Modern horses (family Equidae) are represented by just one large-bodied and single-toed genus, yet their earliest ancestors were dog-sized animals with three or four toes on each foot. Reducing the number of toes, or digit reduction, can be found in lineages as distantly related as theropod dinosaurs and hopping desert rodents. But why? Through their evolutionary history, different species of horses can be found far-flung in geography, living in habitats from open grassland to forested hills, increasing or decreasing in body size, grazing or browsing, reducing digits or not reducing digits, and representing many possible combinations of the above variables. To understand how and why horses reduced their toes, we are using the densely sampled evolutionary history of the Equidae to investigate questions at two scales: first, at the small and detailed scale of individual morphology and biomechanics, and second, at the sweeping scale of whole speciation and extinction events that occur over continents and through millions of years. Specifically, we are investigating this classic evolutionary story by combining a unique suite of experimental (e.g. locomotor dynamics in living perissodactyls) and musculoskeletal modeling approaches (e.g. limb loading via finite element analysis), with a detailed macroevolutionary study of the patterns and processes underlying equid morphological evolution.
Tapir locomotion data collection at the Franklin Park Zoo. Tapirs are living perissodactyls that retain three digits on their forefeet and four digits on their hind-feet, thus making them an excellent modern analogue to investigate the biomechanics of digit reduction. A - The ever so cute Abby the tapir. B - Tapir runway set-up. Images copyright Brianna McHorse.