Intermale-competitions of giraffoid, foreground: Discokeryx xiezhi, background: Giraffa camelopardalis. Credit: Wang Yu and Guo Xiaocong
Fossils demonstrate that head-bashing combat contributed to the development of the long necks of giraffes.
The authors of the study propose an alternative theory for the origin of the long necks of modern giraffes: giraffes needed them for head-bashing combat they used in competition for mates. This theory is supported by an analysis of an early giraffe ancestor’s unique head and neck fossils, which include disk-shaped helmet-like headgear and highly complex head-neck joints.
Since Charles Darwin originally proposed the ideas of adaptive evolution and natural selection, the distinctively long neck of the modern giraffe—the tallest land animal and largest ruminant on Earth—has long been seen as a quintessential example of these processes. According to popular belief, food rivalry led to neck elongation, which enabled giraffes to forage for treetop leaves in the African Savannah woodlands much beyond the reach of other ruminant species.
Modeling of high-speed head-butting in Discokeryx xiezhi using finite element analyses, with (A) and without (B) the complicated joints between cranium and vertebrae, showing the stable (A) or over-bending (B) head-neck articulation. Credit: IVPP
Others, however, have put forward the “necks-for-sex” theory, which contends that intermale competition-driven sexual selection may also have had a role in the development of the elongated neck. Shi-Qi Wang and colleagues claim that remains of extinct giraffe species can shed light on these evolutionary mechanisms.
Here, Wang and his team report and describe a new species of Miocene giraffoid, Discokeryx xiezhi. The fossils, dated to roughly 17 million years ago, indicate that this ancient giraffoid species had helmet-like headgear and particularly complex head and neck joints.
The fossil community in the Junggar Basin at ~17 million years ago. Discokeryx xiezhi are in the middle. Credit: Guo Xiaocong
According to the researchers, these peculiar morphological characteristics show an adaption for fierce head-butting behavior. In fact, the authors suggest that Discokeryx xiezhi may have possessed the most optimized head-butting head and neck adaptation yet identified in vertebrate evolution.
Moreover, tooth enamel isotope data from these fossils suggest that the species also likely filled a specific ecological niche in the ecosystem unavailable to other contemporary herbivores. In total, the scientists suggest that early giraffoid evolution is more complex than previously known, where, in addition to competition for food, sexual combat likely played an important role in shaping the group’s long and uniquely adapted necks.
Reference: “Sexual selection promotes giraffoid head-neck evolution and ecological adaptation” by Shi-Qi Wang, Jie Ye, Jin Meng, Chunxiao Li, Loïc Costeur, Bastien Mennecart, Chi Zhang, Ji Zhang, Manuela Aiglstorfer, Yang Wang, Yan Wu, Wen-Yu Wu and Tao Deng, 3 June 2022, Science.
DOI: 10.1126/science.abl8316
