The giraffe (Giraffa camelopardalis) cervical vertebral column: a heuristic example in understanding evolutionary processes?

Abstract

The current study considers the osteological morphology of the giraffe (Giraffa camelopardalis) vertebral column, with emphasis on evaluating both the adaptive and constraining features compared with other ungulates as a heuristic example in understanding evolutionary processes. Vertebral columns of giraffes varying in age from calf to adult were studied in order to understand the potential evolutionary scenarios that might have led to the modern phenotype. Data from the giraffe sample were then compared with the results from several other ungulate species, including the okapi and two species of camelids that also have visibly elongated necks. Our results show that the elongated neck of the modern giraffe appears to specifically result from evolutionary changes affecting the seven cervical vertebrae, independent of the remainder of the vertebral column. The cervical vertebrae comprise over half of the length of the total vertebral column in the giraffe. The increases in cervical vertebrae lengths also appear to be allometrically constrained, with alterations in the overall length of the neck resulting from the elongation of the entire cervical series, rather than from a single vertebra or subset of vertebrae. We place our results in the context of hypotheses concerning the origin and evolution of the giraffe neck, and the evolution of long necks in a broader sense.

From fetus to adult—an allometric analysis of the giraffe vertebral column

“…The study of these relationships is known as allometry (Huxley, '32). Badlangana et al (2009) compared the giraffe allometrically to other ungulates and concluded that the long neck is a result of the elongation of the entire cervical series, independent of the rest of the vertebral column, thereby supporting Lankester's ('08) view. They also proposed a punctuated evolutionary scenario by which the long neck in modern giraffes could have developed over a brief period in geological time through the caudal shifting of specific Hox genes in the presomatic mesoderm.…”

“…A second criterion is the origins of the roots of the brachial plexus (Burke et al '95, Giffen and Gillett, '96). Because the eighth postcranial vertebra of the giraffe articulates with a rib and is attached to the sternum, Solounias' view has been contested by Mitchell and Skinner (2003) and Badlangana et al (2009). Furthermore, there seems to be developmental constraints that fix the number of cervical vertebrae at seven in mammals (with the exception of the manatee (Trichechus) and sloths (Bradypus and Choloepus)) (Galis, '99;Narita and Kuratani, 2005).…”

“…Some of the criteria used by Solounias to define vertebral identity of the caudal most cervical vertebrae and first thoracic vertebra tend to be variable in mammals. They can be based on the presence of transverse foramina in cervical vertebrae (Turner, 1847, Badlangana et al, 2009, on the length, position, and shape of the spinous and transverse processes (Solounias, '99, Badlangana et al, 2009), and on the position and orientation of the articular facets on the articular processes (Lankester, '08, Solounias, '99) (Fig. 1A, B).…”

“…Conclusions about skeletal growth patterns and scaling require that measurable changes in the skeleton be compared with changes in another measure of growth. Typically, the measure of growth has been body mass, although vertebral column length has also been used (Badlangana et al, 2009). The study of these relationships is known as allometry (Huxley, '32).…”

Orexinergic bouton density is lower in the cerebral cortex of cetaceans compared to artiodactyls

“…Cetacean limbs and necks have been reduced so much that they no longer have a significant locomotor function, whereas in artiodactyls a range of neck and limb proportions are evident (Badlangana et al, 2009). While both groups require substantial food ingestion, cetaceans are carnivorous, while the artiodactyls are herbivorous.…”