Biomechanical components of the plant-insect herbivore arms race: A model test in leaf-cutter ants

Insects and plants have been locked in an evolutionary arms race spanning
350 million years and whilst much work focuses on the chemical components
of this ancient contest, mechanical aspects remain largely understudied.
We test a simple biomechanical model that relates the force required to
cut thin leaf-like tissues to their mechanical properties, and the
geometry of the cutting tool. To remove confounding effects of tool shape
across size, we utilise leaf-cutter ant mandibles as a model system, who
are also both an economically and ecologically relevant pest species. We
measured the force required to cut both natural and pseudoleaves across
mandible sizes (mass) and wear states (using pristine vs worn mandibles)
using a custom built setup based on a fibre optic force sensor. To
quantify the geometry of the cutting edge we measured the mandible cutting
edge radius and wedge angle using focused ion-beam milling (FIB) and
scanning electron microscopy (SEM). The results substantially support the
model, enabling quantitative predictions. Fracture toughness is identified
as a key mechanical defence trait for plants, edge radius as the critical
geometric property of the insect mandible, and wear consequently emerges
as a key modulator of cutting-forces, elevating it up to 5-fold above a
physical minimum.