Why do giraffes have long necks? Until now, it was assumed that a long neck was favored evolutionarily, since the animals could use it to reach higher-growing leaves. However, a fossil find of an extinct species of giraffe now suggests a different explanation: According to this, a long neck could have been helpful in courtship fights between males, in which they headbutt each other. Thus, the evolution of this extraordinary anatomical feature would not only be due to foraging advantages, but also to sexual selection.
The long neck of giraffes is considered a classic example of adaptive evolution: Individuals who happened to have a longer neck had better access to high-altitude food sources and thus had an evolutionary advantage and were able to reproduce more, so that this trait became established over time. An alternative explanation is that a long neck may also have been important in male-female competition. So far, however, there has been a lack of fossil evidence for this theory.
Complex cervical vertebrae
A team led by Shi-Qi Wang from the Chinese Academy of Sciences in Beijing has now found fossils that support the theory of the sexual evolution of the giraffe’s neck. The researchers examined the remains of a newly discovered extinct species of giraffoid and found that the animals appeared to have particular anatomical adaptations for courtship matches between males. The newly described species Discokeryx xiezhi lived in China around 16.9 million years ago.
The fossils, a complete skull and four cervical vertebrae, were discovered on the northern rim of the Junggar Basin in northwest China’s Xinjiang region. “Discokeryx xiezhi had many unique features, including the development of a disk-like bony plate in the center of the head,” describes Wang’s colleague Tao Deng. In addition, the cervical vertebrae were exceptionally strong and complex in shape. “This made Discokeryx xiezhi particularly well adapted to headbutts,” the researchers write. Comparisons with other animal species, both living and extinct, that also fight with headbutts revealed that Discokeryx xiezhi was perhaps the best adapted for this type of fighting even of all vertebrates.
nutrition in the grasslands
“Both the living giraffes and Discokeryx xiezhi belong to the Giraffoidea, a superfamily,” explains Wang. “Although their skull and neck morphologies differ greatly, both are associated with male courtship fights, and both have evolved in an extreme direction.” Modern-day giraffe bulls typically fight by banging their necks together, not their heads like Discokeryx xiezhi. In both cases, however, the male with the longer neck has the advantage and has a better chance of winning over the female and passing on his genes.
Using the preserved fossil teeth, the researchers also analyzed how Discokeryx xiezhi likely fed. “Stable isotopes in tooth enamel have revealed that Discokeryx xiezhi lived in open grasslands and possibly engaged in seasonal migrations,” says co-author Jin Meng of the American Museum of Natural History in New York. The Xinjiang region where Discokeryx xiezhi lived 16.9 million years ago was drier than other areas at the time. Instead of dense forest, sparse grassland prevailed here.
Sexual selection as the main driving force
From the authors’ point of view, the harsh environmental conditions could have led to Discokeryx xiezhi developing particularly aggressive fighting behavior and adapting accordingly. Similar conditions prevailed when the genus Giraffa, to which all modern giraffes belong, arose: About seven million years ago, the East African plateau changed from a forested environment to open grasslands, and the direct ancestors of giraffes had to adapt to the new conditions – what may also lead to increased competition and fighting between males.
“Combat between males was probably the main driving force behind the evolution of the giraffe’s long neck,” the authors write. “The resulting ability to reach food at higher altitudes was likely an added benefit of this development.”
Source: Shi-Qi Wang (Chinese Academy of Sciences, Beijing) et al., Science, doi: 10.1126/science.abl8316