Growth Pattern and Functional Morphology of the Cervical Vertebrae in the Gerenuk (Litocranius walleri): The Evolution of Neck Elongation in Antilopini (Bovidae, Artiodactyla)
Long necks have evolved independently in several different taxa, but the processes underlying the evolution of this trait are not yet fully understood. In this study, we examined the skeletal mechanism underlying the neck elongation in the tribe Antilopini (Bovidae, Artiodactyla). We calculated the growth patterns of the cervical vertebrae in the gerenuk (Litocranius walleri), which possesses the longest neck in this tribe, and compared it with those in two related species. The growth rates of the vertebrae were not significantly different between species, suggesting that the long neck of the gerenuk has resulted from the elongation of the cervical vertebrae during the fetal or juvenile stage. The morphology of the cervical vertebrae of gerenuks differed from that of the closely related, relatively long-necked dama gazelle (Nanger dama), with protrusions occurring on the dorsal surface of the ventral arch of the atlas. This implies that gerenuks possess a well-developed transverse ligament of the atlas that functions to hold the dens of the axis against the atlas. We also found that the atlas lies in close proximity to the neural spine of the axis in the gerenuk, suggesting that hyperextension of the atlantoaxial joint is osteologically limited in this species. While foraging on high foliage, gerenuks flex and extend their necks freely in a bipedal posture without moving their entire body. These morphological characteristics peculiar to the gerenuk enhance the rigidity of the atlantoaxial joint and decrease the risk of subluxation of the joint during this unique foraging behavior.
KeywordsAntilopini Cervical vertebrae Gerenuk Neck elongation Scaling pattern
We are grateful to Ms. Eleanor Hoeger and Ms. Eileen Westwig (AMNH) for access to skeletal specimens under their care. We feel strong gratitude toward Dr. Nikos Solounias (New York Institute of Technology) for helping our investigation at AMNH. We thank Dr. Shin-ichiro Kawada (National Museum of Nature and Science, Tokyo) for allowing us to access the taxidermied specimens of the gerenuk. We thank Dr. Yasuhisa Nakajima (The University of Tokyo) for helpful discussions. We thank Dr. Eric Snively (University of Wisconsin) for improving our English.
This study was supported by JSPS fellowship for young scientists (Research project no. 14 J04974).
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