July 03, 2012

Leafcutter ants retire when they grow old

Leaf-cutter ants "retire" from
their cutting role when they grow old, switching to carrying when their jaws
blunt with age.

A close up shot of a leafcutter ant - Jeff Cremer
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

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

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.

A leafcutter ant carrying a leaf - Jeff Cremer

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.

Greater good

"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.

Source: http://news.bbc.co.uk/earth/hi/earth_news/newsid_9306000/9306830.stm

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