Horses can leap over high hurdles, gallop at speeds of up to 70 kilometers per hour and haul around up to nearly 1,000 kilograms of body weight — and all with just one big toe on each foot. Now, a new study published August 23 in Proceedings of the Royal Society B helps explain why: Streamlined digits improved horses’ strength and speed.
Along with zebras and donkeys, horses are among the few single-toed creatures in the animal kingdom. Scientists have long suspected that horses’ single, hoofed toes helped them run farther and faster over grasslands, letting them flee predators and find fresh forage. But the hypothesis that having one big toe is better than having several, biomechanically speaking, has never been directly tested.
“This study takes an important step” toward resolving why horses shed digits during their early evolution, says Karen Sears, an evolutionary biologist at UCLA.
Ancient horses had a lot of toes to lose. The dog-sized Hyracotherium, which lived about 55 million years ago, had four toes on its front feet, and three on its back feet. Merychippus, which lived about 10 million years ago, resembled a modern horse but had three toes, including one long middle digit with a protective, toenail-like hoof at the end. The only surviving horse genus, single-toed Equus, emerged about 5 million years ago.
“If you look closely, you can still see the vestigial remnants” of a bone that would have led to a side toe on a modern horse’s foot, says Brianna McHorse, a paleontologist at Harvard University.
To retrace the evolution of horse toes, McHorse and colleagues used CT scans to capture the internal structure of fossilized foot bones from 12 kinds of extinct horses. They also analyzed the feet of the closely related Central American tapir, which are oddly toed like Hyracotherium. A computer simulation then let researchers estimate how the bones would respond to the stresses of locomotion for each species, such as jumping over a hurdle or accelerating into a gallop. Then the scientists compared what happened when they applied the animal’s full body weight to just to the central toe, or spread it among multiple toes.
Side toes significantly increased the early horses’ ability to bear their own weight, the team found — the central toe of early horses would have fractured without help from other toes. As the era of modern horses approached and side toes dropped away, however, the middle toe bone grew thicker and hollower. These changes made the single-toed foot nearly as sturdy — resistant to bending and compression — as multiple toes.
As horses’ legs grew longer, the extra toes at the end of the limb would have been “like wearing weights around your ankles,” McHorse says. Shedding those toes could have helped early horses save energy, allowing them to travel farther and faster, she says. The study can’t determine what changes came first — whether bulking up the middle toe drove the loss of side toes, or the loss of side toes caused changes in the middle toe.
Horses aren’t the only animals to have lost toes or fingers to the evolutionary chopping block. “Digits have been lost many times in animals that walk, run, hop and fly,” says Kim Cooper, a biologist at the University of California, San Diego. Modeling how forces of locomotion act on an animal’s bones — living or extinct — could help scientists understand why.
Editor’s note:This story was updated August 28, 2017, to clarify the nature of the vestigial remains seenin modern horses. Karen Sears’ affiliation was also corrected. She is now at UCLA, not the University of Illinois at Urbana-Champaign; an update September 12, 2017, corrected the species of tapir included in the study.
FAQs
So how did horses end up with single-toed hooves? Over millions of years, many horse species lost most of their side toes. The middle toe evolved into a single large hoof, while the other toes became smaller and ultimately functionless. Only one species in this scene, the grazing Dinohippus, has a single hoof.
How did horses lose their toes? ›
As horses' legs grew longer, the extra toes at the end of the limb would have been “like wearing weights around your ankles,” McHorse says. Shedding those toes could have helped early horses save energy, allowing them to travel farther and faster, she says.
What does it mean when a horse toes out? ›
Horses that have toes that point outward (toed-out) are called splay-footed. These splay-footed horses travel with an inward hoof flight path referred to as winging or dishing in. Another structural deviation in the front legs is that in a horse that is base-narrow.
How does a horse lose its hoof? ›
Occasionally, a young foal will have a hoof stepped on by another horse and lose the hoof capsule. In some cases of laminitis, and other conditions causing loss of blood flow to the hoof, the hoof capsule may simply detach, become loose and fall off. This is a grave sign and usually necessitates euthanasia.
