Waders (Scolopacidae) surviving despite malaligned leg fractures in the wild: kinematics of bipedal locomotion

Abstract Background Bone fracture frequencies and survival rates are essential parameters in skeleton evolution, but information on the functional consequences of naturally healed fractures is scarce. No leg bone fracture healing in the wild has been reported so far from long-legged Charadriiformes (waders), which depend on bipedal locomotion for feeding. Methods We documented a healed but malaligned tarsometatarsus fracture in a wild Willet (Tringa [Catoptrophorus] semipalmata), and a malaligned tibiotarsus fracture in a Curlew (Numenius arquata) skeleton from a museum collection. Functional consequences of the malalignments were evaluated by kinematic analyses of videos (Willet) and in silico 3D modeling (Curlew). Results The Willet’s left tarsometatarsus exhibited an angular malalignment of 70°, resulting in a limping gait that was less pronounced at high than at low walking speed. The bird seemed unable to club the toes of the left foot together, apparently a secondary effect of the deformity. The Curlew’s tibiotarsus showed an angular and an axial malalignment, causing the foot to rotate outwards when the intertarsal joint was flexed. Despite the severe effects of their injuries, the birds had survived at least long enough for the fractures to heal completely. Conclusions Somewhat unexpectedly, leg fractures are not necessarily fatal in long-legged waders, even if deformities occur in the healing process. Bipedal locomotion on vegetated grounds must have been impeded due to the bone malalignments in both analyzed cases. The birds probably alleviated the impact of their handicaps by shifting a larger proportion of their activities to vegetation-free habitats.