Osteology
Bonn (1823), Der Skelete der Wanderkauer.

Something odd has been happening this week on Facebook. Person after person have been changing their profile picture to one of a giraffe. I have a passing interest in giraffe (my PhD concerns reconstructing aspects of their evolution). On the surface, giraffe are a daft mixture of ridiculousness and grace. Dig a just few millimetres deeper and you’ll find that they are a sack of intrigue and mystery.

One of the enduring questions is that of their unusual neck. Brian Switek has written a cool post on the why their neck may have evolved. I’d like to bring forward some ideas on the anatomy. Here is the riddle:  how many neck bones do giraffe have? When I say neck bone, I’m talking about neck vertebrae.

The usual answer you’ll hear is that they have the same number as us, or any other mammal for that matter. The actual answer to the question? We’re not sure. This is why science is great; it is totally ok to be unsure.

Spoiler – giraffe have the same number of neck vertebrae as we do. Seven. But a better answer would be sevenish. They blur the definition of what constitutes a neck vertebra. As a side note, there are some mammals out there that definitely do have different numbers of neck vertebrae than the standard seven.

The last of the neck vertebrae in most of us (called Cervical 7, or C7) represents a junction between our neck and our chest. Because of this, C7 is usually quite unique and specialised in terms of its shape and the job it performs. In contrast, most of the other cervical vertebrae are repeated copies of eachother; running from C3 to C6. The first chest vertebra (Thoracic 1, or T1) is defined at its most basic level as the first vertebra to be attached to a pair of ribs. There are a few other T1 norms, defined by factors such as shape and position relative to other objects like nerves and blood vessels.

As long ago as 1908, Sir Ray Lankester (seen here in true early 1900’s anatomist style) noted that there was something odd going on with C7 in giraffe. In most mammals C7 has some features that resemble a transition from the flattened neck vertebrae to the tall and spiney thoracic vertebrae. In giraffe though, this transition state doesn’t really happen. In fact C7 looks more similar to the repetitive C3-C6 series.

Nikos Solounias in 1999 expanded this point by arguing that perhaps the first thoracic vertebra is in fact really a neck vertebra. What followed on was a massive backbone identity crisis. The argument for this was a systematic and logical construct. Many of the attributes we’d expect to find in C7, we actually find instead in the next vertebra down.

The implication is this: the shapes and functions of the neck vertebrae in giraffe are similar as in other mammals, but have been shifted back by one vertebra. C7 is usually a transitional neck to chest bone, but in the case of giraffe, this transitional duty has been waived so it can act the same as one if its C3-C6 friends. This essentially gives giraffe an extra neck vertebra. But I can’t escape one basic truth.

T1 in giraffes has a pair of ribs attached. At the most fundamental definition this makes it pretty impossible for T1 to be a true neck bone. So giraffe get no extra vertebrae for Christmas. But what’s clear is that there is definitely some blurring and bending of the rules of what features define a neck vs thoracic vertebrae.

The rebuilding of the neck to its modern state was a huge undertaking; there must have been some serious evolutionary drive to push giraffe this far. What’s left for us in the present day is to marvel at the engineering behind the anatomy, or watch the brutality of how males use these as weapons in nature. Or perhaps to simply change our profile picture.

Chris.

Lankester, R. (1908). “On certain points in the structure of the cervical vertebrae of the okapi and giraffe” Proceedings of the Zoological Society of London 1908:320-334 http://www.biodiversitylibrary.org/item/97671

Solounias, N. (1999). “The remarkable anatomy of the giraffe’s neck.” Journal of Zoology 247(2): 257-268.

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