Giraffe evolution mystery solved by bizarre fossil

Before Charles Darwin identified natural selection as the process by which species evolved, French naturalist Jean-Baptiste Lamarck put forward his own theory of evolution. Lamarck suggested that acquired characteristics were inheritable.

Lamarck famously cited giraffes as an example. Giraffes stretched their necks to browse higher in trees, posited Lamarck, and the continuation of that habit resulted in a gradual lengthening of the limbs and neck – traits which would then be passed on to the next generation. And voila! Long necks on giraffes.

Conversely – and more correctly – Darwinian evolution proposes that giraffe ancestors would have had slight variations in neck length. Those with longer necks may have been more successful in getting food, hence the attribute is “selected” through the more long-lived, healthier, longer-necked giraffes.

End of story.

But discussions about Lamarckism vs Darwinism aside, we’re still learning about giraffe evolution. And new research published in Science suggests that long necks may have more to do with courtship than obtaining a meal.

In mating rituals, rival male giraffes swing their long necks, hurling their heavy skulls at competitors. Bulls target weak spots on their opponents with their cranial weaponry – ossicones (the skin-covered bone structure atop a giraffe’s head) and osteomas (bony growths on the animal’s skull).

Cartoon of two giraffes wearing bowler hats
Science communicator Ivy Shih points out that the osteomas look like a bowler hat. Credit: Ivy Shih

A longer neck means greater force and damage to the rival.

Researchers from the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) of the Chinese Academy of Sciences studied fossils of strange early giraffoid Discokeryx xiezhi.

Fossils, including a full skull and four cervical vertebrae, belonging to Discokeryx xiezhi were analysed. The 17 million-year-old fossils were found in the Junggar Basin in Xinjiang, China.

Discokeryx xiezhi lived in the Miocene geological epoch which spanned 23 to around 5 million years ago. The Miocene saw the emergence of the first kelp forests and grasslands.

Beginning over 40 million years after the extinction of the large dinosaurs, the age of mammals was in full swing. The first dogs, bears, hyenas and sabre-toothed cats in Smilodon’s family appeared. Primitive antelope, deer, elephants and giraffes appeared.

More on evolution: Megalodon and great white shark competed for food

Discokeryx xiezhi featured many unique characteristics among mammals, including the development of a disc-like large ossicone in the middle of its head,” says co-author of the paper Professor Deng Tao from IVPP. Deng said the single ossicone resembles that of the xiezhi, a one-horned creature from ancient Chinese mythology which gives its name to the ancient mammal.

The researchers say the animal’s joints between its head and neck, and between the very stout cervical vertebrae, are the most complex of any mammal. These articulations, the team found, were adaptations to high-speed head-to-head impact. They found this structure was far more effective than even that of extant animals who engage in head-on combat, such as musk oxen. In fact, Discokeryx xiezhi may have been the best adapted vertebrate to utilise head impact of all time.

“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 lead author Wang Shiqi.

Comparing the horns of giraffoids, cattle, sheep, deer and pronghorns, the team found greater diversity among giraffes. They also found greater extremes in Giraffoidea, indicating more intense courtship struggles.

“Stable isotopes of tooth enamel have indicated that Discokeryx xiezhi was living in open grasslands and may have migrated seasonally,” says co-author Men Jin. The researchers suggest that the less habitable grasslands may have pushed animals like Discokeryx xiezhi to engage in fighting behaviour as a survival-related stress caused by the environment. The genus Giraffa, to which the modern giraffe belongs, emerged around 7 million years ago in similar environments. The authors suggest that similar extreme struggle and sexual selection is what led to the rapid elongation of giraffe necks.

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