Leaf-cutter ants switch jobs when their tools wear down

When leaf-cutter ants wear out their cutting mandibles, they switch jobs, according to a new study published in December 2010. Researchers from the University of Oregon found that as leaf-cutter ants get older, their razor-like mandibles  – those little appendages near the ant’s mouth – get dull, they cut through leaves half as fast and spend twice the energy doing it. Eventually, they transition from cutting leaves to the job of carrying leaves.

As you would expect, cutting leaves is a big part of a leaf-cutter ant’s life. Found mostly in Central and South America, leaf-cutter ants harvest fresh leaves and carry them in pieces back to their nests, where they grow an edible fungus on the leaves. The fungus is the main source of food for the colony. If you’ve ever stumbled across a leafy stream of these ants headed from a leaf back to their colony, you’ve probably marveled at the tireless determination of these creatures – especially if you know that the leaves they’re carrying can be up to several times their body weight.

How do ants walk upside down?

The scientists had suspected that very small organisms, like leaf-cutter ants, experience a lot of wear on their “tools” because they are cutting on such a small surface area. The team created an instrument that could measure the force required to slice a leaf with a mandible, and observed the time it took the ants to cut the leaves. The time varied depending on how worn down their mandibles were. In their paper, the scientists wrote that if all of the ants’ mandibles were in pristine condition, it would have taken them half as much time to cut up their leaves. They also observed for the first time that wear on mandibles affects the division of tasks – which ants carry leaves versus cutting leaves.

The researchers are not sure exactly how ants decide to switch careers, but it makes their miniature society more efficient.  The title of the paper, in case you are wondering, is wonderfully self-explanatory: “Leaf-cutter ants with worn mandibles cut half as fast, spend twice the energy, and tend to carry instead of cut.”

January 6, 2011

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