Mammals in the equine family are the only one-toed animals still in existence

The ancestors of animals in the Equus genus (i.e. horses, donkeys, and zebras) had four toes on their front feet and three on the back. But as horses shifted from “forest to grassland environments,” the animals evolved to have just one toe, according to The Guardian.

In a 2017 study, scientists at Harvard University found that as the horse’s body mass increased over time, “the central toe became larger and more robust, allowing it to withstand greater bending forces and expend less energy as they walked.”

The length of horses’ legs also evolved to be longer, allowing them to travel greater distances to seek resources.

Brianna McHorse, the lead author of that study, told The Guardian, “it is very energetically expensive to have a bunch of toes on the end of that leg. If you get rid of them then it costs less, energetically speaking, to swing that leg for every step.”