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How the Giraffe Got Its Long Neck

by on May 19, 2016 10:30 AM

How the giraffe's long neck and tremendous height evolved has been a mystery to scientists since Charles Darwin considered the species' development in the 1800s. It was a source of fascination for thousands of years before that, historical discoveries have shown.

Now research led by Penn State has begun to unravel that mystery. The genomes of the giraffe and its closest living relative, the okapi found in the African rainforest, have been sequenced for the first time, uncovering the first clues about how the giraffe's most-distinctive features evolved.

"Okapi’s gene sequences are very similar to the giraffe’s because the okapi and giraffe diverged from a common ancestor only 11-to-12 million years ago — relatively recently on an evolution timescale," said Penn State professor of biology Douglas Cavener, who led the research team with Morris Agaba of the Nelson Mandela African Institute for Science and Technology in Tanzania.. "In spite of this close evolutionary relationship, the okapi looks more like a zebra and it lacks the giraffe’s imposing height and impressive cardiovascular capabilities. For these two reasons, Okapi's genome sequence provides a powerful screen that we have used to identify some of the giraffe's unique genetic changes."

The researchers compared gene-coding sequences for the giraffe and okapi to more than 40 other mammals, including humans, to identify the genetic changes responsible for the giraffe's distinctive characteristics, which in addition to its height and neck length include the ability to sprint up to 37 miles per hour.

They found 70 genes that showed signs of adaptation, more than half of which code for proteins which regulate the skeletal, cardiovascular and nervous systems. These are the types of genes the researchers predicted would be necessary for the giraffe's unique development.

Because of the way the giraffe has evolved, its heart has to pump blood two meters vertically to provide enough blood to the brain, which is possible because its heart has an especially large left ventricle. It also has blood pressure twice as high as other mammals.

Several genes that showed signs of adaptation regulate development of the cardiovascular system or control blood pressure. Some of these control cardiovascular and skeletal development, suggesting that both the giraffe's height and unique cardiovascular system evolved together through changes in a few genes.

The giraffe's long neck and legs -- which have about the same number of bones as the legs and necks of humans and other mammals -- evolved to have extended vertebrae and bones. 

"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," Cavener said. The researchers found among the 70 identified genes those that are known to regulate both of those functions.

Cavener and Agaba say they next want to test the function of some of the identified genes that they believe may be responsible for the giraffe’s unique characteristics. 

"We hope that the publication of the giraffe genome and clues to its unique biology will draw attention to this species in light of the recent precipitous decline in giraffe populations," Cavener said. "While the plight of the elephant — giraffe’s shorter companion in the African savannah — has received the lions share of attention, giraffe populations have declined by 40 percent over the past 15 years due to poaching and habitat loss. At this rate of decline, the number of giraffes in the wild will fall below 10,000 by the end of this century. Some giraffe subspecies already are teetering on the edge of extinction."

Geoff Rushton is managing editor for Contact him at [email protected] or find him on Twitter at @geoffrushton.
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