The extent to which the different molar morphologies of two similar-sized mammals, Petaurus breviceps (sugar glider) and Dasyuroides byrnei (kowari), could affect the digestion of larval insect nitrogen was investigated. Nitrogen digestibility of Tenebrio larvae treated with a proteolytic enzyme (trypsin) increased with decreasing larval piece size. However, it was found that the molars of P. breviceps, with their minimal shearing capacity compared with D. byrnei, can only compress rather than finely comminute Tenebrio larvae. As a result, P. breviceps have less efficient nitrogen digestibility than D. byrnei whose shearing molars can finely comminute larvae. Petaurus breviceps molars are inefficient at breaking down insects possibly because of functional constraints, where the molars must perform several different tasks with competing morphological demands, or because of phylogenetic constraints.