Why does the third toe carry most of a perissodactyl’s body weight?

The third toe carries most of a perissodactyl’s body weight because the order is characterized by a mesaxonic limb structure, meaning the axis of symmetry for the limb passes directly through the third or middle digit.1

Throughout the evolutionary history of perissodactyls, this third digit became increasingly dominant, growing longer and thicker to support the animal's weight. As this digit assumed a more vertical position, the other toes were gradually reduced or lost, forcing the third toe to bear the primary load. In highly specialized forms, such as the modern Equidae, the second and fourth digits have been reduced to vestigial splints, leaving the third toe as the sole functional weight-bearing digit. This specialization allows for greater rigidity and efficiency in movement, which is essential for these animals.1