Meeting Abstract
Digit reduction is common in vertebrates, from theropods to marsupials. Extant horses (Equidae) are the most extreme living example, with a single digit on each leg. Although the evolution of a single toe is remarkable, the selective driver has rarely been quantitatively tested. The structural modifications necessary for high-performance locomotion on a single digit are also under-explored. Our work combines ecological, biomechanical, and phylogenetic comparative methods to investigate the evolution of digit reduction from multiple angles. The loss of side toes in horses has long been posited as a response to the spread of grasslands in the Miocene. Our work modeling niche overlap and diversification dynamics shows a more complicated story, though: at broad temporal and spatial scales, three-toed and single-toed horses are not found in significantly different habitats, yet speciation and extinction rates across equid lineages do correlate with environmental shifts. Furthermore, lineages with reduced digits show lower extinction rates, suggesting some adaptive role. On an individual level, we have also demonstrated how positive allometry in the center digit can compensate for loss of important weight-bearing function in the side digits, which may have been a response to increasing body size or pressure for improved locomotor economy. Integrating methods from multiple fields has begun to reveal the complexity of digit reduction in equids, lending support to some historical hypotheses (such as the influence of body size) and rejecting others (such as habitat partitioning at a broad scale). Data from individual disciplines can answer small pieces of the evolutionary puzzle of digit reduction, but a holistic answer requires synthesizing data—both big and small—from multiple fields.
