Hydration- and scale-dependent fracture energy in hierarchical biological composites

• Fracture resistance in biological composites is governed by hydration- and scale-dependent energy dissipation rather than a single intrinsic toughness value. • Hydration controls which crack-tip dissipation mechanisms are activated, thereby shaping the physical meaning of measured fracture resistance. • An energetic framework reconciles disparate fracture measurements across biological materials and length scales. • Experimental trade-offs between hydration fidelity, crack acuity, and site specificity constrain access to biologically relevant fracture energy. Hierarchical biological composites achieve exceptional damage tolerance through a combination of structural organization and environmentally activated dissipation mechanisms. Fracture toughness is widely used to quantify this performance, yet its physical meaning in biological materials is often obscured by their strong sensitivity to hydration and length scale. In these systems, water modifies molecular mobility, interfacial interactions, and microstructural deformation, thereby controlling which energy dissipation mechanisms are activated during crack advance. Here, we critically examine fracture mechanics studies across biological and engineered materials to show that hydration and length scale fundamentally govern the physical meaning of measured fracture resistance. As a consequence, fracture cannot be interpreted solely through stress-based metrics but is more appropriately understood as an energy-controlled process whose magnitude and physical origin depend on both hydration state and structural scale. We propose that fracture resistance in biological composites should be interpreted through hydration- and scale-dependent fracture energy rather than as a single intrinsic material constant. Using arthropod exoskeletons as a representative system, we examine how hydration governs crack-tip dissipation, process-zone development, and the apparent form of measured fracture resistance. We further show that existing fracture measurements predominantly probe hydration extremes, leaving intermediate, functionally relevant states poorly constrained. Finally, we discuss experimental trade-offs between hydration fidelity at the crack tip, crack acuity, and structural site specificity that limit access to biologically relevant dissipation mechanisms across length scales.