A first glimpse at the influence of body mass in the morphological integration of the limb long bones: an investigation in modern rhinoceroses

Abstract

The appendicular skeleton of tetrapods is a particularly integrated structure due to the shared developmental origin or similar functional constraints exerted on its elements. Among these constraints, body mass is considered strongly to influence its integration but its effect on shape covariation has rarely been addressed in mammals, especially in heavy taxa. Here, we propose to explore the covariation patterns of the long bones in heavy animals and their link to body mass. We investigate the five modern rhinoceros species, which display an important range of bodyweight. We used a 3D geometric morphometric approach to describe the shape covariation of the six bones composing the stylopodium and zeugopodium both among and within species. Our results indicate that the appendicular skeleton of modern rhinos is a strongly integrated structure. At the interspecific level, the shape covariation is roughly similar between all pairs of bones and mainly concerns the muscular insertions related to powerful flexion and extension movements. The forelimb integration appears higher and more related to body mass than that of the hind limb, suggesting a specialization for weight support. The integration of the stylopodium elements does not seem to relate to body mass in our sample, which suggests a greater effect of shared developmental factors. Conversely, the covariation of the zeugopodium bones seems more associated with body mass, particularly for the radius‐ulna pair. The fibula appears poorly integrated with other bones, especially within non‐Rhinoceros species, which may represent a case of parcellation due to a functional dissociation between the hind limb bones. The exploration of the integration patterns at the intraspecific level also highlights a more prominent effect of age over individual body mass on shape covariation within C. simum. This study lends support to previous hypotheses indicating a link between high body mass and high integration level.

Limb myology and muscle architecture of the Indian rhinocerosRhinoceros unicornisand the white rhinocerosCeratotherium simum(Mammalia: Rhinocerotidae)

“…The muscles of the forelimb had a total PCSA higher than those of the hindlimb in all our specimens, whereas in highly cursorial horses, the hindlimb has a higher total PCSA than the forelimb, although PCSA data are absent for four muscles of the horse forelimb (Tables 4, 5). All of these inferences are consistent with the higher degree of integration linked to mass observed between the bones of the forelimb in rhinoceroses, compared to those of the hindlimb (Mallet et al, 2020). The large PCSA shown by the muscles of the forelimb, required for body support, may drive the bones' shape towards similar adaptations (e.g.…”

“…The unusual form and function of rhinoceros limbs emphasise the need for a comprehensive anatomical study of their limb muscles, to better understand how their limbs sustain their large body weight. This would complement the extensive work recently performed on the morphology of rhinoceros limb bones (Mallet et al, 2019(Mallet et al, , 2020Mallet, 2020;Etienne et al, 2020). In terms of both qualitative myology and quantitative architecture, rhinoceroses have been poorly studied.…”

“…However, the two species present several differences, like a different body profile: C. simum has a low-hanging head whereas R. unicornis carries its head higher (Dinerstein, 2011). They also display notable differences in limb bone shape (Guérin, 1980;Mallet et al, 2019Mallet et al, , 2020Etienne et al, 2020), and they live in different habitats, C. simum preferring open flatlands while R. unicornis is found in semi-open floodplains, swimming easily (Dinerstein, 2011). C. simum usually displays size dimorphism, with males larger than females, whereas R. unicornis displays dimorphism only in captivity, not in the wild, although size dimorphism in rhinos is difficult to quantify (Dinerstein, 2011).…”

Phenotypic integration in feliform carnivores: Covariation patterns and disparity in hypercarnivores versus generalists

“…2018; Mallet et al. 2020) and vertebrae (Randau and Goswami 2017a,b; Jones et al. 2018; Arlegi et al.…”

The evolutionary biomechanics of locomotor function in giant land animals

“…2 ), rhinoceroses have much more compact, robust proximal limb bones (femur and humerus). Bakker (1971) , Christiansen and Paul (2001) and Paul and Christiansen (2000) added that it is the ‘flexed’ limb posture of rhinos that confers their speed, as compared with the ‘columnar’ posture of elephants (following the morphological logic of Osborn, 1900 ; for a new morphometric perspective, see Mallet et al, 2019 , 2020 ; also Etienne et al, 2021 ). There is little question that the limb posture of rhinos and elephants at top (or any) speed is different, but there is much left to be understood about the locomotion of rhinos.…”