The fossils have been found on the northern margin of the Junggar Basin, in Xinjiang, China, and are from about 17 million years ago, from the early Miocene. The entire skull and all four vertebrae would have belonged to Discokeryx xiezhi , a rare early giraffe. The scientists responsible for the study, which has been published in Science , have studied Discokeryx xiezhi in depth and point out that the long neck of male giraffes is so that they can better impact other males when they fight for the female . Come on, they headbutt each other in the competition for the female.
Giraffes’ skulls are equipped with ossicones , which are the skin-covered bony protrusions on the top of their heads. Well, the huge neck would serve to launch the head against the rival and hit him with these bones in a weak part. Impacts can cause serious injury and sometimes even death. Therefore, the longer the giraffe’s neck, the more damage it will inflict on the opponent.
” Discokeryx xiezhi had many unique features among mammals, such as the development of a large disc-shaped ossicle in the center of the head,” said Professor Deng Tao.
According to the researchers, the cervical vertebrae of Discokeryx xiezhi are very robust and have the most complex joints between the head and neck and between the cervical vertebrae of any mammal. The team showed that the complex joints between the skull and cervical vertebrae of Discokeryx xiezhi were specially adapted for high-speed head butting. They found that this structure was much more effective than that of modern animals, such as musk oxen, which are also trained for head impacts. In fact, Discokeryx xiezhi may have been the vertebrate best adapted to these blows.
This research, therefore, helps to understand how the peculiar neck of giraffes, which can measure up to two to three meters, evolved. An evolution that would be motivated mainly by the social hierarchy and the fights between males that take place at the time of courting the female.
“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 fights and both have evolved in an extreme direction,” he said. Wang Shiqi, first author of the study.
The research team compared the morphology of the horns of various groups of ruminants, including giraffes, cattle, sheep, deer, and pronghorn. They found that giraffe horns are more diverse than those of other groups , with a tendency for extreme differences in morphology, indicating that courtship fights are more intense and diverse in giraffes than in other ruminants.
The scientists also analyzed the ecological environment of Discokeryx xiezhi and the niche it occupied. When this animal existed, the Earth was in a warm period and, in general, had a high density of forests. However, the Xinjiang region, where Discokeryx xiezhi lived, was somewhat drier than other areas because the Tibetan Plateau to the south had risen dramatically, thus blocking the transfer of water vapor.
“Stable isotopes of tooth enamel have indicated that Discokeryx xiezhi lived in open grasslands and may have migrated seasonally,” said Meng Jin, another author. For the animals of the time, the environment of the grasslands was more arid and less comfortable than that of the forest. The violent fighting behavior of Discokeryx xiezhi may be related to the stress of surviving in this more unfavorable environment .
When the genus giraffa appeared, an environment similar to that of Discokeryx xiezhi existed . About seven million years ago, the East African Plateau also changed from a forested environment to open grassland, and the direct ancestors of the giraffes had to adapt to the new changes . It is possible that, among the ancestors of giraffes during this period, mating males developed a way of attacking their competitors by swinging their necks and heads. This extreme struggle, supported by sexual selection, thus led to the rapid elongation of the giraffe’s neck over a period of two million years until it became the current genus, giraffa .
Referencia: Wang Shi-Qi et. al. 2022. Sexual selection promotes giraffoid head-neck evolution and ecological adaptation. Science. DOI: 10.1126/science.abl8316
