Chinese researchers offered a new explanation for the evolutionary mystery of giraffes' long necks: They were elongated by head-bashing combat in competition for mates.
The study published in the journal Science on Friday revealed that the tallest land animal on Earth used its two-to-three-meter-long swinging necks as a weapon in the male courtship competition.
It is commonly believed that competition for food stretched the giraffe's neck, allowing giraffes to browse for treetop leaves in the African Savannah woodlands that were well outside the reach of other ruminant species.
In 1996, two zoologists proposed the "necks-for-sex" hypothesis instead of the "necks-for-food," causing controversy.
In the same year, a team from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) under the Chinese Academy of Sciences discovered a 17-million-year-old fossil in the vast Gobi wilderness in northwest China's Xinjiang Uygur Autonomous Region.
The fossil features disk-shaped headgear equipped with a helmet-like horny cap, and particularly complex head and neck joints.
It is named "Discokeryx xiezhi" since its single ossicone recalls the xiezhi, a one-horned creature from ancient Chinese mythology.
Wang Shiqi with IVPP and his colleague revealed in the new study that the strange beast's horns could serve as a cushion in the collision and the joints between the skull and cervical vertebrae could effectively protect the neck from breaking in the violent collisions.
Then, they analyzed the beast's inner ear structures, finding them distinct from the ox and the deer, and instead consistent with the extant giraffe.
"Both living giraffes and Discokeryx xiezhi belong to the Giraffoidea, a superfamily," said Wang, the paper's first author. "Although their skull and neck morphologies differ greatly, both are associated with male courtship struggles."
The researchers said that Discokeryx xiezhi, an ancestor of the giraffe, lived in a drier grassland at a time when the Qinghai-Tibet Plateau to the south had been rising dramatically, blocking the transfer of water vapor.
For animals of the time, the grassland environment was more barren and less comfortable than the forest environment, forcing Discokeryx xiezhi to take violent fighting behavior for survival, the researchers said.
It is similar to the East African Plateau about 7 million years ago when a forest environment degenerated into open grassland, prompting the giraffe ancestors to adapt to the new environment and become taller.
It is the courtship struggle that led to the rapid neck elongation over a period of 2 million years, and by dint of that advantage, giraffes occupied a relatively marginal, but rewarding ecological niche -- feeding on high foliage out of reach of zebras and various antelopes, according to the study.
"Feeding may be an evolutionary outcome, and sex may be the pathway that leads to this outcome," said Wang.