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Why do hippopotamuses count as artiodactyls despite having four toes?

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reticulated giraffe — artiodactyl Reticulated giraffe (*Giraffa reticulata*), Kenya.

Hippopotamuses are classified as artiodactyls (even-toed ungulates) because they exhibit a paraxonic condition, meaning the axis of symmetry of their limbs passes between the third and fourth toes.1

While it is true that hippopotamuses have four toes on each foot, all of which touch the ground, the defining anatomical characteristic of the order Artiodactyla is not merely the number of toes, but the weight-bearing axis of the foot. In artiodactyls, this axis is positioned between the third and fourth digits. This contrasts with perissodactyls (odd-toed ungulates), where the axis of symmetry passes through the third (middle) toe.12

Therefore, despite having four toes, the hippopotamus maintains the structural limb alignment characteristic of the even-toed ungulate group.12