Ant's head balancing act revealed by video

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An ant's neck plays a critical role in balancing large objects

By Jody Bourton
Earth News reporter

The extraordinary ability of tiny ants to carry huge loads has been known for some time.

But scientists have now employed high speed video cameras and revealed how the ants use controlled head movements to maintain their stability.

The researchers also discovered that an ant's neck plays a critical part in balancing the load, a feature previously unknown in any insect.

Details are published in the Journal of Comparative Physiology A.

Balancing act

Grass-cutting ants (Atta vollenweideri) carry plant fragments many times heavier and longer than themselves.

These ants harvest grass, hauling fragments considerable distances back to their nest, sometimes exceeding 100m.

We did not expect the neck joint to be involved Karin Moll University of Cambridge

Ants maximise the amount of grass they transport by selecting long fragments, the researchers explain.

But this is not an easy thing to do; in order to carry the cargo, workers need to both lift and balance their load.

"In this study we could experimentally show that workers carrying long fragments often fell over," says Karin Moll from the University of Cambridge in the UK, who undertook the study with Dr Walter Federle, also from Cambridge University, and Dr Falvio Roces from the University of Wurzburg in Germany.

To find out how the ants maintained their stability, the research team used high speed video cameras to measure the insects' head movements.

They gave the ants pieces of paper soaked in orange juice to mimic grass leaf fragments and measured the angles at which they carried them.

Leaf lift

"In order to maintain stability the combined centre of gravity of ant and load needs to remain above the area of the supporting legs," Ms Moll says.

The researchers found that the ants carried long fragments at a steeper angle than short fragments of the same mass.

"Workers do not hold fragments differently, but change fragment positions by up and down movements of their head at the neck joint," she says.

That allows the ants to adjust their "leaf position" even if they are walking up or down hills or over objects.

When walking uphill, workers carried fragments at a significantly steeper angle than ants on a horizontal trail, the researchers wrote in the journal.

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When walking downhill, the ants carried fragments at a significantly shallower angle.

The study has provided the researchers with a further understanding of "ant biomechanics" and has yielded some surprising results.

Biomechanics is the term used to describe mechanical principles of living organisms.

"We did expect the ants to have evolved an adaptation that enables them to carry long grass fragments," say Ms Moll.

"However, we did not expect the neck joint being involved in the adjustment of the load position, as this is not known to be involved in load carriage in any other insect."

Ms Moll explains that the study underlines the importance of considering biomechanical factors in theories of how ants forage, especially if large loads are involved.

The team hopes to conduct further work to explore the detailed function of the head movements, and how ants may sense and perceive carrying unequally balanced objects.

Grass-cutting ants occur in the Chaco region of north Argentina and Paraguay and belong to a subgroup of leaf-cutting ants that specialise in harvesting grass leaves.

SOURCES Read more about the ants' head-balancing act at the Journal of Comparative Physiology A

Leaf-cutting ants are fungus growers, which means that harvested leaves are not eaten by the workers themselves, but serve to cultivate a symbiotic fungus within the nest, which is the main nourishment source for the ant larvae.