Where Did Horses’ 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.
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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

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Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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