Supernumerary rectus capitis posterior muscle in Hylobates agilis

The neck is the junction between the head and thorax, and one of its primary functions is to maintain a balanced head. In animals that practice ricochetal brachiation, such as gibbons, head stability may be especially important because this energetic form of arboreal locomotion requires the rapid assessment of the location and stability of the superstrate. The gibbon cervical musculoskeletal system should be adapted to providing head stability. The suboccipital region was of particular interest as these muscles are significant contributors to head extension and rotation, both important aspects of primate head stabilization during locomotion. Brachiation in particular, involves much larger ranges of rotation than other forms of primate locomotion. Furthermore, this region has not been previously described in gibbons. The posterior neck and back of a single Hylobates agilis and one Symphalangus syndactylus were dissected. The H. agilis individual presented a supernumerary rectus capitis posterior muscle. The muscle originates from the spinous process of the second cervical vertebra and inserts on the inferior nuchal line of the occiput, similar to rectus capitis posterior major. The supernumerary muscle, however, is lateral and deep to rectus capitis posterior major and deep to obliquus capitis superior. The supernumerary muscle is similar in location to the rectus capitis posterior medius muscle described in cats. Supernumerary suboccipital muscles have also been described in humans. This muscle has not, however, been identified in any described nonhuman primate taxa including: macaques, baboons, and great apes nor was not present in the siamang. Thus, it is unclear if this aberration is an individual or species‐specific variation. The supernumerary muscle inserts more laterally than rectus capitis posterior major and this placement potentially offers greater mechanical advantage in head rotation when activated unilaterally. Such an advantage might offer greater or more efficient head stability during brachiation.