Why Does a Giraffe’s Long Neck Have the Same Number of Bones as a Human Neck?

A Giraffe Has Seven Neck Bones, and Each One Is Very Long

The bones of the neck are called cervical vertebrae. A giraffe has seven of them, the same as a human. A University of Tokyo announcement about giraffe research puts it directly: the giraffe’s neck is far longer than that of most mammals, but its count is seven, like ours [1].

The difference is the size of each bone. The same announcement says that in a fully grown giraffe a single neck bone can be as long as 30 centimeters (about a foot) [1]. That is a ruler’s length for one bone. The wording is “can be,” so it does not mean all seven reach that length.

A short-necked relative gives the same count. The okapi, a forest animal and the giraffe’s only living close cousin, has a neck that looks like an ordinary hoofed animal’s. A 2015 study that measured the neck bones of both species reports seven in each, and describes the giraffe as having the most extreme neck stretching of any ruminant (the group of hoofed animals that includes cattle and deer) [2]. So the two animals differ in how long the bones are, not in how many.

Birds Made Long Necks by Adding Bones, but Giraffes Stretched the Ones They Had

Adding bones is a real way to build a long neck. A team that read the giraffe’s genome, the full set of its genetic instructions, noted that long-necked birds got their length that way. The giraffe went a different route: the neck is long because each of the seven standard mammal neck bones was made longer [3].

How the stretching happens is only partly understood. The genome study says the lengthening is probably due to longer “somites,” the blocks of tissue in a young embryo that later turn into the neck bones, and that it stays in the neck region because of how a group of genes called Hox genes work together. The authors found unusual changes in several genes in giraffes, but finding a change is not the same as showing it set the length, so this part is a well-informed guess [3].

One way to picture the difference is a train. A bird lengthens its train by adding cars. A giraffe keeps seven cars and makes each one much longer. Real bones are not train cars, of course, because they are linked together so the neck can bend. It is only a picture to hold on to.

Almost Every Mammal Has Seven Neck Bones, Whatever the Length of Its Neck

The giraffe is not the odd one out. The giraffe is following a rule. A scientific review describes the seven neck bones of mammals as remarkably constant in number, no matter how long the neck is [5]. People, mice, giraffes and, as far as these studies say, almost all other mammals share it. Birds and reptiles vary a lot more, so mammals are the unusual group, not the giraffe.

Why would a number stay put for so long? In 1999 the biologist Frietson Galis suggested an answer. Changes in the number of neck bones are linked to changes in Hox gene activity, which also help shape the nervous system and affect how cells multiply. Her proposal was that such changes tend to come with serious health problems, including nerve problems, a higher risk of childhood cancer, and stillbirth, so animals with a different number rarely thrive and pass it on [4]. A later review pointed to the same idea and cited evidence that changed counts are quite common at the start of human development but are strongly weeded out before birth or soon after [5].

This is a hypothesis, a leading explanation rather than a proven fact. It also says nothing about any person: having a neck count of seven is normal, and the idea is about the long timescale of evolution, not about anyone’s health today.

Sloths and Manatees Break the Rule, and Slow Lives May Be the Reason

Two groups of mammals do not have seven: sloths and manatees [5]. For sloths the number depends on the kind. Two-toed sloths tend to have fewer than seven and three-toed sloths more. Counts in the studies range from about five to ten, with the exact numbers differing from one study to the next, so here is only the range [5] [6].

One idea for why these animals could get away with it is their slow lifestyle. The review explains that a low metabolic rate (a slow burning of energy) goes with less damage to DNA and, presumably, less cancer. If a changed number of neck bones is risky mostly because of illness, the risk may matter less for animals that live slowly [5]. The word “presumably” is in the source, and it is the right word: this is a likely explanation, not a settled one.

Even in sloths the neck keeps some of its usual pattern. A study of sloth skeletons found that the neck bones still form three groups, with the borders between them shifting as the count changes [6].

The Giraffe’s First Chest Bone Moves Like an Extra Neck Bone

Some people say giraffes have eight neck bones, and there is a small truth in that. In 2016 Megu Gunji and Hideki Endo of the University of Tokyo studied giraffe bodies and skeletons. They found that the first bone of the chest, the one right behind the seventh neck bone, is shaped much like a neck bone in general hoofed animals and bends up and down more freely than the other chest bones. A change in where one neck muscle attaches also lets this bone take part in neck movement [7].

The University of Tokyo’s article about the study says this bone helps the giraffe reach a space about 50 centimeters (about 20 inches) larger at the top end of the neck, and suggests it helps with eating from high branches and drinking low down [8]. The paper itself says only that it may help with high browsing or with fights between males [7].

So should the giraffe count as having eight? The paper notes that an earlier study had raised that possibility. But this bone carries movable ribs, which is why scientists classify it as a chest bone, and the question has stayed disputed [7]. A fair way to put it is that the giraffe has seven neck bones by the usual count, and one chest bone that works like a neck bone. What a bone is called and what it does are two different questions.

Try It: Count the Bumps at the Back of Your Own Neck, and Compare Skeletons

Sit comfortably and tip your head gently forward. With your fingertips, feel down the middle of the back of your neck, using light touch only, and notice the row of small bumps. Do not press hard, twist, or pull. Now compare your short neck with a giraffe’s much longer one: your seven neck bones fit into a short neck, while a giraffe’s seven stretch across a neck much longer than a person’s.

If you have an animal encyclopedia or can look up skeleton photos from a museum website, pick a few animals, such as a mouse, a horse, a giraffe, and a person. Find where the neck ends and the chest begins in each skeleton. Does each neck have the same number of bones? Do any of the long ones look as if they are stretched, and where do you think a chest bone might be helping out?

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