How the giraffe got its long neck

They’re tall, leggy and can run as fast as a car. Now giraffes’ genetic secrets have been unravelled.

Researchers in the US, UK and Africa compared the giraffe genome with that of a series of other animals, highlighting the adaptations behind the giraffe’s extremely long neck and legs, as well as its incredibly strong heart and unique diet.

Giraffes can grow taller than six metres. Their heart pumps blood two metres up their necks to reach their lofty brain, thanks to an enlarged left ventricle and exceptionally high blood pressure – up to double that of other mammals. They can also sprint up to 60 kilometres an hour.

Douglas Cavener, a biologist at Penn State University and co-author of the study, says the mysteries behind the giraffe’s traits have been of interest to scientists for generations.

“The evolutionary changes required to build the giraffe’s imposing structure and to equip it with the necessary modifications for its high-speed sprinting and powerful cardiovascular functions have remained a source of scientific mystery since the 1800s, when Charles Darwin first puzzled over the giraffe’s evolutionary origins.”

The research team, led by Cavener and Morris Agaba of the Nelson Mandela African Institute for Science and Technology in Tanzania, compared the genetic make-up of the giraffe with its closest living cousin, the okapi, along with other mammals including cows, sheep, goats, camels and humans.

The team uncovered 70 genes which show signs of multiple adaptations within the giraffe, compared to the other animals, suggesting these are particularly responsible for the divergence in the giraffe’s characteristics.

The okapi, by comparison, is much shorter than the giraffe, with a stature more closely resembling a zebra. However, researchers say the giraffe has exhibited its singular traits, such as its height, for at least 8,000 years.

Some of the adapted genes highlighted by the study are responsible for skeletal development and cardiovascular performance, suggesting that the changes in a giraffe’s heart and bone structure may have evolved at the same time, all thanks to a few particular genes.

A giraffe’s neck and legs may be especially long, but they have the same numbers of bones as other animals. The researchers also spotted adaptations among genes that control the development of bones, which may account for leg and neck length in the giraffe.

“To achieve their extraordinary length, giraffe cervical vertebrae and leg bones have evolved to be greatly extended,” explains Cavener.

“At least two genes are required – one gene to specify the region of the skeleton to grow more and another gene to stimulate increased growth.

“The most intriguing of these genes is FGFRL1, which has a cluster of amino acid substitutions unique to giraffe that are located in the part of the protein that binds fibroblast growth factors – a family of regulators involved in regulating many processes including embryo development.”

This particular gene informs the development of bones at embryo stage and beyond birth. Humans and mice with defective genes suffer severe skeletal defects.

The researchers are now keen to test these genes in other contexts. The team has already begun testing FGFRL1 in mice to see how the gene may affect spine and leg development.

In terms of metabolism, the researchers also found adapted genes that control a giraffe’s levels of folic acid, which assists in growth and development, and genes involved in breaking down ingested plants. These may allow for the giraffe’s diet of acacia leaves, which are toxic to most animals.

The work was published in Nature Communications.