3D Muscle Architecture of the Tongue of the White-bellied Pangolin (Phataginus tricuspis) Reveals a Muscular Hydrostat.

The tongues of pangolins are among the more derived of other mammalian ant-eating specialists, extending nearly a body length to capture food. Pangolins pack portions of their hyolingual apparatus in their thoracoabdominal cavity. These muscles are responsible for protruding, retracting, and bracing the tongue through a large range of motion akin to that of muscular hydrostats. Using DiceCT and 3D muscle architectural modeling of a cadaveric Phataginus tricuspis (White-bellied pangolin), we show how the arrangement of muscle fascicles in a section of the thoracoabdominal portion of the tongue converges on the structure of a hydrostat. Rostrally, the body of the tongue is occupied by m. sternoglossus, paired, parallel-fibered muscle bellies that run the longitudinal distance of the first 2/3 of the tongue. Upon entering the abdominal cavity, the body tethers to coiled, elongate xyphoid bones via m. xiphisternalis. Xiphisternal muscle fibers envelop the caudal portion of m. sternoglossus and anchor to the distal portions of the xiphoid bones and aid in retracting the tongue. The coiled nature of the xiphisternal bones and attachments of tongue muscles suggests an elastic mechanism may help propel tongue extension. The body of the tongue resides in a glossal tube, an extension of geniohyoid muscles built by layers of circular and longitudinal fibers that modestly, helically wind around the long axis of the tongue, also aiding in tongue protraction. Together, these muscles act as a hydrostat in promoting hyolingual movements in pangolins.