Macroevolution and Biomechanics of Digit Reduction in Horses

Abstract

Horses (family Equidae) are a charismatic group of animals and have captured human imagination for thousands of years. The swiftness and size of horses have made them central in transportation, agriculture, warfare, and art---but the earliest horses were small, leaf-eating, multi-toed creatures. The classic story of their evolution argues that to adapt to grasslands they evolved larger bodies, tall-crowned teeth, and reduced their side toes until a single large hoof remained. However, this linear pattern is oversimplified; recent work has overturned hypotheses about directional evolution in equid body size and tooth morphology. In this dissertation I challenge the directional story of why horses have only one toe, using the perspectives of broad-scale macroevolution and individual biomechanical performance to test the causes and consequences of digit reduction. In the Introduction, I review recent and historical work on digit reduction in horses, surveying the current status of the field, and then lay out a series of next steps that will be necessary to support---or reject---existing hypotheses for why digit reduction evolved. In Chapter 1, I combine engineering beam bending techniques with CT scans of fossil horse metapodials to assess the mechanical consequences of digit reduction; I also introduce the Toe Reduction Index, allowing us to quantify digit reduction in a continuous way for the first time. In Chapter 2, I use the extensive fossil record of horses to investigate how their speciation and extinction dynamics correlate with the diversity of other clades (such as their potential competitors, the artiodactyls) and with various abiotic factors (such as the spread of grasslands). In Chapter 3, I use extant Baird's tapir (Tapirus bairdii) as an analog for the earliest horses, with which it shares a semi-tetradactyl digit state; using high-speed videography, force data, and digit pressure data, I estimate the forces acting on the tapir metacarpals during a walking gait. These results can later provide estimates for forces that would have acted on the distal limbs of multi-toed extinct horses. Finally, in Chapter 4, I briefly explore the mode of digit evolution in horses, testing whether some optimum digit state was favored (e.g., was digit state being pulled toward monodactyly?) and how that may have been influenced by other factors such as diet. The process that gave us today's large-bodied, monodactyl horses was not as straightforward as is shown in many museum exhibits and textbooks. My results first complicate, then make inroads into answering, the question of why horses evolved a single toe. Using the complementary lenses of macroevolutionary and biomechanical approaches, I show how much is left to address in this fascinating evolutionary story.