About project

This is work from my PhD in evolutionary biology at Harvard University.

Modern horses (the family Equidae) fall into just one genus, Equus, which includes domestic horses, donkeys, and zebras. They are quite large and have a single toe on each foot: the hoof. In contrast, the horse’s 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 the jerboas, hopping bipedal 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, I use 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. This work draws from both paleontological and neontological approaches.

Results

Selected papers

McHorse, B.K., A.A. Biewener, and S.E. Pierce. The evolution of a single toe in horses: causes, consequences, and the way forward. 2019. Integrative and Comparative Biology 59(3): 683-655.

Parker, A.K., B.K. McHorse¹, and S.E. Pierce. Niche modeling reveals lack of broad-scale habitat partitioning in extinct horses of North America. 2018. Palaeogeography, Palaeoclimatology, Palaeoecology 511: 103-118.

McHorse, B.K., S.E. Pierce, and A.A. Biewener. Mechanics of evolutionary digit reduction in fossil horses (Equidae). 2017. Proceedings of the Royal Society B: Biological Sciences 284(1861): 20171174.²

Selected media coverage

How horses got their hooves - Trilobites, New York Times
How horses lost their toes - Science News
How the horse became the only living animal with a single toe - The Guardian
Why horses evolved to have only one toe - Inverse Science
Why modern horses have only one toe - Science Magazine

Presentations

• McHorse, B.K., and S.E. Pierce. Diversity dynamics and digit reduction in fossil horses. Society of Vertebrate Paleontology Annual Meeting 2017, Calgary, AB, Canada. Finalist for Colbert Student Poster Prize.
  
• McHorse, B.K., S.E. Pierce, and A.A. Biewener. Beam mechanics of digit reduction in fossil horses. International Congress on Vertebrate Morphology, 2016.
  
• McHorse, B.K., A.A. Biewener, and S.E. Pierce. Evolutionary digit reduction and beam mechanics in fossil and extant horses. Society for Integrative and Comparative Biology, 2016.
  
• McHorse, B.K. and S.E. Pierce. Changing structural properties and morphology through evolutionary digit reduction in the Equidae (Perissodactyla). Society of Vertebrate Paleontology, 2015. Selected to participate in Colbert Student Poster Prize competition.