Researchers for the first time have sequenced the genomes of the giraffe and the okapi – the squatter, short-necked member of the Giraffidae family. Their findings, described in Nature Communications this week, may help explain how giraffes got their legendarily long necks.
To be the world’s tallest animal, giraffes must possess a cardiovascular system that maintains stable blood pressure, a muscle and skeletal system to support body mass that’s elongated vertically, and a nervous system that can rapidly relay signals over long networks of neurons. Over evolutionary time, giraffes have evolved a turbocharged heart that can pump blood 2 meters (6.5 feet) vertically from the heart to the brain.
To withstand this high blood pressure (twice that of other mammals), blood vessel walls in the lower extremities are thickened. Meanwhile, vessels elsewhere are uniquely adapted to deal with pressure changes whenever the animal lowers its head to drink water at a pond. And a large, strong ligament along the back helps sustain the weighty neck.
There are nine known subspecies of giraffes, and their closest relatives are okapis (Okapia johnstoni). The two diverged around 11.5 million years ago.
Okapi. Raul654 CC-BY-SA 3.0
Now, a large international team led by Douglas Cavener of Penn State University has sequenced the whole genomes of two female Masai giraffes (Giraffa camelopardalis tippelskirchi) from Kenya’s Masai Mara reserve and the Nashville Zoo and one fetal male okapi from the White Oak Conservatory. They also compared these gene sequences to that of other mammals, including cows, dogs, and humans.
The team discovered 70 genes with multiple signs of adaptation in giraffes. Several of them code for known regulators of skeletal and cardiovascular development, suggesting that the giraffe’s stature and cardiovascular adaptations co-evolved through changes in a small number of genes.
"To achieve their extraordinary length, giraffe cervical vertebrae and leg bones have evolved to be greatly extended," Cavener explained in a statement. Despite being so much taller, giraffes actually have the same number of neck bones as we do. According to 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."
A gene called FGFRL1 is known to regulate both of these functions, and mutations in this gene lead to skeletal and cardiovascular defects in humans and mice. Three other genes involved in the development of body structures – HOXB3, CDX4, and NOTO – show significant changes in giraffes compared with other mammals.
Additionally, genes for metabolism may help explain the giraffe’s unusual diet of acacia leaves and seedpods. While highly nutritious, these also contain toxins.
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