Why do giraffes have long necks? It’s a question that has puzzled both schoolchildren and evolutionary scientists since Charles Darwin first set out his theory of evolution in the 1800s. Now, new research published in Nature Communications has shed some light on the lofty question by comparing the animal’s genes with its closest relative, the reclusive African okapi.
Giraffes can reach around six metres in height and need a huge left ventricle in their heart to pump blood a full two metres up to their brain, making their blood pressure twice that of other mammals. The okapi on the other hand is a rather more normal 1.5m tall and at first glance look more like a zebra, with distinctive black and white stripes on their legs. By using genetic sequencing to compare the two animals with that of more than 40 other mammals, the team from Penn State University and the Nelson Mandela African Institute for Science and Technology in Tanzania were able to find 70 genes that were responsible for the adaptations in the giraffes which made them differ from the okapi.
“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,” says Douglas Cavener, who led the research team. “Okapi’s genome sequence provides a powerful screen that we have used to identify some of the giraffe’s unique genetic changes.”
Of these 70 genetic differences more than half of them were responsible for the development of skeletal, cardiovascular and nervous systems, which are all vital in understanding the differences between the giraffe and okapi.
“To achieve their extraordinary length, giraffe cervical vertebrae and leg bones have evolved to be greatly extended,” Cavener says. “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.” Both of these genes were discovered among the clutch of differences found.
Another interesting find is the gene that helps the giraffe metabolise the volatile fatty acids produced when food ferments in its stomach. The highly nutritious but toxic acacia leaves and seedpods that form the main part of a giraffe’s diet are normally toxic to other animals, but a gene was discovered that might have helped the giraffe evolve a resistance to the toxins.
Next up for the team is to use CRISPR technology to introduce the gene that is responsible for the giraffe’s long neck into mice to test how it affects growth and bone development. Although we’re not expecting to see long-necked mice anytime soon, the results of the study will be available on the Giraffe Genome Project website.
“Giraffe populations have declined by 40 per cent over the past 15 years due to poaching and habitat loss,” says Cavener. “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.
“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.”