Giraffes grew longer necks to battle mating rivals, mysterious fossil suggests

BEIJING, China — Why does the giraffe have such a long neck? A new study suggests that it may have to do with love and evolution! With the help a strange fossil, researchers in China say the giraffe grew its long neck for headbutting battles with rival lovers.

Scientists say they needed them for head-banging combat in competition for mates rather than for reaching for food in tall trees. An analysis of an early giraffe ancestor’s unique head and neck fossils suggests an alternative explanation for the evolution of modern giraffes’ long necks. The fossil included disk-shaped, helmet-like headgear and highly complex head-neck joints.

The characteristic neck of the modern giraffe – the tallest land animal and largest ruminant on Earth – has long been considered a classic example of adaptive evolution and natural selection since Charles Darwin first penned the concepts.

It’s a common belief that competition for food drove neck elongation and allowed giraffes to browse for treetop leaves in the African Savannah woodlands that were well outside the reach of other ruminants. However, others have argued a “necks-for-sex” hypothesis, suggesting sexual selection driven by competition among males may have also contributed to neck evolution.

Early giraffes
Intermale-competitions of giraffoid, foreground: Discokeryx xiezhi, background: Giraffa camelopardalis. (Credit: IVPP)

Bigger neck, better fighter?

It was the discovery of strange fossils, roughly 17 million years-old, that revealed the key driving forces in giraffe evolution, according to the Chinese research team.

As scientists began to increase their observations of giraffe behavior, they realized that the long neck of the species could also serve as a weapon during male courtship displays — possibly revealing the key to the giraffe evolutionary mystery. Researchers say that the longer the animal’s neck is, the greater the damage it can inflict.

Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences conducted their study on Discokeryx xiezhi, a strange early giraffoid.

The fossils analyzed in the study, published in the journal Science, were discovered in the northern margin of the Junggar Basin of Xinjiang. The fossils include a full skull and four cervical vertebrae.

“Discokeryx xiezhi featured many unique characteristics among mammals, including the development of a disc-like large ossicone in the middle of its head,” says corresponding author Professor Deng Tao in a media release.

Giraffes have the best bone structure for headbutts?

Study authors explain that the single ossicone resembles that of the xiezhi, a one-horned creature from ancient Chinese mythology — giving the fossil its name. Prof. Deng adds that the cervical vertebrae of Discokeryx xiezhi have the most complex joints between the head and neck and between the cervical vertebrae of any mammal known to man.

The team found that the complex movements between the skull and cervical vertebrae of Discokeryx xiezhi were capable of withstanding high-speed, head-to-head impacts. They also discovered that these giraffe ancestors were more effective than other animals, such as the musk oxen, at adapting to head impacts. In fact, the team believes the Discokeryx xiezhi may have the best vertebrate ever when it comes to withstanding a head impact.

“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 struggles and both have evolved in an extreme direction,” says study first author Dr. Wang Shiqi.

Giraffe evolution
The fossil community in the Junggar Basin at ~1700 million years ago. Discokeryx xiezhi are in the middle. (Credit: IVPP)

The research team compared the horns of several groups of ruminants, including giraffoids, cattle, sheep, and deer. They found that horn diversity in giraffes is far greater than in other groups, with some extreme differences in the shapes and sizes, indicating that their mating battles can be more intense than other ruminants.

Ancient giraffes lived in a harsh land

Study authors also analyzed the ecological environment of Discokeryx xiezhi and the places the species occupied. During this time, researchers believe the Earth was in a warm period and was home to dense forests. However, in the Xinjiang region, the land was drier than other areas because the Tibetan Plateau to the south had been rising dramatically. This blocked the transfer of water vapor in the area.

“Stable isotopes of tooth enamel have indicated that Discokeryx xiezhi was living in open grasslands and may have migrated seasonally,” says co-corresponding author Dr. Meng Jin.

The team says this grassland environment was more barren, making it less comfortable than in the forests. With that in mind, the violent fighting behavior of Discokeryx xiezhi may have been the result of survival-related stress in the harsher environment.

The researchers also believe a similar environment existed on Earth at the dawn of the Giraffa species. Around seven million years ago, the East African Plateau changed from a forest landscape to an open grassland, forcing the direct ancestors of modern-day giraffes to adapt.

Prof. Deng says it’s possible that mating males developed a way of attacking their rivals by swinging their necks and heads. This extreme struggle, the team adds, led to the rapid elongation of the giraffe’s neck. Overall, scientists believe this process took around two million years.

As an added bonus, Giraffa were now in a position to feed on taller trees.

South West News Service writer Stephen Beech contributed to this report.

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