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For generations, giraffes' long necks have been used as a prime example of the operation of natural selection. However, a newly analyzed fossil girraffoid provides evidence for a less wholesome explanation: A long-extinct species appears to have evolved a head and neck perfectly designed for battle, and it looks like giraffes followed in their footsteps.
According to the standard story, ancestral giraffes with longer necks than their peers could reach more nutritious leaves at the tops of trees, and therefore were more likely to survive and reproduce. Their offspring inherited this trait, leading to ever-longer necks.
Survival of the fittest is not the only evolutionary driver, however. Darwin was aware sexual selection also plays a role, and since his day, further complications have also been recognized. Some zoologists have theorized the struggle for mates could have contributed to giraffes' mighty necks, and new evidence presented in the journal Science supports the case.
Male giraffes swing their heads at each other to claim dominance and mating opportunities. They may not be familiar with the physics of torque, but as a species, they worked out that longer necks mean more hitting power for the head. Males also use the force of a well-swung skull to make females urinate so they can check their fertility. So romantic!
So, did the necks evolve for reaching treetops, or to whack against rivals? In the normal course of things, it can be hard to distinguish causes, particularly for a trait that evolved millions of years ago. However, Professor Deng Tao of the Chinese Academy of Sciences and co-authors got lucky with a 16.9 million-year-old fossil found in the Junggar Basin, Xinjiang. They named the newly described species Discokeryx xiezhi – Discokeryx is among the oldest members of the superfamily, known as giraffoids, now restricted to two species of giraffe.
"Discokeryx xiezhi featured many unique characteristics among mammals, including the development of a disc-like large ossicone [horn-like structure] in the middle of its head," said Deng in a statement.
Its ossicone was perfectly honed by evolution to be effective in head-to-head battering contests. Indeed, the authors conclude it was better at its job than equivalent structures in the skulls of modern animals such as musk-oxen that compete for dominance in similar ways. Discokeryx's neck vertebrae were thickened to provide support in such battles, while the head-neck joints are the most complicated ever seen in mammals, presumably for the same reason.
The authors conclude giraffes have been using head-butting as a tool to win mates since long before they had such epically long necks. Moreover, they note; “Fossil giraffoids exhibit a higher degree of diversity in headgear morphology than any other pecoran group; such a diversity, associated with the complex head-neck morphology, likely indicates the intensive sexual combats between males in the evolution of giraffoids.”
When modern giraffes started evolving five million years ago, the use of the head and neck as a weapon ran deep in their ancestry. Males would have benefited from longer necks so they could apply more force. Access to leaves beyond other animals' reach was probably secondary, although the much shorter-necked Discokeryx's teeth indicate it also fed on tree leaves.
Although Discokeryx butted heads in a warmer and wetter world than today, the Junggar Basin was drying out thanks to Tibet's rise, making for an ecosystem like today's East African Savannah.
The name xiezhi was chosen for the resemblance to the mythical Chinese animal of the same name, while the genus name combines Disco (round plate) and keryx (meaning horn). Sadly, there's no evidence Discokeryx liked to boogie; if anything it seems the males were fighters, not dancers.