Do horse feet grow back? ›
Unfortunately, just like our own fingernails, the hoof wall is mostly composed of dead tissue, so it can't mend and heal. Instead, damaged sections have to be regrown and replaced, and even with the best supplement and farrier care, that's going to take time.
Why are horses destroyed when they break a leg? ›
Do you have to euthanize a horse if it breaks its leg? Often the only humane option after a horse breaks its leg is to euthanize it. This is because horses have heavy bodies and delicate legs, and broken leg bones are usually shattered making surgery and recovery impossible.
Do horses feel pain when their hooves are cut? ›
The external structures of the hoof don't have nerves or blood vessels, so correct trimming doesn't hurt the horse. Excessive trimming though, can be painful and cause complications, so leave it to an experienced professional farrier that you trust.
How to fix a horse that toes out? ›
This is accomplished by trimming the impact side (inside of foot) lower than the outside. In order to maintain the trimming performed, a straight bar shoe is often applied. Additional hoof wall at the heel is removed before applying the shoe so a space is created between the heel of the foot and the shoe.
What is horse knuckling? ›
“Knuckling” is used to describe a deformity of the leg of a horse caused by a contraction of the posterior tendon of the fetlock.
Why did horses have 4 toes? ›
The distant ancestors of modern horses had hooved toes instead of a single hoof, which vanished over time, according to researchers. The animals, such as the Eocene Hyracotherium, had feet like those of a modern tapir: four toes in front and three behind, each individually hooved with an underlying foot pad.
The most distal bone in the horse's leg is the coffin bone.
This critical bone has other names, such as distal phalanx, third phalanx, or even P3 for the abbreviation fans. The coffin bone is the hoof shaped bone that attaches to the laminae in the hoof.
Can a horse recover from losing a hoof? ›
Most hoof wounds respond to proper therapy and recover without a hoof wall defect. The time lapse between injury and definitive care is critical. The amount of hoof loss and the exposure of deep foot structures are less critical. Such cases should be handled as a high priority, if not on an emergency basis.
Can horses feel their hooves? ›
Do Horses Feel Pain in Their Hooves? Yes, horses have some nerves in their hooves that register pain. However, a horse's hooves are also very tough and resilient, so they can withstand a lot of wear and tear. And the outer portion of the hoof (the hoof wall) has no nerves, so it doesn't have sensation.
Why do horses no longer have toes? ›
How horses—whose ancestors were dog-sized animals with three or four toes—ended up with a single hoof has long been a matter of debate among scientists. Now, a new study suggests that as horses became larger, one big toe provided more resistance to bone stress than many smaller toes.
Why do wild horses not need shoes? ›
Wild horses amble long distances daily, usually over rough grassland, which gradually builds up hard hooves. Domestic horses usually grow weaker hooves because of intermittent exercise, often over softer, damper ground, and sometimes exacerbated by an unbalanced diet.
How did horses shave their hooves in the wild? ›
A domestic horse is unable to wear their hooves down as nature intended. Wild horses maintain their own hooves by moving many kilometres a day across a variety of surfaces. This keeps their hooves in good condition as the movement across abrasive surfaces wears ('trims') the hooves on a continual basis.
When did the three-toed horse go extinct? ›
Mesohippus, genus of extinct early and middle Oligocene horses (the Oligocene Epoch occurred from 33.9 to 23 million years ago) commonly found as fossils in the rocks of the Badlands region of South Dakota, U.S. Mesohippus was the first of the three-toed horses and, although only the size of a modern collie dog, was ...
Why did horses evolve to have single-toed hooves? ›
How horses—whose ancestors were dog-sized animals with three or four toes—ended up with a single hoof has long been a matter of debate among scientists. Now, a new study suggests that as horses became larger, one big toe provided more resistance to bone stress than many smaller toes.
Did the Dawn horse have toes? ›
Diet: Herbaceous plants (bushes, young tree shoots)
Its back was rounded and it may have had a striped coat for camouflage. It lived in the Old World and in North America. Eohippus had 4 toes on each front foot and 3 toes and a splint bone on the hind feet.
