The mastication system of the giant anteater (Myrmecophaga tridactyla) and the southern tamandua (Tamandua tetradactyla) were functional-morphologically compared by three dimensional image analysis. The anteaters medially and laterally rotate the mandibular bones to control the elongated tongue and to house the relaxing tongue. From the three-dimensional CT images of the two species, we point out that the shape and size of oral cavity can be changed by the rotated mandibular bones. The volume of the oral cavity can be bilaterally widened by the medial bending of the dorsal part of both mandibles. The masseter muscle, the superficial temporal muscle and the lateral pterygoid muscle act as main motors of the mandible rotation. These muscles contribute to the medio-lateral movements of the mandibular bones in the two species. The digastric muscle consisting of a short and thin bundle is vestigial. The dorso-ventrally lower body of mandible and the incomplete zygomatic arch avoid the collision of the mandible with the cranium during the rotation movement. The two species are commonly equipped with the elongated skull and the derived mastication mechanism. We suggest that the distribution pattern of the mastication muscles has not drastically changed in the evolutionary history within Vermilingua.