Manus and pes range from plantigrade to digitigrade. Their method of locomotion ranged from plantigrade to digitigrade. A plantigrade foot is the primitive condition for mammals; digitigrade and unguligrade locomotion evolved later. Most birds, except loons and grebes, are digitigrade, not plantigrade. They are quadrupedal and, when on the ground, they walk digitigrade and plantigrade. Whereas the aardvark is considered digitigrade, it appears at time to be plantigrade. All canids are digitigrade, meaning they walk on their toes. In contrast, digitigrade animals walk on their distal and intermediate phalanges. Indohyus is a genus of extinct digitigrade artiodactyl known from Eocene fossils in Asia. It ran digitigrade, and upright on its hind legs. Foxes are digitigrade, and thus, walk on their toes. Animal species such as cats that walk on their toes are described as being "digitigrade". The agouti is fleet-footed and antelope-like, being digitigrade and having hoof-like nails. A digitigrade () is an animal that stands or walks on its digits, or toes. Vortigaunts have mottled green skin and digitigrade legs, allowing them to move quickly.