Morphological and Biomechanical Adaptations of Salamanders Across Habitat Gradients
Open Access DepositedHabitat transitions expose animals to new physical demands that often promote morphological changes. Salamanders (Urodela) are a diverse order of amphibians, consisting of over 800 extant species with extensive habitat diversity that provide a useful system to study how habitat transitions shape morphological evolution. Salamander evolution has involved multiple transitions between water and land, with subsequent diversification into different terrestrial microhabitats. Many terrestrial lungless salamanders (Plethodontidae) occupy microhabitats that involve climbing on trees or rock structures. While some salamander lineages have morphologies that reflect adaptations for their habitats, the rest of the order displays a weak relationship between habitat and body shape. To better understand the factors that shape morphological evolution in salamanders, I investigate potential morphological and biomechanical adaptations to their locomotor system along water-land and terrestrial-climbing gradients. Aquatic and terrestrial environments impose conflicting demands on limb bones that promote increased buoyancy control and load bearing capacities, respectively. Some salamanders have complex life cycles with aquatic larvae that metamorphose into adults with different degrees of terrestriality. These species experience episodes of conflicting selective pressures that may constrain morphological evolution. In my first chapter, I show that the adults of aquatic species have denser limb bones, while those of terrestrial salamanders have stiff yet lightweight bones. The limb bones of multiphasic species evolved faster and were as morphologically diverse as those of solely aquatic and terrestrial species. My findings indicate that complex life cycles increase morphological diversity by promoting alternate evolutionary trajectories within the same environment. Climbing enables animals to exploit the vertical axis of their environment but exposes them to more pronounced gravitational forces, which could promote attachment mechanisms that prevent the animal from falling. However, climbing salamanders lack morphological adaptations typically found in other tetrapods, like claws or adhesive toe pads. In my second chapter, I show that some plethodontids (Aneides and Plethodon) that regularly climb have their own suite of morphological traits (e.g., longer limbs, larger feet, and toes suited for gripping) that might improve climbing abilities. This alternative suite of traits further supports that there is more than one way to be a climber. Additionally, there are many climbing salamanders that lack these traits and may rely more on behavioral adjustments to climb. In my third chapter, I show that four species of salamanders from my second chapter modify their limb kinematics in similar ways to climb on vertical inclines. However, climbing species with longer limbs and larger feet climb faster than those with shorter limbs and smaller feet. These results indicate that behavioral changes enable salamanders to climb without morphological adaptations, but morphologies that increase surface attachment enable exceptional climbing abilities. The data presented here highlight stronger relationships between habitat and morphology in salamanders than previously documented. However, the relationships appear strongest among closely related species, compared to the weak relationship across the whole order. Further studies are needed to investigate the ecomorphological trends within and across salamander clades to better inform our understanding of the factors that promote and constrain morphological evolution.
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