Ecological Drivers of Carnivoran Body Shape Evolution

AbstractMorphological diversity is often attributed as adaptations to distinct ecologies. Although biologists have long hypothesized that distinct ecologies drive the evolution of body shape, these relationships are rarely tested across macroevolutionary scales in mammals. Here, I tested hypotheses that locomotor, hunting, and dietary ecologies influenced body shape evolution in carnivorans, a morphologically and ecologically diverse clade of mammals. I found that adaptive models with ecological trait regimes were poor predictors of carnivoran body shape and the underlying morphological components that contribute to body shape variation. Instead, the best-supported model exhibited clade-based evolutionary shifts, indicating that the complexity and variation of body shape landscape cannot be effectively captured by a priori ecological regimes. However, ecological adaptations of body shapes cannot be ruled out, as aquatic and terrestrial carnivorans exhibited opposite allometric patterns of body shape that may be driven by different gravitational constraints associated with these different environments. Similar to body size, body shape is a prominent feature of vertebrate morphology that may transcend one-to-one mapping relationships between morphology and ecological traits, enabling species with distinct body shapes to exploit similar resources and exhibit similar ecologies. Together, these results demonstrate that the multidimensionality of both body shape morphology and ecology makes it difficult to disentangle the complex relationship among morphological evolution, ecological diversity, and phylogeny across macroevolutionary scales.

Evolutionary and morphological patterns underlying carnivoran body shape diversity

The diversity of body shapes is one of the most prominent features of phenotypic variation in vertebrates. Biologists, however, still lack a full understanding of the underlying morphological components that contribute to its diversity, particularly in endothermic vertebrates such as mammals. In this study, hypotheses pertaining to the evolution of the cranial and axial components that contribute to the diversity of carnivoran body shapes were tested. Three trends were found in the evolution of carnivoran body shapes: (1) carnivorans exhibit diverse body shapes with intrafamilial variation predicted best by family clade age, (2) body shape is driven by strong allometric effects of body size where species become more elongate with decreasing size, and (3) the thoracic and lumbar regions and rib length contribute the most to body shape variation, albeit pathways differ between different families. These results reveal the morphological patterns that led to increased diversity in carnivoran body shapes and elucidate the similarities and dissimilarities that govern body shape diversity across vertebrates.

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

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.

Shared extremes by ectotherms and endotherms: Body elongation in mustelids is associated with small size and reduced limbs

An elongate body with reduced or absent limbs has evolved independently in many ectothermic vertebrate lineages. While much effort has been spent examining the morphological pathways to elongation in these clades, quantitative investigations into the evolution of elongation in endothermic clades are lacking. We quantified body shape in 61 musteloid mammals (red panda, skunks, raccoons, and weasels) using the head-body elongation ratio. We also examined the morphological changes that may underlie the evolution toward more extreme body plans. We found that a mustelid clade comprised of the subfamilies Helictidinae, Guloninae, Ictonychinae, Mustelinae, and Lutrinae exhibited an evolutionary transition toward more elongate bodies. Furthermore, we discovered that elongation of the body is associated with the evolution of other key traits such as a reduction in body size and a reduction in forelimb length but not hindlimb length. This relationship between body elongation and forelimb length has not previously been quantitatively established for mammals but is consistent with trends exhibited by ectothermic vertebrates and suggests a common pattern of trait covariance associated with body shape evolution. This study provides the framework for documenting body shapes across a wider range of mammalian clades to better understand the morphological changes influencing shape disparity across all vertebrates.

Motion analysis of non-model organisms using a hierarchical model: Influence of setup enclosure dimensions on gait parameters of Swinhoe's striped squirrels as a test case

In in-vivo motion analyses, data from a limited number of subjects and trials is used as proxy for locomotion properties of entire populations, yet the inherent hierarchy of the individual and population level is usually not accounted for. Despite the increasing availability of hierarchical model frameworks for statistical analyses, they have not been applied extensively to comparative motion analysis. As a case study for the use of hierarchical models, we analyzed locomotor parameters of four Swinhoe's striped squirrels. The small-bodied arboreal mammals exhibit brief bouts of rapid asymmetric gaits. Spatio-temporal parameters on runways with experimentally varied dimensions of the setup enclosure were compared to test for their potentially confounding effects. We applied principal component analysis to evaluate changes to the overall locomotor pattern. A common, non-hierarchical, pooled statistical analysis of the data revealed significant differences in some of the parameters depending on enclosure dimensions. In contrast, we used a hierarchical Bayesian generalized linear model (GLM) that considers subject specific differences and population effects to compare the effect of enclosure dimensions on the measured parameters and the principal components. None of the population effects were confirmed by the hierarchical GLM. The confounding effect of a single subject that deviates in its locomotor behavior is potentially bigger than the influence of the experimental variation in enclosure dimensions. Our findings justify the common practice of researchers to intuitively select an enclosure with dimensions assumed as "non-constraining". Hierarchical models can easily be designed to cope with limited sample size and bias introduced by deviating behavior of individuals. When limited data is available-a typical restriction of in-vivo motion analyses of non-model organisms-density distributions of the Bayesian GLM used here remain reliable and the hierarchical structure of the model optimally exploits all available information. We provide code to be adjusted to other research questions.