Our horses’ ancestors had four toes in front and three in back. Over the generations, they dropped to the modern-day single toe that ends in the hoof. We’ve known this for a while. But it’s only recently that scientists have finally understood why.
A single-toe format “outweighed the costs” of multiple-toe formats as horses gained body mass and grew longer legs, said Brianna K. McHorse, a PhD candidate in the Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology at Harvard University, in Cambridge, Massachusetts.
“There was some selective pressure, or set of pressures, that conspired to push (some species of) horses in the direction of having a single toe,” she said.
Specifically, that pressure was to reduce bone stress in the legs as horses became heavier and to reduce the extra toes’ drag effect on a long limb. As the middle toe lengthened to become the cannon bone, the toes around it probably didn’t bear much weight anymore. But they would have had their own weight—and resistance, not being very “aerodynamic.”
“The benefits of a single toe obviously outweighed the costs in the ancestors of today’s horse species, including zebra and wild asses,” McHorse said.
In their study, McHorse and colleagues studied possible biomechanics models of the way ancient horses would have moved, starting with the dog-sized Hyracotherium genus (early Eocene, 55.8–33.9 million years ago) with its 14 toes total. Altogether, they examined the internal bone geometry of 12 fossil horse genera, covering the whole realm of evolutionary changes in horse feet and body sizes, all the way to modern-day Equus. They also tested how locomotor bone stresses changed across the evolutionary line. And they invented a new measurement method to evaluate load-bearing in the side toes—what they called the “toe reduction index” (TRI).
They focused their models on two primary speeds or gaits—a trotting gait (or equivalent) and high-speed acceleration or jumping, McHorse said. They found that, particularly in the higher speeds or jumping, most of the fossil bones would have endured significantly higher bone stress than modern horses if the side toes didn’t carry some of the weight. But starting with Hipparion and its relatives, which had already reduced their side toes considerably, bone stress became similar to what it is in modern horses, even without the side toes carrying some weight.
The side toes would have been useful in earlier horse genera, partly for stability, she said. For example, Hyracotherium had a TRI of 0.89 on a scale of 0 to 1—meaning it probably bore a substantial amount of weight on the side toes. But about 20 million years later, Parahippus (early Miocene, 23.03 to 5.3 million years ago) had a TRI of 0.61. That might have been because Parahippus lived on North American grasslands where it needed to cover a lot of ground every day. The extra toe drag would have been disadvantageous in that case. In Equus, which has no side toes, the TRI is 0.
“Our results support the hypothesis that increasing body mass was a potential driver of digit reduction because a single, robust digit could resist the increased bending forces better than three smaller digits of the same total bone mass,” the researchers stated in their report.
However, other factors, such as the effects of side toes on inertia and speed, probably also played a role, they added.
This doesn’t mean horses evolved in a way that’s superior to other animals. It just means this evolution worked for them in their particular situation. “It’s generally not a good idea to think of anything as ‘more evolved’ or ‘evolutionarily superior’,” McHorse said. “Evolution just happens.”
It also doesn’t mean it’s created a flawless solution. “Evolution is no guarantee of being foolproof,” she said. “In fact, this (among plenty of other examples!) points to exactly what I’ve said previously, that evolution doesn’t work toward some perfect ideal.”
But knowing how they evolved could help scientists and veterinarians work toward better management of horses’ imperfections, said McHorse. “Hopefully as we learn more about how and why they evolved their unique structure, we can have more insight into modern issues such as lameness,” she said.
The study, “Mechanics of evolutionary digit reduction in fossil horses (Equidae),” was published in the Proceedings: Biological Sciences.