A bony growth in elephants' feet is a sixth "toe" that helps the world's heaviest land mammal keep its balance, scientists said on Friday.
The growth protruding from the back of elephant's feet was discovered in the 18th century when a Scottish surgeon dissected one of the creatures for the first time.
Researchers had been baffled by the piece of bone but a new study in the US journal Science claims to have solved the mystery.
While not a true toe, the growth has developed the same function as a toe -- giving the behemoths much-needed help in supporting their colossal weight as they lumber across African plains and through Asian jungles.
"It is performing the function of a toe in supporting the elephant's weight," lead author Professor John Hutchinson, of Britain's Royal Veterinary College, told BBC radio.
"It is a little weird piece of bone that has been elongated during evolution into quite a long piece of bone."
The researchers used techniques including X-rays, dissection and an electron microscopy in their study.
The elephant's five conventional toes point forwards but the extra "toe" points backwards to give extra support.
A similar phenomenon can also be observed in pandas and moles, according to scientists. A panda's "sixth finger" helps it to grip bamboo, while the extra digit helps moles to dig.
The researchers studied elephant fossils to discover when the bone first appeared and believe it was around 40 million years ago, as elephants got bigger and became more land-based.