Shell damage and mandible mechanics in the ant Messor wasmanni

Ant middens represent accumulations of discarded materials and provide insight into colony-level foraging and processing behavior. While middens of granivorous ants are typically dominated by plant material, animal remains are occasionally present, yet their origin and mechanical basis remain poorly understood. Here, we investigated the occurrence of gastropod shells in an external midden of the harvester ant Messor wasmanni and evaluated ant mandible mechanics to provide some background for shell fracturing. Midden material was collected during three sampling events and sorted by material type and size. Snail shells were identified to either genus or species level, fracture locations were documented, and representative shells were fractured experimentally using major worker mandibles to quantify shell-breaking forces. The forces varied widely depending on snail species, growth stage, and fracture location, with juvenile shells and mechanically weak regions fracturing at substantially lower forces than adult shells or their reinforced regions. Nanoindentation revealed strong regional and caste-specific differences in mandible mechanical properties, with highest hardness and Young's modulus at the masticatory margin of major workers. Elemental analyses showed enrichment of zinc and other metals in cutting edges, consistent with enhanced wear resistance. This study integrates ecological, mechanical, and material data to provide a mechanistic framework for interpreting snail shell occurrence in ant middens.