T oughness is crucial to the structural function of bone. Usually, the toughness of a material is not just determined by its composition, but by the ability of its microstructure to dissipate deformation energy without propagation of the crack1. Polymers are often able to dissipate energy by viscoplastic flow or the formation of non-connected microcracks2. In ceramics, well-known toughening mechanisms are based on crack ligament bridging and crack deflection3 . Interestingly, all these phenomena were identified in bone4–12, which is a composite of a fibrous polymer (collagen) and ceramic nanoparticles (carbonated hydroxyapatite)13–16. Here, we use controlled crack-extension experiments to explain the influence of fibre orientation on steering the various toughening mechanisms. We find that the fracture energy changes by two orders of magnitude depending on the collagen orientation, and the angle between collagen and crack propagation direction is decisive in switching between different toughening mechanisms.