A few genes gave rise to many unique adaptations.
Its long neck and legs have elevated the giraffe to the status of world’s tallest living mammal. But its towering height poses several big challenges.
When giraffes raise their heads, all of the blood in their brains wants to rush downward. To avoid getting dizzy all the time, giraffes have developed a specialized circulatory system that keeps the blood in place when they straighten up. Their large, powerful hearts also help pump blood up the steep slope of the neck to reach the brain.
Their closest relative — the okapi — resembles a zebra, and lacks the giraffe’s impressive height and modifications that come along with it.
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Researchers in the United States and Tanzania decoded the genomes of the giraffe and the okapi to gain insight into the mystery of how the giraffe evolved its long neck and legs. Their study was published in the journal Nature Communications.
Along with giraffe and okapi genomes, the researchers looked at gene sequences of more than 40 other mammals, including sheep, cows, and humans, to help determine what genetic differences were associated with the giraffe’s unique features.
Genes involved in development and physiology tend to be very similar across different mammalian species, reflecting their conserved functions. However, the researchers found about 70 that were specific to giraffes. Two thirds of these genes were found to have roles in development and physiology, including aspects of the giraffe’s unique cardiovascular system. Four of the genes were related to development of the spine and legs.
“All of these genes in the giraffe — we have them ourselves. What made giraffes unique is just to tinker with them a bit and alter them in subtle ways,” study co-author Douglas Cavener, from Pennsylvania State University told Nature News.
Some of the identified genes are actually involved in regulating both skeletal and cardiovascular development, which could indicate that changes in only a small number of genes are driving many adaptations in the giraffe.
The researchers also considered how genes might be related to the giraffe’s unusual diet. Elevated stature enables giraffes to feed on acacia leaves, which are nutritious but also contain toxins.
Several of the identified genes were found to be involved in metabolism, as well as growth and cardiovascular function. This suggests that the “giraffe’s unique features may have co-evolved to elevate its stature, adapt its metabolism for more toxic food sources and adapt its cardiovascular and nervous system to the increased demands imposed by its unique morphology,” the authors wrote.
Though this study identified genes that are associated with many of the giraffe’s adaptations, the results don’t explain why these traits would have been favored by natural selection. The mystery of how the giraffe got its long neck remains unsolved — but we have some new clues to examine.
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