By Emma Brennand
Earth News reporter
Leaf-disks collected by the forager ants are transported back to the colony
Central American leaf-cutter ants "retire" from their cutting role when they grow old, switching to carrying when their jaws blunt with age.
Leaf-cutter ants start their lives with razor-like jaws, or mandibles, to cut through the leaves they harvest.
But as these "wear out", the insects tend to carry the leaves cut by their younger counterparts.
The findings suggest that individual ants can extend their useful lifespan as their skills decline.
They are reported in the journal Behaviour Ecology and Sociobiology.
The US-based scientists discovered that older ants were significantly less efficient at cutting leaves.
They estimated these older colony members' "worn-out teeth" halved the speed at which the entire colony was able to harvest leaves.
The researchers, from the University of Oregon and the Oregon State University, supports previous research showing the survival of a leaf-cutter colony depends on the efficiency of its workers.
The leaf-disks collected by the forager ants are transported back to the colony where the sap can be harvested for food.
The gathered leaves are also used as a surface to grow the fungus that is consumed by the colony.
"Cutting leaves is hard work," explains the University of Oregon's Dr Robert Schofield, who led the study.
"Much of the cutting is done with a V-shaped blade between teeth on their mandibles.
"This blade starts out as sharp as the sharpest razor blade that humans have developed."
It is believed that leaf-cutter ants' mandibles also contain zinc-enriched biomaterials, which strengthen them.
Over time, however, these razor-sharp blades become blunt and less efficient.
The ants cut the leaves with a V-shaped blade on their jaw
The researchers measured the wear on the mandible cutting blade in the ant species
Atta cephalotes from a colony in Soberania National Park, Panama.
Dr Schofield and his team used electron microscopy to compare the pristine teeth of laboratory-reared pupae with the worn teeth of the wild forager ants.
By comparing the radius of the mandible teeth they found the blade of a cutter ant to be 340 times duller than the pristine blade of a pupa.
The study revealed that leaf-cutter ants with highly worn mandibles had difficulties cutting and anchoring leaves.
Individuals with the most worn teeth, which had less than 10% of the cutting blade, exclusively carried leaves rather than cut them.
The team estimated that, because of this age-related wear, a colony spent twice the energy cutting leaves than it would if all the ants had sharp mandibles.
The findings support the idea that wear and fracture can be significant problems for insects as well as larger animals.
"This study demonstrates an advantage of social living that we are familiar with," says Dr Schofield.
"Humans that can no longer do certain tasks can still make very worthwhile contributions to society."
Leaf-cutter ants live in colonies that have a very developed structure with a strict hierarchy.
This level of social organisation is described as eusocial.
As well as benefitting the colony, the researchers believe that this ability to change jobs may also lead to longer life spans in social insects compared to their solitary cousins.