An analysis of an early giraffe ancestor’s unique head and neck fossils – disk-shaped helmet-like headgear and highly complex head-neck joints – suggests an alternative explanation for the evolution of modern giraffes’ long necks: giraffes required them for head-bashing combat they used in competition for mates, the study’s authors say. The characteristic long neck of the modern giraffe – the tallest land animal and largest ruminant on Earth – has long been considered a classic example of adaptive evolution and natural selection since Charles Darwin first penned the concepts. It’s commonly believed that competition for food drove neck elongation and allowed giraffes to browse for treetop leaves in the African Savannah woodlands that were well outside the reach of other ruminant species. However, others have argued a “necks-for-sex” hypothesis, suggesting sexual selection driven by intermale competition may have also contributed to neck evolution. According to Shi-Qi Wang and colleagues, fossils of ancient giraffe species can help to clarify these evolutionary mechanisms. Here, Wang et al. 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. According to Wang et al., these peculiar morphological characteristics show an adaption for fierce head-butting behavior. In fact, the authors suggest that D. 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, Wang et al. 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.
Sexual selection promotes giraffoid head-neck evolution and ecological adaptation
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