DOI: 10.5061/dryad.r2280gbqh
title: Biomechanical components of the plant-insect herbivore arms race: A model test in leaf-cutter ants

description: 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.
description: # Biomechanical components of the plant-insect herbivore arms race: A
 model test in leaf-cutter ants Dataset DOI:
 [10.5061/dryad.r2280gbqh](10.5061/dryad.r2280gbqh) ## Description of the
 data and file structure We have submitted all data presented within the
 manuscript, including the average steady-state cutting, fracture, and
 spacing forces for the pseudoleaf force dataset (Pseudoleaf_forces.csv).
 The thickness corrected average steady state cutting, fracture, and
 spacing forces for the natural leaf force dataset (Leaves_forces.csv). The
 mandible cutting edge radius measurements were taken at varying
 percentages of total tooth height, measured in ImageJ
 (FIB_SEM_Radius.csv), and the mandible cutting edge wedge angle was also
 taken at each tooth height obtained in ImageJ (FIB_SEM_Angle.csv). ###
 Files and variables #### File: FIB\_SEM\_Radius.csv **Description:** 
 Cutting-edge radius and wedge angle were obtained using focused ion-beam
 milling (FIB) and scanning electron microscopy (SEM) Zeiss Auriga FIB-SEM
 at 30KV acceleration voltage and 240pA current. Trapezoidal cuts were
 milled on the second tooth of each mandible (counted from distal).
 Subsequent analysis was completed in ImageJ. Cutting edge radius was
 defined as the radius of the smallest circle tangential to the two lines
 that still fits within the cross section.  ##### Variables * Mass (mg):
 Ant worker mass the mandible was obtained from * Age: Callow (pristine) or
 Forager (worn) * Location (%): Tooth height  * Radius (nm): Mandible
 cutting edge radius #### File: FIB\_SEM\_Angle.csv **Description:** 
 Cutting edge radius and wedge angle were obtained using focused ion-beam
 milling (FIB) and scanning electron microscopy (SEM) Zeiss Auriga FIB-SEM
 at 30KV acceleration voltage and 240pA current. Trapezoidal cuts were
 milled on the second tooth of each mandible (counted from distal).
 Subsequent analysis was completed in ImageJ. Wedge angle was defined by
 placing two lines along the straight part of the cutting edge, and then
 measured utilising the inbuilt angle tool. ##### Variables * Mass
 (mg): Ant worker mass the mandible was obtained from * Age: Callow
 (pristine) or Forager (worn) * Location (%): Tooth height  * Angle
 (degrees): Mandible cutting edge wedge angle #### File: Leaves\_forces.csv
 **Description:**  Mandibular cutting forces were measured with a
 custom-built fibre-optic force setup. For each measurement the average
 steady-state cutting force was taken over a 2mm region. Forces presented
 here have been corrected for leaf laminar thickness.  ##### Variables *
 Name: Mandible ID * Species: Ant species (Atta cephalotes) * Mass_mg:
 Worker body mass (mg) * Age: Days after eclosion * Age_min: Days after
 eclosion * Age_class: Callow (pristine, c) or Forager (worn, f) * Colony:
 Laboratory colony * Side: Side of head capsule * Plant: Plant type *
 Cutting_force_mN: Average cutting force (1st pass, mN)  *
 Friction_force_mN: Average friction (spacing) force (2nd pass, mN) ####
 File: Pseudoleaf\_forces.csv **Description:**  Mandibular cutting forces
 were measured with a custom-built fibre-optic force setup. For each
 measurement the average steady-state cutting force was taken over a 2mm
 region. ##### Variables * Name: Mandible ID * Species: Ant species (Atta
 cephalotes) * Mass_mg: Worker body mass (mg) * Age: Days after eclosion *
 Age_min: Days after eclosion * Age_class: Callow (pristine, c) or Forager
 (worn, f), pseudoleaf thickness (200 - 200 micron, 400 - 400 micron) *
 Colony: Laboratory colony * Side: Side of head capsule *
 Cutting_force_mN_10to1: Average cutting force for 10:1 Pseudoleaves (1st
 pass, mN) * Friction_force_mN_10to1: Average friction (spacing) force for
 10:1 Pseudoleaves (2nd pass, mN) * Cutting_force_mN_4to1: Average cutting
 force for 4:1 Pseudoleaves (1st pass, mN) *
 Friction_force_mN_4to1: Average friction (spacing) force for 4:1
 Pseudoleaves (2nd pass, mN)