How elephants evolved a sixth toeS

While an elephant's foot may seem to be a cylinder, it's actually a fully pentadactyl cluster of toes, smooshed together into a small space. If you look at a skeleton of their appendages, an elephant is resting its immense weight on five digits, with a soft, fleshy pad rounding out the rear of the foot. It's this mound of flesh at the back that's proven hard to understand, as it supports a significant chunk of an elephant's weight without the advantage of a bone structure.

What new research has revealed is that elephants have co-opted another part of their hands and feet into the structural role of bones. Elephants' sesamoid bones, attached to the back of their big toes, have evolved into a "sixth toe." Over the course of an elephant's life, the sixth toe starts as cartilage, and converts to bone years after the rest of the animal's bones have hardened.

The researchers' reconstructions show that these "predigits" are not just passive bearers of weight, but also partly retain the ability to move and assist with motion. When elephants ancestors moved from water to land, the shift transformed their stance from flat-footed to tip-toed, necessitating the evolution of these not-quite-digits which serve to make these creatures stable - without requiring even more massive legs.

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