Biomechanics of raptorial dorsiflexion and tensile material properties of the m. tibialis cranialis tendon in the hindlimbs of hawks and owls.

Skeletal morphology and tendon properties reveal the underlying systems that facilitate movement and, ultimately, behavior in animals. Of all terrestrial vertebrates, birds are the most diverse in terms of species. Birds often interact with their prey via the hindlimbs, making these portions of the body rich in ecomorphological information. The m. tibialis cranialis is the primary dorsiflexor of the ankle, and in raptorial birds, this muscle critically aids in both prey acquisition and transport. Here, we assessed the biomechanical implications of tarsometatarsal morphology and the tensile properties of the m. tibialis cranialis tendon among accipitrids and strigids to assess potential differences in prey acquisition strategies and load capacities. Examination of tarsometatarsal morphology reveals significantly more robust and much more distally positioned insertion points for the m. tibialis cranialis in strigids, resulting in comparatively increased mechanical advantage. Though acquisition strategies among focal raptorial birds differed, results of tensile properties indicate no significant differences in mass-specific load-carrying capabilities. However, results suggest that strigids only require approximately half the torque required by accipitrids to lift a given resistance, even when adjusting for mass categories. The elastic modulus of m. tibialis cranialis tendons was low across examined birds, ranging from 17.2 to 128 MPa. Our central conclusions are that tarsometatarsal morphology indicates differences in mechanical lever arm advantage between the two groups, and that hindlimb tendons of arboreal birds of prey are relatively elastic, with significantly lower moduli than those of cursorial birds and mammals, suggesting functional roles in shock absorption and strain energy storage during lifting and transport of prey.