Charles Darwin held up giraffes as a prime example of natural selection, his theory that’s often summarized as “survival of the fittest.” Giraffes with comparably longer necks could reach food high up in trees, which gave them an advantage over other animals and members of their own species with shorter necks. These longer-necked individuals thrived and reproduced more, leading to generations of giraffes with their signature lengthened anatomy.
Now, though, researchers believe they’ve discovered another piece of the evolutionary giraffoid puzzle. Yes, giraffes may have evolved to be able to reach food at higher elevations, but their long necks may also be the result of fierce competition for mates, according to new research published this week in Science by an international team of paleontologists and paleobiologists.
In 1996, paleontologists in northern China discovered an unusual skull and a few neck vertebrae For many years, researchers simply called the mysterious animal “guài shòu,” or “strange beast.” Now, scientists have given the mammal a name—Discokeryx xiezhi—and they’ve pieced together a rough outline of how the animal may have lived some 16.9 million years ago.
Discokeryx xiezhi was an early relative of today’s giraffes, but more like a cousin and not a direct ancestor. This giraffoid was about the size of a big sheep and had a sturdy skull with a one-inch-thick plate as its forehead, and very thick neck vertebrae. It lived in what is now northern China during the warm, wet mid-Miocene, about 5.3 to 23 million years ago, munching on grasses and leafy plants.
When they weren’t grazing, males were likely fighting, using their ultra-robust head and neck setup to bash each other while competing for mates. Researchers have surmised that Discokeryx xiezhi likely had the strongest, most complex head and neck joints of any mammal, ever. They compared the giraffoid to dinosaurs, musk oxen and several types of sheep—which all have a propensity for using their heads in battle—and determined that it could likely out-headbutt them all.
“To the best of our knowledge, D. xiezhi exhibits the most optimized head-butting adaptation in vertebrate evolution,” the researchers write in the paper.
This find gave them new insight into why present-day giraffes may have evolved such long necks—to reach food, yes, but also for sexual competition. Though they don’t exactly headbutt each other, modern male giraffes do swing their necks around violently to bash their heads into each other’s bodies. The discovery of Discokeryx xiezhi suggests this fighting style and competition for mates may be a contributing factor in their long-neck evolution.
"Both living giraffes and Discokeryx xiezhi belong to the Giraffoidea, a superfamily,” Shiqi Wang, a paleontologist with the Chinese Academy of Sciences and one of the study’s authors, says in a statement. “Although their skull and neck morphologies differ greatly, both are associated with male courtship struggles and both have evolved in an extreme direction.”
Of course, that doesn’t explain why female giraffes also have long necks, nor why both sexes have such long limbs. Scientists believe a number of factors, and likely not one specific catalyst, prompted their evolutionary trajectory.
“In reality, it was likely a combination of natural selection ... for a particular dietary preference and sexual selection in that lineage that drove the evolution of modern giraffe necks and limbs,” Advait Jukar, a paleobiologist at Yale University who was not involved in the study tells Scientific American’s Rachel Nuwer.
Either way, as Rob Simmons, a biologist and ecologist at the University of Cape Town who was not involved in the study, tells Science’s Elizabeth Pennisi, if Darwin were alive today, he’d likely be “bowled over” by this latest bit of evolutionary research.