One-Toed Wonder: Why Horses Rely On Only One Digit

by | 08.27.2017 | 8:40am

How the horse evolved from the multi-toed animal of ancient times to the one-toed, hooved equine we are familiar with today is being investigated by scientists, and their findings are intriguing. While the earliest equids were the size of dogs, they had three toes on their front legs and four toes behind; it has been determined that the pressures of increasing body weight and shrinking side toes forced horse's middle toes to become stronger and better able to withstand force, reports HorseTalk.

Scientists from Massachusetts' Harvard University notes that many textbooks state that the cause for the reduction in toes was because of the takeover of grasslands from forests, which caused both an increase in equine size and a reduction in digits. But the scientists pondered: What was the mechanical consequence from standing on one toe and when did the side toes cease to have a purpose?

To get answers, the researchers performed micro-CT scans of 12 fossil horse species, and then used engineering “beam bending” analysis to calculate how much stress each species' lower leg bones were experiencing during normal movement and high-speed running.

The stress data were then compared to the fracture stress of bone. Bone stress was assessed in two ways: One way was to assume body weight load on the center digit (which is what modern horses experience) and the second was to scale the stress load relative to the size of the side toes, assuming that the larger the toes were, the more weight they took off the central digit.

The research results shows that for early horses, the side bones were critical and bore some of the animal's weight to combat high levels of stress on the main digit, potentially avoiding bone fracture.

As the horses grew in size and weight, the side toes shrunk and the middle digit changed its internal geometry, allowing the horses to eventually stand on one toe. The bone of the load-bearing digit was eventually moved farther from the center of its cross section, which allowed it to resist bending even more. As horses evolved, they also gained more bone, helping to resist compression as well.

Evolving from multiple digits all the way to one is unusual; horses are the only living single-toed animal. Even though they have one digit on which they stand, they can complete many athletic endeavors, including running, jumping and endurance riding, among others.

Read more at HorseTalk.


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