CAREER: Evolutionary biomechanics and functional morphology of salamander locomotion

How tetrapods, four footed animals, became terrestrial was a pivotal event in vertebrate evolution that set the stage for the diversification of tetrapods thereafter. The locomotor capabilities of early tetrapods are often modeled with extant salamanders since the latter have a generalized tetrapod body plan. Yet, salamanders exhibit tremendous morphological diversity across environments, providing a framework to assess the mechanical requirements for terrestrial locomotion by comparing morphological change across carefully matched evolutionary lineages.

The greater effects of gravity may impose biomechanical constraints that preclude certain salamanders from moving on land, but the habitat that salamanders occupy differs between developmental strategies. Metamorphosis involves the development of an animal across two or more distinct life stages but can be biphasic (aquatic larvae to terrestrial adults) or multi-phasic (aquatic larvae to terrestrial juveniles to aquatic adults) whereas direct development remains in one environment.

Thus, biomechanical constraints may be stronger in terrestrial direct developers than biphasic metamorphers since the former do not experience an aquatic stage. This project will integrate physiology, engineering, and evolutionary biology to examine how the interplay between habitat preference and developmental strategy affects the relationship between the structure and function of tissues (e.g., bones) and whole-organism performance (e.g., locomotion).

Students will receive research training through a new Course-Based Undergraduate Research Experience on Organismal Form and Function and Professional Research Experience for Post-baccalaureates in Biology program to broaden the participation of learners from historically excluded communities. In addition, “Salamander Safaris” will be hosted during Amphibian Week to promote the participation of girls in STEM.

Locomotion places some of the highest physical demands (‘loads’) on bones and failure to withstand loads could cause fractures or even death in an animal, yet how bones evolved to support the loads imposed by aquatic vs. terrestrial environments is not well understood. Phylogenetic comparisons of whole-bone mechanics across ecologically diverse species will advance knowledge of how habitat and developmental strategy has shaped the evolutionary morphology of salamander limb bones.

Investigators will quantify in vivo bone loading during terrestrial walking through synchronized 3D kinematics and kinetics. Investigators will then apply these loading data to collect the first dynamic measures of limb bone strength by integrating mechanical property testing and 3D digital image correlation. Finally, they will combine these techniques to examine how bone mechanics is affected by water-land and land-water transitions within a lifetime by comparing juveniles and adults from species with different developmental strategies (i.e., direct, biphasic, multiphasic).

Bone strength is predicted to be highest in terrestrial direct developers lowest in paedomorphic aquatic salamanders, and intermediate in biphasic metamorphic salamanders. Stronger bones likely assist terrestrial species to withstand internal (muscle) and external (ground reaction forces) loads and then transfer this energy into propulsion. Compared to the femur, the humerus is expected to evolve at faster rates based on the multi-functional role of forelimbs (e.g., digging, reproduction, locomotion) that is likely less constrained compared to hindlimbs whose primary role is for generating propulsion.

Findings from this work will contribute new insights into the mechanical requirements of becoming terrestrial.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.