Tetrapod Functional Morphology and Secondary Aquatic Invasions
The invasion of land by early tetrapods is arguably one of the most profound events in vertebrate history. However, equally profound but less often studied is the return of terrestrial vertebrates to highly or exclusively aquatic lifestyles. This secondary shift back to the water is often accompanied by characteristic morphological changes, including the flattening of the shaft of the long bones in the limb. Such morphological changes have largely been attributed to intuitive changes in loading regime between land and water, with reduced body support demands in aquatic habitats. However, actual load magnitudes on land versus in water have not been quantified until recently. My work investigates potential mechanisms for how flattening of the limb long bones may come about by collecting limb bone strain data from turtles during walking and swimming. I use turtles as a model system due to the presence of their body shell, which prevents axial bending. This ensures that limb bones loads are the result of muscular and environmental forces and are not confounded by bending along the animal's spine.
Femoral Loading: Strain data from the femur of semi-aquatic Trachemys scripta (slider turtle; Family: Emydidae) confirm the expected reduction in load magnitudes during swimming compared to terrestrial walking. In addition, torsional (twisting) loads on the femur were also significantly reduced during swimming compared to walking, which may imply a potential mechanism for achieving a flattened shape. Whereas bones that are round in cross-section resist twisting well, flattened bones do not. However, in an environment in which torsional loads are not prevalent, animals may be released from the need to maintain a round cross-section, thus permitting the shift to a more hydrodynamically advantageous flattened shape.
Humeral Loading: Data from the humerus of the semi-aquatic Pseudemys concinna (river cooter turtle; Family: Emydidae) show similar patterns to those observed in the slider femur discussed above. These data indicate that changes in terrestrial and aquatic loading regime are associated more with a reduction in overall strain magnitude rather than changes in load orientation (change in torsional loads). Thus, despite similar reductions in shear strain between terrestrial and aquatic locomotion, these reductions are produced by different mechanisms.
Retention of Swimming Kinematics: Specialized performance in a particular habitat often comes at the cost of high performance in contrasting habitats. However, retention of ancestral performance ability in specialist species has been largely unexplored. Despite descending from aquatic ancestors, some turtle lineages have transitioned to exclusively terrestrial habits. Two of these include species in the genus Terrepene (box turtles) and the family Testudinidae (tortoises). Although both of these taxa have evolved terrestrial lifestyles, tortoises have been terrestrial for a much longer period of time, 50 million years, compared to box turtles, which diverged from aquatic ancestors only 5 million years ago. Comparison of swimming kinematic variables of the forelimb and hindlimb of tortoises (Testudo horsfieldii) and box turtles (Terrepene carolina) to two semi-aquatic emydid turtles (Trachemys scripta and Chrysemys picta) indicates functional convergence of the forelimb in in box turtles and tortoises, but the box turtle hindlimb retains ancestral swimming motions.
Phylogenetic Limb-Bone Scaling: Although strain data from turtle humeri and femora indicate a potential mechanism for how flattened limb bone shapes may be achieved, such a mechanism is difficult to assess without considering bone morphology across the functional spectrum exhibited in turtles (including terrestrial walkers, aquatic rowers, and marine flappers/underwater flight). In order to address this topic, I am comparing the size and shape relationships of turtle limb bones among four families of functionally diverse turtles in the context of their evolutionary relationships. I collected data from skeletal specimens housed at the American Museum of Natural History, the Carnegie Museum of Natural History, the Florida Museum of Natural History, the Smithsonian National Museum of Natural History, and the Chelonian Research Institute. Analyses are on-going.
Student Opportunities in the Young Lab
Many of my projects involve mentoring undergraduate students with an interest in research. These students are involved with many aspects of projects, from project development and construction of equipment to data collection and analysis to manuscript development and publication. Many students involved in projects with me have the opportunity to present their work at conferences and/or co-author scientific publications.
Thermal Acclimation in a Semi-Aquatic Salamander
A ubiquitous challenge for ectotherms is dealing with changes in temperature, both spatially and temporally. Such variation may negatively impact organismal fitness unless individuals are capable of responding to such changes; these responses may include behavioral and physiological responses. Theory predicts that highly variable thermal environments should lead to reduced thermal sensitivity in ectotherms, resulting in greater acclimation capacity. To test this theory, I exposed salamanders of the species Desmognathus brimleyorum (Ouachita dusky salamander; Family: Plethodontidae) to constant and variable thermal conditions in the lab. Following an 8-week acclimation period, I then tested thermal sensitivity of metabolic rate and swimming performance across a range of temperatures. Swimming performance did not show a significant difference between treatments (see figure at left showing individual performance curves in black and mean performance in red), however a limited degree of acclimation of metabolic rate was observed.
Interestingly, individuals responded to treatments differently depending on which source population they originated from. Furthermore, tail morphology appeared to differ between populations, though sample sizes were too small to statistically confirm this difference. I would like to revisit this system to further investigate the extent of morphological diversity between populations and to determine what environmental elements may be driving these differences.