Body shape varies drastically across vertebrates, making it an effective trait to study when trying to understand macroevolutionary patterns of phenotypic variation. Body shape has been quantified in many ectothermic clades, but rarely in mammals. The goal of our research is to quantify body elongation in the family Sciuridae, as this area has been understudied thus far. Squirrels (Sciuridae) can be sorted into three distinct ecotypes based on life history and locomotion: ground, tree, and gliding. This leads to questions regarding differences in body shape between ecotypes in their respective environmental niches, and how differences in elongation correlate to different types of locomotion. We hypothesize that tree squirrels will be the most elongate, followed by ground squirrels, then gliding squirrels due to ecological and functional adaptations. To determine the potential differences in elongation between ecotypes, we will calculate the head-body elongation ratio (hbER) from skeletons held at natural history museums. We will use phylogenetic comparative methods to compare hbER between the three ecotypes. Thus far, our preliminary data shows both gliding and ground squirrels to have a statistically significant difference in hbER from tree squirrels. Tree squirrels are the most elongate, followed by ground, then gliding squirrels. We hope to further test differences between the hbER of ground and gliding squirrels with an increased sample size. Research on correlations between robustness and bone density in this clade is already underway, which will complement our results on elongation ratios between ecotypes.

Bats are the only mammals capable of powered flight and have correspondingly specialized body plans, apparent in the limbs. These specialized morphologies are thought to be the result of adaptations for the demands of flight; the skeletal elements of the bat forelimbs are elongated in order to support flight membranes and increase aerodynamic efficiency, whereas bat hind limbs are relatively short and specialized for hanging and catching prey in flight. Due to a deficient fossil record, the evolution of bat flight is still not fully understood but is hypothesized to be the result of an ancestral transition from gliding to flying. This hypothesis is plausible considering the morphological similarities between bat and glider forelimbs (both elongated) and the contrast between bat and glider hind limbs (shorter versus elongated). In this study, I collected linear measurements of the forelimb and hind limb skeletal elements of bats to add to a dataset of gliding, arboreal, and terrestrial mammals. I then fit evolutionary models to the data to test the hypothesis that A) selective pressures for flight drove the evolution of bat forelimb skeletal elements from glider-like forelimbs and that B) bat hind limbs evolved to become morphologically distinct from those of other mammals. Based on this hypothesis, I predict that A) bat and glider forelimb trait optima will fall progressively farther from arborealist optima and B) bat hind limb trait optima will be located in a unique region of morphospace. Preliminary results show that forelimb long bone lengths have evolved to be progressively longer from arborealists to gliders to flyers, supporting my hypothesis. This research helps address the longstanding question of how bats may have evolved flight from ancestral gliding mammals.

In vertebrates, differences in limb morphology are often the result of adaptions to locomotion. While previous researchers have examined the external shape of skeletal elements, there have been relatively fewer studies examining internal bone structure despite its potential significance to locomotor biomechanics. This study aims to help fill this gap by quantifying internal differences in forelimb skeletal morphology of squirrels (Sciuridae) across three locomotor ecologies: ground, tree, and gliding. We test the hypothesis that forelimb internal bone structure reflects adaptations to these ecotypes. To test our hypothesis, we micro-CT scanned the humeri of 61 species of squirrels and conducted bone structure analyses in the open-source software 3D Slicer. We assessed cortical bone composition by measuring material properties including global compactness (bone density), diaphysis (shaft) elongation, and second moment of area (bending ability). Based on biomechanical demands,we predict that A) gliders will have relatively less compact long bones with more elongated diaphyses due to the gravitational/aerodynamic constraints of gliding and B) ground squirrels will exhibit highly compact long bones with more robust diaphyses to gain more force while digging burrows​​. Preliminary results support our prediction that larger ground squirrels exhibit relatively more compact, robust, and bend-resistant humeri in accordance with their digging locomotion. This research furthers the understanding of diversity in forelimb morphology across mammals and the connection between forelimb morphology and locomotion. This study also lays the groundwork for future biomechanical and behavioral work to examine the evolutionary ties between form and function.