Three-dimensional CT image analysis of the digging system in the aardvark.

We examined the bone movement in the forepaw and hind paw in the aardvark (Orycteropus afer) by using three-dimensional (3D)-computed tomography (CT) techniques and osteometrical methods to confirm the functional adaptation of the extremities as a digging system. The four metacarpal bones could be strongly bent from the distal carpal bones. The distal end of the second and third metacarpal bones possessed enlarged smooth articulation surfaces that allowed the proximal phalanx to bend at a sharp angle. However, the articulation surface was not well-developed in the distal end of the fourth and fifth metacarpal bones and the proximal phalanx could bend at smaller angle in these two lateral digits. The proximal phalanges sharply crook from the metatarsal in the first, second, third and fourth digits in the hind paw. We suggest that the medial two digits in the forepaw directly contribute to the crushing, when these proximal phalanges crook in the phase of power stroke. In contrast the lateral third and fourth digits may act as sweeper of the crushed soil. These suggestions regarding the different functional adaptation between medial two digits and lateral two digits are consistent with the anatomical data of the forearm musculature. In the hind paw, we suggest that the second, third and fourth digits are functionally similar and that the hind paw may not act as a crushing apparatus but as a running motor or soil-sweeper similarly using these main three digits.