Functional cervicothoracic boundary modified by anatomical shifts in the neck of giraffes.

Megu Gunji,Hideki Endo

doi:10.1098/rsos.150604

Abstract

Here we examined the kinematic function of the morpho- logically unique first thoracic vertebra in giraffes. The first thoracic vertebra of the giraffe displayed similar shape to the seventh cervical vertebra in general ruminants. The flexion experiment using giraffe carcasses demonstrated that the first thoracic vertebra exhibited a higher dorsoventral mobility than other thoracic vertebrae. Despite the presence of costovertebral joints, restriction in the intervertebral movement imposed by ribs is minimized around the first thoracic vertebra by subtle changes of the articular system between the vertebra and ribs. The attachment area of musculus longus colli, mainly responsible for ventral flexion of the neck, is partly shifted posteriorly in the giraffe so that the force generated by muscles is exerted on the cervical vertebrae and on the first thoracic vertebra. These anatomical modifications allow the first thoracic vertebra to adopt the kinematic function of a cervical vertebra in giraffes. The novel movable articulation in the thorax functions as a fulcrum of neck movement and results in a large displacement of reachable space in the cranial end of the neck. The unique first thoracic vertebra in giraffes provides higher flexibility to the neck and may provide advantages for high browsing and/or male competition behaviours specific to giraffes.

Highlights

The mammalian vertebral column comprises successive units organized into five series that are highly conserved and recognized: cervical, thoracic, lumbar, sacral and caudal
Our results showed that the decreasing point of the vertebral mobility was shifted posteriorly in giraffes, and that the intermediate mobility was observed in T1 of the giraffe and C7 of okapi
In the light of the above evidence obtained from previous genetic studies, we considered the possibility that the mutations of Hox5 and/or Hox6 specific to the primaxial domain would have resulted in the transformations of the musculoskeletal structure in the neck of giraffes

Summary

Introduction

The mammalian vertebral column comprises successive units organized into five series that are highly conserved and recognized: cervical, thoracic, lumbar, sacral and caudal. The number of cervical vertebrae has remained constant at seven for at. Least 200 Myr, despite variable counts in other regions of the mammalian vertebral column and in the 2 cervical vertebrae of other vertebrate classes [1,2,3]. The long neck of giraffes follows the cervical constraint; each cervical vertebra is prominently longer than that of the common short-necked ruminants, including okapi [4,5]

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Discussion

Conclusion