How The Giraffe Got Its Long Neck (Pictures)

Giraffes may have grown their distinctive six-to-ten-feet-long necks to better swing their heavy, knobbled skulls around like weapons during courtship contests, a study has argued.

While the exact process that led to the evolution of the giraffe’s long neck has long been uncertain, it had always been assumed that the driver was evolution to reach high foliage. However, this may not be the case, a new study has proposed — with the ability to nibble on the top of trees perhaps more of an incidental benefit. Instead, researchers have proposed, giraffes grew long necks to give themselves better weapons in courtship contests, building on an ancient ancestor who had a disk-shaped “helmet” on their head that they used to headbutt each other.

In their study, palaeontologist Professor Tao Deng of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing and his colleagues studied a strange-looking early giraffoid species called Discokeryx xiezhi.

In particular, the team analysed the fossilised remains of D. xiezhi — including a complete skull and four neck vertebrae — that were unearthed on the northern margin of the Junggar Basin in northwest China’s Xinjiang region.

According to the researchers, the specimens date back to some 17 million years ago, during the early Miocene epoch.

Prof. Deng said: “Discokeryx xiezhi featured many unique characteristics among mammals, including the development of a disc-like large ossicone in the middle of its head.”

Ossicones are skin-covered bone structures, superficially similar to horns, that can be found on the heads of modern giraffes and male okapi.

In fact, the ancient giraffoid gets its species name from resemblance of its single ossicone to that of the “xiezhi”, a one-horned creature from Chinese mythology.

The team note that the neck vertebrae of D. xiezhi are very stout and have the most complex joints of any known mammal between both the head and neck and between the vertebrae.

Furthermore, the researchers’ analysis indicated that these complex articulations were particularly well suited to engaging in high-speed, head-to-head impacts — more so even than modern animals like musk oxen that are adapted for banging heads.

In fact, the team said, D. xiezhi may well be the vertebrate best adapted to head impacts from all of life’s history.

Paper author and vertebrate palaeontologist Professor Shiqi Wang, also of IVPP, said: “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.”

Next, the researchers compared the horn shapes and sizes of several groups of ruminants, including not only giraffoids but also cattle, deer, sheep and pronghorns.

The team found that the giraffe family have more extreme horn differences between species than other groups — suggesting that courtship struggles are more intense and diverse in giraffes than in other ruminants.

According to the researchers, D. xiezhi lived during a time when the Earth was warm and generally densely forested — although the Xinjiang region where the ancient giraffoid lived was comparatively drier in the Miocene as a result of the rising of the Tibetan plateau to the south block the influx of water vapour.

Paper co-author and palaeontologist Dr Jin Meng of the American Museum of Natural History said: “Stable isotopes of tooth enamel have indicated that Discokeryx xiezhi was living in open grasslands and may have migrated seasonally.”

The grassland environment, the team explained, would have been more barren and less comfortable than the forest settings enjoyed elsewhere during the early Miocene.

Accordingly, the violent fighting behaviour of D. xiezhi may have been driven by environmentally-driven stressors.
A similar environment existed when the genus Giraffa first emerged around seven million years ago, the researchers explained.

At this time, the East African Plateau also changed from a forested environment to open grassland, which would have forced the direct ancestors of giraffes to adapt accordingly.

The researchers propose that mating males developed a way of attacking their competitors by swinging their heads and necks.

It was this extreme struggle — supported by sexual selection — that led to the rapid elongation of the giraffe’s neck over a period of two million years.

And while this would then have made modern giraffes ideally suited to feeding on high foliage, it was the courtship competition brought about by the challenging nature of their environment that led to their extreme neck evolution.