1. Why Is a Giraffe's Neck So Long If It Has the Same Number of Neck Bones?
When you look up at a giraffe in a zoo, the first thing you notice is the length of its neck. You might think it must have many more neck bones than other animals. However, the giraffe has only 7 cervical vertebrae, just like humans [1]. A study in 2016 reported that the giraffe’s extreme neck length is not because it has more bones. Instead, the 7 vertebrae that all mammals have grew longer in the vertical direction [2].
Since the number is the same, but each bone is longer, the next question is: what internal changes made these 7 bones grow? The clue lies in comparing the giraffe with a short-necked relative.
2. Comparing the Genomes of Giraffes and Their Short-Necked Relative, the Okapi
To find clues about neck length, researchers looked at the okapi, the closest living relative of the giraffe. Okapis have short necks and are not tall. It is thought that the two species split from a common ancestor about 11.5 million years ago (estimated between 11 and 12 million years ago) [2]. The okapi is not an ancestor of the giraffe, but a cousin.
In 2016, scientists decoded and compared the genomes (the full set of genetic information) of two female Masai giraffes and one male okapi for the first time [4]. They found 70 genes that showed notable changes in the giraffe [3]. More than half of these genes were blueprints for proteins involved in the development and function of the skeleton, heart, blood vessels, and nerves [3]. This means the changes were not just in bones. Genes related to pumping blood and nerves were also involved. Reports suggest that the giraffe’s body shape may have evolved in parallel with adaptations in its circulatory system through changes in a small number of genes [4]. However, not all 70 genes are necessarily responsible for neck length.
3. Which Bone-Growth Genes Changed Significantly in Giraffes?
Among the genes with notable changes were members of the homeobox gene family, which are involved in the growth of neck and leg bones. Genes such as HOXB3, CDX4, and NOTO were reported to differ markedly in giraffes compared to other mammals [2]. You do not need to memorize the names. The important point is that genes related to bone growth changed noticeably in giraffes.
However, these are only "candidates" for the cause of the long neck. It has not been proven that these genes cause the long neck. Once candidates are found, the next question is: when and how does the neck grow?
4. When Does the Giraffe's Neck Grow So Long? Evidence That It Happens After Birth
When does the neck grow? A paper from 2010 reported that the extreme lengthening of the cervical vertebrae happens after birth, not during fetal development [5]. It also states that the second through seventh cervical vertebrae (C2 to C7) lengthen by the same proportion [5]. This means that instead of one specific bone growing unusually long, they all lengthen together at the same ratio.
It is important to note that this is a report from one paper. It is too early to say "the neck is short at birth." What can be said is that the extreme growth happens after birth. During this growth, the blood vessels and nerves that send blood to the head must also keep up with the neck’s length. The fact that many of the 70 changed genes are related to blood vessels and nerves aligns with this view [3]. Have any of these genes been tested for their actual function?
5. What Happened to Mice That Were Given Giraffe-Type FGFRL1 Genes?
The gene FGFRL1 had a concentration of 7 amino acid replacements unique to giraffes. These changes are not found in other ruminants [6]. In 2021, researchers created mice with the giraffe-type FGFRL1. These mice had significantly higher bone density. Even when given a drug that raises blood pressure (angiotensin II), they did not develop high blood pressure [6].
This experiment showed effects on bones and blood pressure. It was not an experiment to see if the neck would grow. A Penn State explanation notes that putting in the giraffe FGFRL1 does not result in mice with long necks [3]. Still, the fact that changes in one gene affect both bones and blood pressure aligns with the view that body shape and the circulatory system changed in parallel [4].
6. Why Genes Alone Cannot Fully Explain a Giraffe's Long Neck
Did FGFRL1 make the neck long? The researchers themselves state that mutations in this gene alone are not enough to explain the neck’s elongation. They also mention the involvement of other genes and the limitations of putting giraffe genes into other species [6]. A Penn State explanation introduces the researchers’ speculation that changes in homeobox genes and FGFRL1 might be two "ingredients" for making necks and legs longer [3]. This is speculation, not proof.
Let’s separate the questions. This article asks, "What changed inside the body?" The question "Why did it evolve to be long?" is different. There are theories that it is to eat leaves higher up than competing herbivores, and theories that males use their necks to fight. A 2010 review suggests that both mechanisms likely played a role [7]. The answers to both the mechanism and the reason are still in progress. The full set of genes that determine neck length is not yet known.
7. Try It Yourself: Compare an Okapi and a Giraffe and Count Three Differences
Look at okapis and giraffes in an encyclopedia at home. Compare their necks, legs, and height. Researchers who compared genomes looked at these two animals. Imagining how visible differences connect to the differences in 70 genes makes the scientific words feel closer.
You can also gently touch the back of your own neck. Like the giraffe’s neck, the human neck also has 7 bones lined up [1]. Please touch it gently and only within a range that does not hurt. Keep in mind how close the next discovered gene might bring us to the answer about neck length.