A maladaptive intermediate form: a strong trade-off revealed by hybrids between two forms of a snail-feeding beetle

Although trade-off curves between fitness components are essential in theoretical studies of ecological specialization, few empirical studies have actually determined these curves experimentally. Using the snail-feeding carabid beetle Damaster blaptoides, which is endemic to the Japanese archipelago, we estimated the trade-off curve for feeding success with alternative foraging behaviors that are linked to varying morphology. First, we crossed a stout-bodied and a slender-bodied subspecies and produced their F-1 and backcross hybrids, which exhibited intermediate body shapes. Then we compared the snail-feeding success of these beetles. Stout beetles could eat small snails by crushing shells, whereas slender beetles could eat large snails by inserting their heads into shells. Although hybrids with intermediate body shapes attempted to employ both strategies, they frequently failed at both. The relationship between feeding success rate and beetle body shape was represented by an inward bending curve, which implies a strong trade-off that can cause disruptive selection, leading to ecological specialization. We suggest that the intermediately shaped beetles were maladapted for snail-feeding and that disruptive selection may have played an important role in the morphological divergence of these beetles.