With its lanky legs and towering neck, the giraffe is a record-breaker: At 4.5 to 5.7 meters, it’s the tallest land animal on the planet. Even newborn calves are giants by human standards, entering the world at about 2 meters tall. Now, scientists have sequenced the genome of the giraffe—and that of its close cousin the okapi—to unravel the genetic mysteries behind the animal’s unique physique.
To pump blood all the way up to the lofty perch of its brain, a giraffe’s heart has an enlarged left ventricle, and its blood pressure is 2.5 times greater than a human’s. Okapi, a horse-sized herbivore native to central Africa’s tropical forests, don’t share the giraffe’s long neck or cardiovascular power. These key physical differences, in contrast with their close genetic ties—the two are the only currently living members of the Giraffidae family—make the okapi genome “the perfect screen for identifying things that were unique in the giraffe,” says molecular biologist and study co-author Douglas Cavener of Pennsylvania State University, University Park.
Cavener and an international team of colleagues sequenced the whole genome of two Masai giraffes—a subspecies of giraffe—and a single okapi. Then, they compared the protein-coding regions of the giraffe and okapi genomes to see how they differed. Next, the scientists compared the giraffe and okapi genes with several dozen other animals like cattle and mice, to get a better picture of which genetic differences separate them from other mammals. There are nine subspecies of giraffe, so to be sure that the giraffe variations were common within the species, the researchers also sequenced and compared several genes from two other giraffe subspecies.
The scientists identified 70 genes with variations unique to giraffes, they report online today in Nature Communications. About half the genes are known to regulate growth and development, including of the skeleton and the circulatory system, “the two fundamental things that really change in [the] giraffe,” Cavener says. The scientists speculate that some of the genes might work together to elongate each vertebra in a giraffe’s neck—though much longer than ours, they number only seven, just like in humans.
A mutation of one of the 70 genes—its function in giraffes still unknown—causes defects to both bone and cardiovascular development in humans and mice. That means the same genes may be responsible for more than one of the giraffe’s unusual attributes, the researchers say.
The next step, says evolutionary geneticist Hopi Hoekstra of Harvard University, who was not involved in the study, is to run laboratory tests to see whether the giraffe gene variants produce the physical differences that the researchers predict. That could be done in mice, for example, a study Cavener says is in the works. Until the scientists also inspect the non–protein coding regions of the giraffe genome—areas with regulatory or unknown functions—“they’re still a far way away from discovering everything that makes a giraffe a giraffe,” Hoekstra adds. Still, comparing the genomes of two creatures that are as closely related but as physiologically different as the giraffe and okapi was a novel choice, she says.
Researchers may have to act fast if they want to continue solving the giraffe’s genetic riddles. The creature’s numbers are plummeting across Africa—40% of the population has been lost since 2000, the study authors note, thanks to hunting and habitat loss. “It would be a travesty to lose this magnificent animal when we are just beginning to understand its genetic code,” says quantitative ecologist Derek Lee of the Wild Nature Institute in Weaverville, North Carolina.