Animals

This Is How Horses Turned Their Toes Into Hooves

New research helps to explain how horses evolved to become the only living animals with just one toe per limb.

Brianna McHorse is a Harvard doctoral student who has always been interested in the animal mentioned in her family’s surname.

“I’m fascinated by the incredible athletic performance of modern horses, and as a paleontologist, I'm interested in how skeletal shape influences the way an animal moves in life, and how that changes through time,” McHorse, who has ridden horses for many years and works in the labs of Stephanie Pierce and Andrew Biewener at Harvard’s Department of Organismic and Evolutionary Biology, told Seeker.

“Combining those [interests] with the incredibly rich fossil record of horses, plus their status as a classic evolutionary story, offers a fruitful area of study that hasn't received very much quantitative attention yet,” she added.

That area of study has now received attention, as McHorse and her colleagues have just conducted the most extensive investigation to date on how movement and load-bearing stresses acted on early horse ancestors as they evolved to become the world’s only living monodactyl, or single-toed animal.

The findings, published in the journal Proceedings of the Royal Society B, suggest that, at least for horses, having just one sturdy toe per limb — hooves are technically toes — can be better than having many toes, as humans and numerous other animals do.

McHorse notes that the early ancestors of horses had four toes on each front limb and three toes on each back limb. At this time in horse history, roughly 55 million years ago, such animals like those in the genus Hyracotherium were about the size of a small dog and lived in forests that covered much of North America.

It has long been known that changing climatic conditions allowed grasslands to expand. Selective pressures resulting from the new open terrains then drove increases in the body mass of horse ancestors and caused them to lose all but one toe per limb. Many questions have remained, however, such as what the underlying mechanical consequences were of standing on just a single toe.

To help answer these questions, McHorse, Pierce, and Biewener performed micro-CT scans of 12 fossil species in the horse family tree. The scientists then used an engineered “beam bending” analysis to calculate how much stress each species’ lower leg bones were experiencing during regular movement and high-speed running. The stress data were then compared to the fracture stress of bone.

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The researchers now believe that, as horses evolved, they soon lost their fourth toes on their front limbs, leaving them with three toes on each limb. Each was “not quite a hoof, but not quite a claw either,” McHorse said, adding that the toes were “more like what a living hyrax has — sort of thick, modified nails.”

Hyracotherium probably had some sort of pad under each foot, and would have had a less upright foot posture than living horses, which essentially stand on tip-toe all of the time.

“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,” Pierce explained.

“The bone within the load-bearing digit of later horses was distributed farther away from the center of its cross-section, allowing it to better resist bending,” she continued. “The total amount of bone also increased, allowing it to better resist compression as well as bending, which are of critical importance for animals with large body sizes.”

Horses first evolved into monodactyls at least five million years ago, and there were trends toward significant digit reduction in the lineage before then. As a result, humans didn’t influence these anatomical changes in horses, according to the researchers.

“I believe cave art depicting horses has been dated to around 30,000 years ago, but that would have been long before actual domestication,” McHorse said.

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Ancestors of other modern animals, such as deer, were subject to similar climate and environmental changes as the early horses. Today’s deer and other hoofed animals, however, have two toes per limb instead of just one. Their anatomy and weight-bearing needs placed the axis of symmetry per limb right between their two digits, as opposed to the arrangement in horses, where the axis of symmetry now runs right down the middle of the single digit.

“That means to maintain symmetry, you wind up with two toes,” McHorse said, explaining why cows, pigs, sheep, antelopes, deer and other animals have this number of digits.

While horses are the world’s only living monodactyl, they were not the only animals to evolve only one toe per limb. A now-extinct group of South American animals called Prototheriidae evolved an even smaller single toe per limb than horses today have. These animals were horse-like in their build and foot structure, but were not closely related to horses. The similarities between them and horses were just “a remarkable example of convergent evolution,” McHorse said.

She still marvels at horses, which can run, jump, and quickly dodge sideways — all on one toe per limb. As she and her team demonstrate, horses took the common evolutionary theme of digit reduction and literally ran with it.

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