Trade-offs between force and fit: Extreme morphologies associated with feeding behavior in carabid beetles

We explored how functional trade-offs in resource handling strategies are associated with the divergent morphology of predators. The malacophagous carabid Damaster blaptoides shows two extreme morphologies in the forebody; there is an elongate small-headed type and a stout large-headed type. A feeding experiment showed that the small-headed type obtained a high feeding performance on snails with a thick shell and a large aperture by penetrating the shell with its head. In contrast, the large-headed type showed a high feeding performance on snails that had a thin shell and a small aperture, and they ate these prey by crushing the shell. The large-headed, strong-jawed beetles are efficient at shell crushing but are ineffective at shell entry; the large mandibles and musculature that allow for shell crushing make the beetle's head too wide to penetrate shell apertures. On the other hand, small-headed, weak-jawed beetles crush poorly but can reach into shells for direct predation on snail bodies. These findings are hypothesized to be functional trade-offs between force and fit due to morphological constraints. This trade-off would be a primary mechanism affecting both resource handling ability in animals and phenotypic diversity in predators and prey.