Evolution of Tetrapod Limb Bone Design: Testing the Relationship Between Bone Loads and Locomotion
Richard W. Blob Clemson University
The limb bones of tetrapods show enormous diversity in shape. It has long been thought that these shape differences were closely related to differences in loads that animal limb bones have to support during locomotion. However, research on the relationship between limb bone loading and limb bone design in animals that use terrestrial locomotion has been dominated by studies of specialized mammals and birds, and the patterns observed in these species might not be true for all vertebrates. For example, some amphibians and reptiles use unconventional types of locomotion, such as jumping in frogs and slow walking in turtles. Studies of how these species deal with the loads placed on their limbs will dramatically expand knowledge of the factors that determine limb bone design. Such studies will provide critical tests of how the loads of locomotion relate to bone shape and the strength of bone material. Such studies will also test whether patterns seen in mammals and birds are evolutionarily primitive features of all vertebrates. This project will address these questions about the form, function and evolution of limb bones by comparing limb bone design with limb bone loads in species from five major groups of vertebrates (frogs, salamanders, turtles, lizards, and basal mammals) that use different forms of terrestrial locomotion (jumping, slow walking, and fast running). Three main types of data will be collected. (1) Forces, stresses, and strains on the femur in nine species (evaluated from surgically implanted strain gauges and force plate data synchronized with high-speed video of limb movements). (2) Mechanical properties (resistance to failure) of the femur in the same species. (3) External and internal measurements of femur shape from a broad range of vertebrate species. These studies will provide the first measurements of limb bone loads in most of these species. The comparative methods used in this project will explicitly analyze limb mechanics data in an evolutionary context. This is a novel approach that will test whether correlations between locomotor style and limb bone loads, shape, and mechanical properties represent new functional adaptations, rather than traits retained from primitive ancestors. These studies will provide tests of long-standing ideas about evolution of bone and locomotion, and will move the field toward comprehensive evolutionary explanations of limb bone properties. This work will enhance training and opportunities for several young scientists, including a postdoctoral researcher, undergraduate and graduate students, and two new faculty members (one from an exclusively undergraduate institution). Results from this work also will be incorporated into instruction for students at two colleges and into materials provided to local science teachers. This project will also provide data for clinical fields (e.g., orthopedics) by examining how the skeleton responds to varying environments and mechanical demands. Finally, by testing hypotheses about correlations between form and function in an evolutionary context, this research will provide a foundation for a broad range of future integrative studies of functional evolution.
