You might've known that horses are ungulates, or hoofed mammals. But did you know that those hooves are considered toes? Yeah, each horse leg has one big toe, carrying all that weight — and now, scientists officially know why.
Horses' single-digit hooves make them a bit unusual, because while all land-based vertebrates have a five-toed lineage, many of them "lost" toes due to evolution. Millions of years ago, horses were small and had three or four toes. But over time, horses needed to be able to move faster to escape predators and find food. Now, horses — along with zebras and donkeys — have a sole, lonely digit on each leg.
In the past, researchers have posited that the single, big toe helps with weight distribution and allows for easier running. But in 2017, researchers at Harvard University finally put the hypothesis to the test. In their study, they confirmed that that having one strong, thick toe helped carry the weight of the horse as it evolved into the galloping equine it is today.
The researchers scanned fossils of the foot bones of 12 extinct horse types, ranging from ones 55 million years old to species in Equus, the genus that contains all modern horses. Then they created computer simulations that let them test how the horses' bones would react to jumping, running and general frolicking when the weight is spread amongst toes or centered on one toe.
The study authors found that while side toes were useful for bearing weight and preventing the main toe from fracturing when horses were smaller, the central toe eventually became more hollow and thick, which made it just as stable as multiple toes. Eventually, those side toes "dropped" away, shedding unnecessary weight from horses' slender, elongated legs — and the horses were free to traverse greater distances with more speed and energy.