Networks of cross-linked and bundled actin filaments are ubiquitous in the cellular
cytoskeleton, but their elasticity remains poorly understood. We show that these
networks exhibit exceptional elastic behavior that reflects the mechanical properties of individual filaments. There are two distinct regimes of elasticity, one
reflectingbendingof single filaments and a second reflectingstretchingof entropic
fluctuations of filament length. The mechanical stiffness can vary by several decades with small changes in cross-link concentration, and can increase markedly
upon application of external stress. We parameterize the full range of behavior in
a state diagram and elucidate its origin with a robust model.