One of the key aspects of species biology is locomotion, which determines many important behavioural activities such as foraging, hunting, escaping from predators, or migrating [1–3]. Therefore, the study of locomotor adaptations in living and extinct species is crucial to understanding their role in present and past ecosystems [4, 5].
Natural selection has led to morphological adaptations in the postcranial skeleton, which have been largely treated in the literature as “ecomorphological indicators” of locomotion modes in living species. Thus, several studies on locomotor evolution in mammals have used limb indicators of ecological adaptations to determine paleobiological aspects in extinct species [6–14]. However, natural selection is not always the only factor in shaping morphological traits [15–17]. It is important to investigate the effects of different potential sources of variation prior to identifying possible ecomorphological correlates, such as phylogenetic inheritance [15–17] or allometry [18–23].
This study investigated the influence of phylogeny, allometry, and locomotor behaviour in shaping the morphology of the hind limb bones (i.e., femur, tibia, and pelvic girdle bones) in mammalian carnivores (extant and extinct taxa from the order Carnivora plus some taxa from the closely-related order Creodonta). We used mammalian fissiped carnivorans (i.e., a paraphyletic group that includes members of the mammalian order Carnivora exclusive of members of the clade Pinnipedia, which were excluded due to their highly aquatic specialization) as a model system for the following reasons: (i) their mode of locomotion is remarkably diverse, including arboreal, terrestrial, and semiaquatic modes [24–28]; (ii) they have a different hunting styles, including pursuing, pouncing, ambushing, or hunting [9, 29–36]; and (iii) their phylogenetic relationships are well characterised .
This article forms part of a wider study on the ecomorphology and evolution of the appendicular skeleton in the order Carnivora with a particular focus on the influence of various factors in shaping the fore- and hind limb bones. We complement the analysis of the forelimb  by studying the evolution of the hind limb. This study will therefore lead to a complete picture of the morphological evolution of all major limb bones of the carnivoran appendicular skeleton as a whole.
Our predictive hypothesis was that there would be many similarities between the evolution of the bone shape of the fore- and hind limbs. However, as these limbs have several functional differences and anatomical peculiarities, we also predicted that there would be some differences in their patterns of evolution. For example, it has been demonstrated that the forelimbs of domestic dogs support a greater proportion of body weight than the hind limbs [39, 40] and this could be the case for all fissiped carnivorans. If this supposition were correct, it would be reasonable to assume that allometry has less effect on the hind limb bones than on the forelimb bones. Furthermore, hind limbs are thought to be more important in providing impulse during acceleration and running than the forelimbs [39, 41, 42] and therefore locomotor behaviour could have a stronger influence on shaping the hind limb than the forelimb. On the other hand, many carnivoran species use their forelimbs for activities other than the ones involved in locomotion, such as grasping, climbing, or manipulating prey [28, 33, 43] and this could also be a potential source of morphological differences between the fore- and hind limbs.
We used 3D geometric morphometrics to characterize the morphology of the hind limb bones (i.e., femur, tibia and the pelvic girdle bones) in order to answer the following questions: i) Is there an allometric effect in shaping the morphology of the hind limb bones; (ii) Is there a phylogenetic signal in all hind limb bones? (iii) Is there an association between locomotor behaviour and the shape of these bones? (iv) What are the evolutionary pathways followed by the hind limb long bones? (v) Is the evolutionary pattern of the hind limb similar to that of the forelimb? (vi) Does the appendicular skeleton – fore- and hind limbs – reflect functional and ecological convergences similar to the way the craniodental skeleton reflects them?