Today's horse is the product of millions of years of evolution, especially in the "toe" department. The oldest equines had five digits, and as the species evolved horses gradually dropped their digit number down to four, three, and then just one. Like their ancient ancestors, modern horses have the genes for five toes. But by the time they’re born, today's equids are down to one toe per foot—the hoof. So where do the extra toes go?

Recent research results suggest that, while modern horses are still partially “programmed” to create five toes in each foot, those four extra toes either don't develop fully or essentially disappear during fetal development.

“We think that the positions of the digits are correct, so digit three (the middle finger) knows what it is and where it's supposed to be,” said Kimberly Cooper, PhD, assistant professor in the department of biological sciences at the University of California, San Diego. “But only the proximal remnants of the second and fourth even initiate. Digits one and five are completely lost.”

In other words, the horse's genetic code still instructs the embryo to create a total of 20 toes (five in each foot) in the early stages of embryonic development. But for reasons researchers still don’t understand—possibly that there “just aren't enough cells to even begin to make those toes,” Cooper said—the first and fifth digits never even start to grow. Meanwhile, the second and fourth toes do start to grow in the unborn horse, but through a process known as apoptosis (or regulated cell death), these two toes are “terminated” and form the splint bones that fuse to the back of the cannon bone, Cooper explained.

Why would evolution do that, you ask? After all, cheetahs still have five digits on their feet, and they run faster than any other land mammal. But Cooper explained that such a speed dynamic wouldn't be advantageous for the horse. As a predator, the cheetah needs all its claws to attack its prey—“Cats would be pretty pathetic hunters if they only had one claw,” she said. But the price for that speed is limited distance: the cheetah overheats very quickly.

Large hooved animals—“ungulates”—are prey animals that eat grass. The extra digits serve them no purpose and would only slow them down over longer distances. “The ungulates elongated their limbs (over time) to increase their stride length so they cover more ground at each stroke,” Cooper said. “And with that longer limb comes a need to decrease the mass located further from the body so that it takes less energy to redirect the swing (from back to front, for instance). Not only is there selection for all these animals to lose digits; they’ve also mostly lost muscle in their hands and feet—muscle that's used for grasping in other animals. But a horse has no need to hold on to anything.”

Still, despite these evolutionary improvements, the old programming sometimes still wins out. On rare occasions, the second and/or fourth digits don’t turn into splint bones during fetal development; in these cases the horse is born with extra hooves, or something smaller that might look like a claw.

“It's my understanding that when any resemblance of an extra toe appears, the material is surgically removed and that animal isn't used for breeding stock,” Cooper said. “But I'd love to get my hands on some DNA from a few of these horses!”

It’s very likely these horses have genetic differences that are visible simply by noninvasive genome sequencing, which could help explain how toes are evolutionarily lost, she said.

Cooper encouraged owners of horses born with extra digits to contact her at kcooper@ucsd.edu.

The study, "Patterning and post-patterning modes of evolutionary digit loss in mammals," was published in Nature.