Adventures Among Ants - Mark W. Moffett [114]
My excitement wasn’t much consolation for Luiz. His back was covered with leafcutter soldiers the size of horseflies, which had latched onto his flesh with scalpel-sharp mandibles, and his shoulder was bleeding where a soldier was busy slicing an especially deep curved groove.2 I indicated to Luiz that he should protect himself by wearing a shirt, but my own defenseless hands were marked with similar crescent moons, painful as paper cuts, as if I’d been clawing through brambles. More soldiers poured out of a hole in the floor of the trench and swarmed up my legs as I helped two of Luiz’s friends pull the ants from his back. Then everyone picked up their picks and shovels and went back to work.
Leafcutter soldiers have few duties. They have been known to use their offensive skills to dice up tough fruit, but mostly they protect the nest and its immediate environs from army ants, hungry armadillos (whose powerful forearms help them tunnel easily into the heart of a nest), and curious entomologists and their unfortunate assistants.3 The damage they had done to Luiz’s back and my hands reminded me of Lophomyrmex bedoti, an otherwise unremarkable ant with jaws almost as fine-toothed as saw blades. In Indonesia, I had seen three workers of this species shear off both antennae, four legs, and the tips of two body spines of a big-headed ant with grisly ease, in less than thirty seconds.4
I would have guessed that leafcutter jaws work with an equal effortlessness, but leaf carving is an arduous activity for these ants, equivalent to the cost of flying in other animals.5 Follow the leafcutters to the source of the ants’ parasols, and you will find the ants on vegetation, cutting arcs like the ones chiseled into my fingers. Rather than using her mandibles like saws or scissors, a leafcutter sticks the terminal tooth of one jaw into the leaf to fix its position as she pushes her other jaw against the leaf edge. She then forces the second jaw into the tissue with a rocking motion, the way people use a lever-type can opener. Leafcutter jaws are also like a can opener in that they get some of their strength and rigidity from metal, having a zinc content of 30 to 40 percent.6
While cutting, the worker herself acts like a geometer’s compass: she anchors her back legs at the leaf margin and moves in an arc around that point, adjusting the size of each fragment by flexing her legs or head to different degrees. By such fine-tuning, she slices off fragments that she (or other ants) can carry: smaller pieces from thicker leaves and larger pieces from thinner ones.7 She can also be precise in her choice of leaf: workers tend to end up on the foliage they cut best. This is often based on their size, which is quite diverse in leafcutter workers, even among those who specialize in cutting leaves. The smallest cutters abandon foliage too tough for them to handle, while larger ones tend to depart from soft foliage after being pushed aside by bustling smaller ants. There’s even some evidence that larger cutters are drawn to recruitment trails that lead to the sturdier foliage.8
Unlike marauders and army ants, leafcutters always move their burdens individually and never need help.9 Because leaves are flat, large pieces have a small mass that a single ant can easily heft, and larger individuals slice and haul larger loads.10 But leafcutters take fragments just a few times their body weight at most. They are capable of hefting heavier fragments, so this may seem inefficient, but small pieces reduce congestion on the trails and inside the nest, speeding up the processing of foliage overall.11 Additionally, because workers carry the fragments vertically and high above their heads, on steep slopes the ants can become unbalanced, causing them to slow down, flip over, and even fall. The ants seem to anticipate when the route home will be uphill, and cut