How leaf-cutter ants find their way home

Written by Thomas Hesselberg

Monday, 31 March 2008

A foraging leafcutter ant returning to its nest. Scientists have shown that ants use a magnetic compass for orientation. Photo courtesy of Scott Bauer, US Department of Agriculture.
Social insects are among the most fascinating creatures on Earth, their complex societies and highly developed mounts, hives and nest are a continuous source of wonder and inspiration. Especially fascinating are the Central and South American leafcutter ants. There colonies can consist of several million individuals of distinct castes with large soldier ants more than 10 times larger than their smaller nest mates. Most impressive, however, is their agriculture or should we say fungiculture. The ants live in a mutualistic relationship with a fungus from which they get all their nourishment. In return they provide the fungus with leaves and other plant material and protect it from pests and moulds. It is while collecting food for their fungus that we get the characteristically columns of ants carrying leaves vertical on their backs. However, it has been something of a mystery how these night active ants find their way back to the nest in the rainforest floor where no polarised light reach.

In a recent paper Andre Riveros and Rob Srgley from the Smithsonian Tropical Research Institute found that leafcutter ants use path integration in combination with a magnetic compass in situations where they cannot use other sensory cues such as landmarks, polarised light or odour trails.

The biologists conducted experiments on the leafcutter ant, Atta columbica, in a rainforest in Panama. Ants were picked up on their way back to the nest and transferred to an artificial magnetic field some distance away to measure their orientation. Care was taken to block any visual landmark cues. Results showed that all control ants exposed to the normal magnetic field orientated towards the nest, whereas half of the ants exposed to a reversed magnetic field orientated in the exact opposite direction of the nest.

In a separate experiment ants were also briefly exposed to a magnetic pulse that disrupted their magnetic compass to see how that affected their orientation. The results showed that ants now orientated in a random direction.

Both experiments strongly suggest that leafcutter ants do indeed use a magnetic compass for orientation, when no other cues are available.

Source:
Riveros, A. J. & Srygley, R. B. (2008) Do leafcutter ants, Atta colombica, orient their path-integrated home vector with a magnetic compass? Animal Behaviour 75, 1273-1281.

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