The idea that horses have more than one toe may seem far-fetched.
But that is what a new study, released in the journal "Royal Society Open Science," is proposing: modern horses have not one toe, but five.
It is a big change from the previously held belief, and the story with which all equestrians are familiar: horses, zebras and other equines started out with a greater number of digits, but lost them through millions of years of evolution until they ended up with one single toe ending in a hoof.
As lead author Nikos Solounias, a professor at the New York Institute of Technology, told AFP:
"We provide evidence that the 'missing' digits are in fact still present. All five digits have merged to form the compact forelimb with hooves we know today."
He compared this phenomenon of extra toes to a tulip that never opens. While the digits aren't visible to the naked eye, bones, fossils, and arteries in embryos of ancient horses tell a different story, revealing traces of the toes that were thought to have vanished, morphed into a single digit.
Some scientists have acknowledged that small splints on the outer edge of the modern horse's metacarpal are remnants of the second and fourth toes, but that the first and fifth have completely disappeared. However, the study argues that ridges on the back of the splint bones correspond to these outside toes.
The researchers also followed the gradual change of the equine limb over 55 million years of evolution, showing that the digits had merged together, and studied dissections of fetal and adult horses, where they discovered a neurovascular network consistent with five digits, not one.
"If there are five fingers, there should be 10 primary nerves and 10 arteries—exactly what we found," Solounias said. "We are suggesting a new paradigm where horse limb evolution is formed by re-shaping, not by loss."
Do you think horses still have five toes? Share your thoughts below.
All photos via Phys.Org/NYITCOM
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