DID GIRAFFES EVOLVE THEIR LONG NECKS FOR COMBAT?
Fossil of ancient giraffe relative reveals skull and neck bones that are highly adapted for throwing brutal headbutts
Giraffes are widely thought to have evolved such long necks in order to feed on high foliage that other animals found difficult to reach. But it may actually have been competition for mates that gave them this unique adaptation, researchers at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences have found.
Giraffes’ necks can measure two to three metres in length. During courtship competitions, the animals fight by using their long necks to whip their heavy skulls, which are topped with hard, horn-like ossicones, into their opponents.
Animals with longer necks are able to land more powerful blows on their opponents, making them more likely to win fights for mates. It’s also believed giraffes with longer necks are deemed to be further up the social hierarchy.
Now, a 17-million-year-old fossil of Discokeryx xiezhi, an ancient relative of modern giraffes, found in Junggar Basin, Xinjiang, western China, suggests that this behaviour may be what led to the animals evolving their trademark long necks.
Though its neck is much shorter than that of modern giraffes, analysis of the fossil shows that D. xiezhi had an incredibly complex series of joints between its head and spine, along with a hard, disc-shaped ossicone on its head that made it particularly adapted for high-speed, head-to-head impacts.
They found this structure was far more effective at absorbing impacts than that of other modern animals adapted to heavy head impacts, such as musk oxen.
The researchers believe that the evolution of this unusual skeletal structure was driven by headbutting courtship competitions. This means it was likely to also be true for the evolution of the unique neck and head anatomy seen in modern giraffes, they say.
“Both living giraffes and Discokeryx xiezhi belong to the Giraffoidea, a superfamily. Although their skull and neck morphologies differ greatly, both are associated with male courtship struggles and both have evolved in an extreme direction,” said the study’s first author Dr Wang Shiqi.
The researchers believe that during the emergence of the genus Giraffa seven million years ago, the direct ancestors of modern giraffes developed a way of fighting by swinging their necks and heads into one another.
Along with sexual selection, this behaviour led to the extreme elongation seen in modern giraffes over the following two million years. This would also have made them particularly well-suited for the niche of feeding on high foliage as a consequence, the researchers say.