Abstract
Strigiformes are an order of raptorial birds consisting exclusively of owls: the Tytonidae (barn owls) and the Strigidae (true owls), united by a suite of adaptations aiding a keen predatory lifestyle, including robust hind limb elements modified for grip strength. To assess variation in hind limb morphology, we analysed how the dimensions of the major hind limb elements in subfossil and modern species scaled with body mass. Comparing hind limb element length, midshaft width, and robusticity index (RI: ratio of midshaft width to maximum length) to body mass revealed that femoral and tibiotarsal width scale with isometry, whilst length scales with negative allometry, and close to elastic similarity in the tibiotarsus. In contrast, tarsometatarsus width shows strong positive allometry with body mass, whilst length shows strong negative allometry. Furthermore, the tarsometatarsi RI scales allometrically to mass0.028, whilst a weak relationship exists in femora (mass0.004) and tibiotarsi (mass0.004). Our results suggest that tarsometatarsi play a more substantial functional role than tibiotarsi and femora. Given the scaling relationship between tarsometatarsal width and robusticity to body mass, it may be possible to infer the body mass of prehistoric owls by analysing tarsometatarsi, an element that is frequently preserved in the fossil record of owls.
Highlights
Strigiformes are an order of raptorial birds consisting exclusively of owls: the Tytonidae and the Strigidae, united by a suite of adaptations aiding a keen predatory lifestyle, including robust hind limb elements modified for grip strength
Comparing hind limb element length, midshaft width, and robusticity index (RI: ratio of midshaft width to maximum length) to body mass revealed that femoral and tibiotarsal width scale with isometry, whilst length scales with negative allometry, and close to elastic similarity in the tibiotarsus
In the case of Strigiformes, we find a differing scaling relationship for each element of the hind limb
Summary
Strigiformes are an order of raptorial birds consisting exclusively of owls: the Tytonidae (barn owls) and the Strigidae (true owls), united by a suite of adaptations aiding a keen predatory lifestyle, including robust hind limb elements modified for grip strength. The Tytonidae are characterized by large heads with a heart-shaped facial disc, round ear openings, and generally have long legs and powerful feet; barn owls may be distinguished from true owls by their sternum, which has a broad carina that becomes slightly narrower ventrally, and the ventral edge of which has slight emarginations on either side[4]. The Strigidae, are characterized by large, round crania with more variable outer ear shape, strong, hooked bills, a round facial disc, a compact body shape, and strong feet with sharp talons; the true owl sternum is narrower dorsally and has two deep emarginations on either side of its ventral edge[4]. Strigiformes have a digit II claw that is larger than digit III and can be distinguished as birds of prey from this character alone[11,15,16,17,18]; they have shorter, more robust toes relative to other raptors, on digits III and IV, in addition to a slightly lower inner claw curvature; they can rotate digit IV in order for digits II and III to oppose digit I and digit IV, creating a functionally zygodactyl foot[11,12]
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