Biologists have long debated whether giraffes extended their necks first and then evolved the heart adaptations to counter the difficulties posed by the long necks.
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Being the world’s tallest animal comes at a price. The giraffe’s towering height and large leopard-like spots make it an iconic animal of the African savanna. Even as other ruminants crane their necks to reach the lower branches of trees, this lanky animal takes advantage of its height to browse on tree crowns.
How does its heart pump blood up to its brain? Blood has to fight gravity to course up the two-metre-long neck to reach the brain. For years, researchers puzzled over how the giraffe achieved this. A 1987 study discovered its muscular heart pumps blood with such force that the animal’s blood pressure is two and half times greater than ours. While this works as long as the giraffe stands upright, fulfilling an everyday need – drinking water – can turn into a life-threatening situation.
To reach water, the giraffe splays its front legs and lowers its head. Hypertension could send blood surging into the brain and the animal could collapse from a stroke. If it survived this and slaked its thirst, it faces another danger. Lifting its head would cause pressure to drop and the giraffe to faint. It has a neat work-around: The jugular vein locks off blood to the head when the animal bends down. And on lifting the head, the vein opens up again.
As anyone suffering from high blood pressure knows, it causes the legs to swell with fluid, a condition called edema. Yet, giraffes have long spindly legs. Their bodies have a couple of tricks to solve this problem. The arteries below the knees have thick walls to bear the pressure, and their hide wraps tightly around their bodies, giving no space for any fluid to collect.
The main cause of disease and failure of kidneys in humans is hypertension. But giraffe kidneys show no signs of damage. The renal capsule, a tough layer of fibrous tissue that envelopes the kidneys, is strong enough to withstand the high blood pressure. The curious thing is: giraffes aren’t born with hypertension. They develop it as they grow and their necks begin extending.
Besides challenging the cardiovascular and renal systems, the long neck also taxes the skeleton and muscles. Nineteen bones support the long necks of flamingos. But the giraffe has the same number of vertebral bones as we have: seven. What it lacks in quantity, it makes up in size. Each vertebra of an adult giraffe is almost a foot long. And yet, the neck is flexible enough that it twists its long neck around and rests it on its rump when asleep.
The nuchal ligament is a band of elastic tissue running down the length of the neck. In giraffes, it is much larger and tougher, the better to hold up the weight of the head and neck. All these adaptations are common to all the nine subspecies of giraffes, even though they separated from each other two million years ago.
What are the genetic underpinnings of these adaptations? How different is the genome of this unique creature compared to other ruminants? A group of molecular biologists led by Douglas R. Cavener from Penn State University, US and Morris Agaba from the Nelson Mandela African Institute of Science and Technology, Tanzania, investigated. The 16-member team sequenced the genes of two Masai giraffes from Masai Mara, Kenya, and an okapi foetus from the White Oak Conservation Center, US. The researchers scoured through the genomes looking for genetic clues.
The okapi is the giraffe’s closest and only surviving relative but there’s little family resemblance. It has none of the giraffe’s looks – no long neck or leopard spots. The reddish brown animal resembles an antelope with the striped legs of a zebra. Biologists think their common ancestor may have had a neck that was longer than an okapi’s but shorter than a giraffe’s.
Cattle ancestors separated from giraffe ancestors 28 million years ago. So the researchers compared giraffe and okapi genes with cattle genes to identify areas of difference. Then by matching the genes of the okapi and the giraffe, the researchers narrowed down the genes specific to giraffes. They estimate the giraffe grew tall and evolved a “turbocharged” heart within the last 11.5 million years, when its lineage split from the okapi’s.
The researchers made a surprise discovery: Of the thousands of genes, no more than 70 genes with multiple adaptations make the giraffe the unique animal it is. Several of these regulate skeletal, cardiovascular, and neural development while others affect metabolism and growth. Four genes may govern the development of the spine and legs, while eight are likely to administer heart functions. The researchers also identified the genes that allow giraffes to digest the nutritious but toxic leaves of the acacia. The genes that control this metabolism also appear to affect the heart.
Biologists have long debated whether giraffes extended their necks first and then evolved the heart adaptations to counter the difficulties posed by the long necks. Since many genes affect more than one function, the researchers suggest the changes didn’t occur independent of each other but in tandem. For instance, one particular gene, FGFRL1, regulates the growth of the skeleton and the cardiovascular system.
The real surprise was the many disparate functions performed by a gene. “That genes involved in mitochondrial function might be connected even remotely to the physiology was quite astounding,” says lead author Morris Agaba. “Mitochondria is supposed to do one thing: produce energy for the cells. Why would mitochondrial genes be involved? Hidden in the complex names and symbols that scientists give to genes and proteins are two genes, FOLR1 and MTHFD1. These two genes are involved in the metabolism of Vitamin B9 that’s mainly found in leafy shrubs. Why would these genes standout in the giraffe and okapi?”
Mutation in FOLR1 is lethal to mice embryos. In humans, it causes defects to develop in myelin, the white sheath protecting the spinal cord, brain, and other nerves, a condition called hypomyelination. Myelin plays a major role in transmission of nerve impulses. In giraffes, the mutations in this gene appear to act in concert with other genes to drive the growth of their unique adaptations.
Mutations in another gene, MDC1, kill defective cells and protect giraffes and okapis from cancer. This gene “exhibits the most radical evolutionary change in giraffe and okapi compared with all other vertebrates,” write the researchers.
“Clearly the unique physiognomy and physiology must have at least some of its background in gene variability, so in this respect it is probably difficult to say that the results are extremely surprising,” says Christian Aalkjær of Aarhus University, Denmark. He wasn’t involved in this study.
While picking through the genes and finding amino acids indicate how giraffes get their long necks, further studies have to confirm their functions. “I would love to see the core long neck gene set refined,” says Agaba, “and if possible transplant the evolution process, so to speak, into a candidate model antelope as proof. We have opened a can of genes, and I believe that something good and useful will come out that may advance our knowledge of nature, our health and that of our ecosystems on a fragile earth.”
The team writes studying the giraffe’s adaptations would provide insights for the treatment of cardiovascular disease and hypertension in humans. The study was published in the journal Nature Communications on May 18, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.