Maximum bite force is an important metric of feeding performance that defines the dietary ecology of many vertebrates. In mammals, theoretical analyses and empirical studies suggest a trade-off between maximum bite force and gape at behavioural and evolutionary scales; in vivo bite force is expected to decrease at wide gapes, and cranial morphologies that enable high mechanical advantage are thought to have a lower ability to generate high bite forces at wide gapes, and vice versa. However, very few studies have confirmed these relationships in free-ranging mammals. This study uses an ecologically diverse sample of bats to document the variation in bite force with respect to gape angle, and applies three-dimensional models of the feeding apparatus to identify the major morphological and biomechanical predictors of the gape-bite force relationship. In vivo and model data corroborated that bite force decreases significantly at wide gapes across species, but there is substantial intraspecific variation in the data obtained from live bats. Results from biomechanical models, analysed within a phylogenetic framework, revealed that species with larger temporalis muscles, higher temporalis stretch factors and high mechanical advantages experience a steeper reduction in bite force with increasing gape. These trends are illustrated by short-faced durophagous frugivores. The results from this study suggest that gape-mediated changes in bite force can be explained both by behavioural effects and cranial morphology, and that these links are relevant for functional analyses of mammal dietary ecology.