The Occasional Hymenoptera: Worn mandibles in leaf-cutters
A new paper in Behavioral Ecology and Sociobiology concludes that when the mandible of certain leaf-cutter ants (Atta cephalotes) becomes sufficiently dulled, instead of continuing to shear leaves, they switch tasks and become carriers.
Attine ants, as we saw once before, cut leaves to bring to the nest, not for food (or at least not primarily for food), but as food for the fungus they cultivate. The division of labor required to maintain a colony of these farming ants is one of the marvels of the animal kingdom. It also represents a difficulty (as does all eusociality among hymenopterans) for evolutionary theory, a difficulty that we will soon (deo volente) address in this series.
The paper by University of Oregon researchers Robert M.S. Schofield, Kristen D. Emmett, Jack C. Niedbala and Michael H. Nesson was the result of a project to show that wear damage to “tools” (such as claws and teeth) tend to be greater problems for smaller organisms. It is entitled “Leaf-cutter ants with worn mandibles cut half as fast, spend twice the energy, and tend to carry instead of cut,” and was available on-line first on December 8. It is open-access until the end of this month only (abstract with pdf downloads and html versions). With the miracle of YouTube they allow you to see how leaf-cutters use their mandibles. In ordinary cuts, one mandible is held stationary and dragged through the cut:
On tougher cuts (across veins) the ants use their mandibles “symmetrically” like a saw (or wire cutter):
The paper goes through a series of intricate biomechanical calculations to show that wear over time reduces the cutting rate of the mandible. They then show that the reduction in cutting rate increases the required energy for cutting leaves. At a certain point the energy necessary for cutting leaves exceeds the energy available from the cut leaves (in the form of the sugary sap). As for the effect on the colony, the authors’ field observations in Panama showed that a colony used 1.8 times as much energy as it would have if all the cutters had pristine mandibles. (Or to put it as they did: if the cutters had pristine mandibles they would have used only 0.56 as much energy.)
They observed in the field that ants with the most worn mandibles no longer cut, but rather were used as leaf carriers. This performance-based job allocation had not been reported in the literature before. When they analyzed the allocation of tasks with the energy-formulas they had developed, they found that the allocation was not optimal but better than a random allocation between cutters and carriers.
The authors conclude from this study that the energy demands from mandible wear is a strong selective pressure. They suggest that task allocation was the response and that this aspect of eusociality accounted for longer life of the social insects over their solitary counterparts.
The press release that announced the paper says that “Schofield was lead author of a study published in 2001 that had identified a family of biomaterials present in mandibular teeth, tarsal claws, stings and other such tools of small organisms. In 2009, a team led by Schofield reported that a similar type of substance empowers the claw tips of striped shore crab and is present on the walking legs of Dungeness crabs.”