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New Research Helps Explain Why Giraffes Have Such Long Necks

Scientists sequenced the giraffe genome for the first time ever.

by Kaleigh Rogers
May 17 2016, 3:00pm

Image: Sadie Hart/Flickr

It goes without saying that giraffes don't really look like any other animal on the planet. Their stilt-like legs, geometric spots, and iconic neck make them one of the most physically-unique species on Earth. Under the surface, the giraffe is even more distinct, with a super-powerful heart and high blood pressure in order to pump blood all the way up its six-foot neck to its brain.

Now, for the first time ever, scientists have sequenced the giraffe's genome, an endeavor which offers clues into how the animal evolved these distinctive characteristics.

We still don't really know why the giraffe evolved its long neck and legs. A number of theories have circulated since Darwin's day, ranging from being able to reach higher food sources during drought periods, to sex selection (i.e. size matters to lady giraffes). But part of the answer to that question lies in how the giraffe evolved these attributes, a puzzle partially solved by sequencing the giraffe's genome.

To suss out which genes are involved in the giraffe's long neck, powerful heart, and gorgeous gams, a group of scientists from universities in the US and Africa compared the giraffe's genome to more than 40 other mammals, according to a paper published Tuesday in Nature Communications. The most illuminating comparison was between the giraffe and its closest relative, and only other living member of the Giraffidae family, the okapi:

An okapi. Image: Eric Killby/Flickr

As you can see, the okapi has a kind of long neck, but not, like, a giraffe-long neck, despite the fact that these two species diverged only 11 to 12 million years ago and share about 19 percent identical genes. By comparing the two, the researchers were able to identify 70 different genes that show multiple sign of adaptation in the giraffe that aren't adapted in the okapi or other mammals.

Unsurprisingly, many of these genes are associated with regulating skeletal, cardiovascular, and neural development, indicating these gene mutations are partly responsible for the giraffe's unique form. In fact, the researchers found that two-thirds of the divergent genes in giraffes are related to either skeletal, cardiovascular, or neural development, and sometimes all three.

But there were some surprises in the paper as well, like that overlap between development areas, which means a handful of mutations could have helped the evolution of different characteristics occur in tandem, a process the researchers estimate occurred over a 15-20 million year period.

"As its neck extended out, its cardiovascular system was also changing in tandem and some of the same genes were actually controlling both processes in concert," Douglas R. Cavener, a professor of biology at Penn State University and co-author of the paper, said in a video.

Image: Nature Communications

The researchers also discovered a number of genes that regulate metabolic function, including an improved ability to digest acacia leaves and seedpods—plants that make up a big part of the giraffe's diet, but can be toxic to other mammals. This, according to the paper, lends evidence to the theory that giraffes evolved these characteristic as an adaptive evolutionary response, to access more food, since so many of the attributes were developing in tandem.

We're still not finished with puzzling over why giraffes necks are so long, and some of the paper's authors have already started experiments injecting one of the gene mutations into mice using CRISPR/Cas9, according to a press release, to determine exactly what role it plays. But at least we have a better understanding of how it got to this point.

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