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Why Horses and Their Ilk Are the Only One-Toed Animals Still Standing

Early horses had 15 toes, but life on the plains led to a stronger center toe, leading to life on four hooves

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smithsonian.com

Animals in the genus Equus, which includes zebras, horses and donkeys, have an unusual claim to fame: They are the only living group of animals with just one toe. But this wasn't always the case. The group's dog-sized ancestors actually had four toes on their front feet and three on their back. Why did they lose their digits? As Nicola Davis at The Guardian reports, researchers may finally have an answer.

In the study, published in the journal The Proceedings of the Royal Society B, Harvard researchers used micro-CT scans to create detailed 3-D images of animal feet. They examined the long bones in the feet of 12 extinct horse species each from a different genus, as well as the bones from a tapir, a large pig-like South American species with four toes on its front feet and three in the back. 

They then used that data to model how much stress those bones experience during running and jumping. How did the center toe handle the body weight? How did the side toes distribute the force?

What they found is that the extra toes in early horse ancestors were necessary; without the toes present to distribute the weight, the stress from running and jumping would be enough to fracture the big toe bone. As horses grew larger, however, the architecture of the big toe bone changed.

"As body mass increased, and side toes shrunk, the middle digit compensated by changing its internal geometry, allowing ever-bigger horse species to eventually stand and move on one toe," Harvard evolutionary biologist and co-author Stephanie Pierce says in a press release. The bone of the digit carrying the load moved further from the center of the foot and grew thicker, helping it to resist bending and breakage.

This finding backs up to the latest theories on why horses grew larger and lost their toes. As Jen Viegas at Seeker reports, 55 million years ago the horse’s earliest ancestor was the dog-sized genus Hyracotherium, which roamed the forests of North America and had four front and three back toes for each limb. The creature lost a front toe due to natural selection, creating a hoof-like claw that likely had a pad on the bottom. As the climate changed, opening vast grasslands in the region, early horses moved onto the plains, with selective pressure leading to a larger body mass. By about five million years ago, this shift led to the strengthening of the center toe and the loss of the outer digits.

This single toe likely helped the animals move faster and more efficiently, according to the lead author of the study Brianna McHorse (yes, that is her real name). "It is very energetically expensive to have a bunch of toes on the end of that leg," McHorse tells Davis. "If you get rid of them then it costs less, energetically speaking, to swing that leg for every step."

Robin Bendrey, a zooarchaeologist from the University of Edinburgh, who was not involved in the study, supports the analysis. "[The study] makes a major contribution to explaining a major evolutionary adaptative trend of the family Equidae and one which ultimately produced an animal that has been so influential in human history," he tells Davis.

Despite its usefulness for horses, having one toe is still evolutionarily unusual. Some species of a type of prehistoric animal that evolved in South America known as liptoterns also lost their digits and developed a horse-like uni-toe. But these creatures have long been extinct, leaving the horse and its ilk the only remaining one-toed critters on Earth. 

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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