Like us, giraffes today have seven neck (or cervical) vertebrae. But their C3 or third cervical vertebra – which is nine times longer than it is wide – is about as long as the humerus bone connecting our shoulders to our elbows. According to a new study published in Royal Society Open Science, the elongation of the giraffe neck occurred in at least two stages. The cervical vertebrate stretched towards the head first, then towards the tail millions of years later.
Why giraffes have such a long neck – for reaching higher plants, specialized fighting, or sexual selection, for example – is still debated. To study how their necks got so long, a team led by Melinda Danowitz, from the College of Osteopathic Medicine at the New York Institute of Technology (NYIT), analyzed and compared 71 vertebrae from 11 species in the family Giraffidae: nine extinct and two living members. These fossil bones were housed in museums around the world, and some were discovered over a century ago.
"It's interesting to note that that the lengthening was not consistent," study coauthor Nikos Solounias also of NYIT said in a statement. "First, only the front portion of the C3 vertebra lengthened in one group of species. The second stage was the elongation of the back portion of the C3 neck vertebra. The modern giraffe is the only species that underwent both stages, which is why it has a remarkably long neck."
On the left, fossil C3 vertebra of Samotherium, an extinct giraffid that lived seven million years ago. This species underwent the first stage of elongation, but not the second stage. On the right, the C3 vertebra of a modern giraffe, which underwent both stages of elongation. Nikos Solounias
The cranial (or top) end of the C3 vertebra stretched initially around 7 million years ago in an extinct species called Samotherium major, a relative of today’s giraffe. This was followed by a second stage of elongation at the caudal (or tail) end just a million years ago. The C3 of Samotherium and a modern giraffe are pictured above.
To their surprise, the team discovered that this elongation preceded Giraffidae. "We also found that the most primitive giraffe already started off with a slightly elongated neck," Danowitz said. "The lengthening started before the giraffe family was even created 16 million years ago." A possible giraffe ancestor called Prodremotherium elongatum was showing signs of lengthening 25 million years ago. That means neck length, the most distinguishing and popular attribute of Giraffa, the team writes, is apparently not a defining feature of the family. Furthermore, as the giraffe's neck was getting longer, the neck of another member of the giraffe family – the okapi of central Africa – was shortening.
An illustration of neck elongation and shortening within the giraffe family. Nikos Solounias and Melinda Danowitz