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Any member of the five species of the Prasinohaema (Greek: “green blood”) skinks that live on the island of New Guinea in the South West Pacific, would have been a shoo-in for J.K. Rowling’s wildly imaginative Harry Potter book series. That’s because besides being the only known land vertebrate to have “vivid lime green” blood, the reptiles also sport green bones, green muscles, and even a green tongue!
The unusual lizards first caught the attention of Louisiana State University biologist and National Geographic explorer, Christopher Austin in 2013 when he was still a graduate student. The researcher was so intrigued by the green-blooded reptiles that he decided to study them further. To his surprise, the scientist discovered that the dark green color is caused by the presence of incredibly high levels of toxic biliverdin in the lizard’s blood.
The bile pigment, which is responsible for the greenish color that you sometimes see in bruises, is a by-product of used hemoglobin. The red protein transports oxygen from the lungs to the body's tissues and carries carbon dioxide from the tissues back to the lungs. When hemoglobin starts to decompose, it is sent to the liver to be broken down into biliverdin and other by-products. These are then immediately channeled to the intestines to be excreted. A high level of biliverdin in the bloodstream only occurs in people whose livers are damaged or in newborns whose livers are still not breaking down old hemoglobin. If left untreated, the excess biliverdin can prove deadly for humans.
However, in the case of the green skinks, the biliverdin does not get removed. Instead, the toxic substance accumulates in the blood stream to such high levels that it even masks the color of the red hemoglobin. While that much biliverdin would kill a human, it doesn't seem to bother the lizards at all.
So why did these reptiles’ blood evolve to this vivid shade of green? That is a question Austin, and graduate student Zachary Rodriguez, are still trying to answer. One theory proposed that the toxic blood might prevent predators from eating the lizards. But this turned out to be incorrect since birds, snakes, and cats appear to enjoy eating the green-blooded lizards, biliverdin notwithstanding. Other popular theories suggest that the color may help shield against cell damage caused by ultraviolet solar rays or provide enhanced camouflage in the foliage.
Austin speculates that the biliverdin may be protecting the lizards from Plasmodium, a parasite that causes malaria in humans, reptiles, and birds. However, to find out for sure, the LSU scientists have to first determine where the legacy of the green blood came from. To accomplish this, they are creating a genealogical tree of the lizards and its relatives. The researchers think this will provide valuable insights into the specific lifestyle, environment, or potential threats, that made the reptiles turn green.
So far, however, their research seems to be raising more questions than answers. For example, the scientists recently discovered that the five species of the biliverdin-tinted lizards are not each other’s closest relatives, as one would expect. They instead appear to have more in common with certain species of red-blooded lizards. This has further deepened the mystery of the evolutionary process of these super-cool lizards, and made the researchers even more determined to unlock their genetic secrets.
Resources: nationalgeographic.com, sciencenews.org, io9gizmondo.com