and after the trees were removed. An image difference analysis showed that the change in the overall panorama following the removal of these trees was relatively little. We achieved this by felling three dead trees, two located along the typical route followed by the foragers of that particular nest and one in a direction perpendicular to their foraging direction. Here, we had an opportunity to slightly modify the natural visual environment around the nest of the nocturnal bull ant Myrmecia pyriformis. The ability of ants to navigate when the visual landmark information is altered has often been tested by creating large and artificial discrepancies in their visual environment.
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