The influence of seed structure and strength on their destruction by granivores is central to understanding the dynamics of granivore-plant interactions. For up to nine seed species, the effects of seed size (cm3), mass (mg), density (mg/cm3) and coat strength (MPa) on the damage inflicted by three post-dispersal granivores (Harpalus pensylvanicus, Anisodactylus sanctaecrucis, and Gryllus pennsylvanicus) were evaluated. Seed destruction rates by G. pennsylvanicus were statistically unrelated to the size and toughness of the seeds. Seed densities significantly affected their destruction by A. sanctaecrucis and H. pensylvanicus, as did seed size, mass, and strength in H. pensylvanicus under choice conditions. The carabid beetles destroyed more of the small, denser seeds with stronger seed coats. The results show that different granivores are able to distinguish the structural strength and physical density of seeds as well as seed size. The relative ability of granivores to detect these seed characteristics offers a way in which diverse communities of post-dispersal insect granivores can persist within a single habitat. The authors redefine how the strength of biological structures should be evaluated in ecological studies, using guidelines commonplace in the field of engineering.