Why giraffes have long necks

Evolution has not provided the giraffe with a long neck because of snacking on fresh tree leaves, but because of fighting other species. That is what a team of mainly Chinese researchers concluded this week in science, based on a 17-million-year-old fossil giraffe skull. The extinct primordial giraffe Discokeryx xiezhi had a helmet-like head that allowed it to pound hard on other males in the battle for a female.

In later species, that pounding would have turned into “neck wrestling,” with male giraffes with a stronger, longer neck having an advantage. The fact that all giraffes – including the females – eventually developed longer necks and could therefore eat fresh tree leaves was a successful side effect.

For centuries, biologists have been fascinated by the giraffe’s neck. Eighteenth-century Jean-Baptiste de Lamarck suggested that giraffes developed increasingly longer necks during their lives as they tried to reach higher and higher leaves and passed that neck length on to their children. In the nineteenth century, Charles Darwin saw in the giraffes a textbook example of natural selection: the giraffes with the longest necks had the advantage because they could eat tree leaves undisturbed, and had the most offspring.

Necks for sex

In 1996, American zoologists Robert Simmons and Lue Scheepers came up with a revolutionary new hypothesis: necks for sex† Giraffes, they wrote, eat just as often from low vegetation as from high vegetation, so they don’t necessarily need that long neck at all. According to them, neck length was especially important for males, who with a strong, long neck could engage in physical battle with rivals in order to win over a female.

Male giraffes also had stronger, larger necks than female giraffes, according to Simmons and Scheepers. But in 2013, their theory was contradicted by three South African researchers in the Journal of Zoolog, who determined, based on measurements taken on zoo giraffes, that there was no apparent difference at all in neck and skull growth between males and females.

Still, the Chinese paleontogens come in science now, after analyzing a recently discovered fossil from northern China, making amends for the necks-for-sex hypothesis. Discokeryx xiezhia previously unknown extinct giraffe-like creature from the early Miocene, lived about 16.9 million years ago and had sturdy cervical vertebrae and a stout discus-like ‘helmet’ made of keratin.

The joints between his neck and skull were “optimized to deal headbutts.”

The joints between its neck and skull were “more complicated than any mammal,” the authors write, and “optimized for headbutting.” Those vertebrae, disc head and strong neck joints served him well in head fights with other males, similar to the neck wrestling that modern giraffe men sometimes do to measure their strength, the researchers say.

Other tooth enamel

The neck of Discokeryx xiezhi was significantly shorter than that of current giraffes. The species did eat ‘different vegetation’ than the other large grazers that lived at the time, the researchers concluded on the basis of the tooth enamel of the ancient giraffe. It is impossible to say whether these were tree leaves.

The headbutters eventually evolved into the neck-wrestling giraffes. The fact that they could also eat from high trees was a nice side effect. Female giraffes, who do not wrestle neck, also benefit from that extra advantage.

Menno Schilthuizen, evolutionary biologist at Naturalis Biodiversity Center and professor at Leiden University, thinks when asked that the sciencearticle provides good evidence that competition between males by cephalopods originated early in the giraffe lineage. “And the neck extension only later. That makes it plausible that the giraffe’s long neck is also a result of sexual competition and only came in handy for foraging in the second instance, rather than the other way around.”

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