Femoral morphology of sciuromorph rodents in light of scaling and locomotor ecology

Abstract

Sciuromorph rodents are a monophyletic group comprising about 300 species with a body mass range spanning three orders of magnitude and various locomotor behaviors that we categorized into arboreal, fossorial and aerial. The purpose of this study was to investigate how the interplay of locomotor ecology and body mass affects the morphology of the sciuromorph locomotor apparatus. The most proximal skeletal element of the hind limb, i.e. the femur, was selected, because it was shown to reflect a functional signal in various mammalian taxa. We analyzed univariate traits (effective femoral length, various robustness variables and the in‐levers of the muscles attaching to the greater, third and lesser trochanters) as well as femoral shape, representing a multivariate trait. An ordinary least‐squares regression including 177 species was used to test for a significant interaction effect between body mass and locomotor ecology on the variables. Specifically, it tested whether the scaling patterns of the fossorial and aerial groups differ when compared with the arboreal, because the latter was identified as the ancestral sciuromorph condition via stochastic character mapping. We expected aerial species to display the highest trait values for a given body mass as well as the steepest slopes, followed by the arboreal and fossorial species along this order. An Ornstein–Uhlenbeck regression fitted to a phylogenetically pruned dataset of 140 species revealed the phylogenetic inertia to be very low in the univariate traits, hence justifying the utilization of standard regressions. These variables generally scaled close to isometry, suggesting that scaling adjustments might not have played a major role for most of the femoral features. Nevertheless, the low phylogenetic inertia indicates that the observed scaling patterns needed to be maintained during sciuromorph evolution. Significant interaction effects were discovered in the femoral length, the centroid size of the condyles, and the in‐levers of the greater and third trochanters. Additionally, adjustments in various femoral traits reflect the acquisitions of fossorial and aerial behaviors from arboreal ancestors. Using sciuromorphs as a focal clade, our findings exemplify the importance of statistically accounting for potential interaction effects of different environmental factors in studies relating morphology to ecology.

The evolution of femoral cross‐sectional properties in sciuromorph rodents: Influence of body mass and locomotor ecology

“…This might indicate that forces acting in any direction increase disproportionately with increasing body mass, necessitating adjustments in the femoral CSP. Furthermore, as Wölfer et al () found isometric scaling for the size of the femoral head in Sciuromorpha, we assume that apposition of compact bone within but not outside of the femoral head is playing a major role in increasing robustness in this region. Concordantly, Mielke et al () discovered positive allometry in the proportion of trabecular volume of overall bone volume in the femoral head in Sciuromorpha.…”

“…It should be noted that at approximately 20% femoral length, cross‐sections already represent the diaphysis instead of these two trochanters in some larger aerial species as the one displayed in Figure . This can be explained by the femoral length scaling with 0.39 in relation to body mass in the aerial group, whereas the distances of the third and lesser trochanters from the center of the femoral head (i.e., the in‐levers of the attaching muscles) only scale with 0.35 and 0.36, respectively (Wölfer et al, ). In comparison, all three variables scale with 0.36 in the arboreal group, whereas the lesser trochanter is located even more distal along the midshaft in the fossorial group (its distance scales with 0.36, the femoral length and the distance of the third trochanter with 0.29 and 0.30, respectively; Wölfer et al, ).…”

“…Figure depicts the phylogenetic tree used in this study. According to Wölfer et al (), the fossorial lifestyle has evolved three times independently and the most recent common ancestor most likely had an arboreal lifestyle. The clade of all aerial species (Pteromyini) is monophyletic and descended from an arboreal ancestor (Rocha, Leite, Costa, & Rojas, ; Steppan, Storz, & Hoffmann, ).…”

“…Afterward, it was tested whether the residuals contain a phylogenetic signal. We took the fully dichotomous molecular phylogenetic tree displayed in figure S2 from Wölfer et al () that is based on the phylogeny provided by Zelditch et al () and complemented with species from the TimeTree database (Kumar et al, ; see supplementary material in Wölfer et al, for more details). This tree was pruned so that it only contained the species of our data set.…”

“…In these differing ecological niches, several preferentially exhibited locomotor behaviors have evolved (termed “lifestyles” in this study). Following the argumentation of Wölfer et al (), we categorized the lifestyles of sciuromorph taxa as arboreal, fossorial, or aerial (i.e., gliding; see Section 2). Table provides a summary of the species and corresponding lifestyle categories used in this study.…”