Kermit the frog used to sing that it was not easy to be green, but that's not the case with some real lizards. They seem so green that even their blood is green.
A study published on Tuesday suggests that this lime-green blood has evolved independently in lizards several times.
Scientists are now trying to understand how these lizards might look like benefit from blood that is green. The answer could provide new insights into human diseases such as jaundice and malaria.
The strange blood was found in Skinks, which live on New Guinea, an island off Australia, and whose bright color is striking. "There's so much green pigment in the blood that it overshadows the bright red color of the red blood cells," says Chris Austin, a biologist at Louisiana State University who has been studying these lizards for decades. "The bones are green, the muscles are green, the tissues green, the tongue and the mucosa green."
The whole green comes from high levels of biliverdin, a toxic waste product that develops during normal red blood cell depletion. In humans, large amounts of a similar bile pigment called bilirubin make people sick with jaundice, but the lizards seem to be untouched by it.
"I find it absolutely remarkable that you have this group of vertebrates, these lizards, Bilverdin, which would kill a human being, and yet they are out there to catch insects and live lizards," says Susan Perkins, researcher at the American Museum of Natural History in New York.
In the past, scientists had assumed that lizards with this green blood must belong to a closely related group. But these lizards do not look the same and have different lifestyles, some lay eggs and others give birth to live cubs.
So Austin, Perkins, and their colleague Zachary Rodriguez decided to study the DNA of 51 Australasian skink species, including six species that have green blood, by studying a kind of lizard family tree.
It turns out that the green-flowered lizards are not the closest relatives to each other, according to a report in the journal Science Advances
What the researchers found instead suggests that the ancestors of all lizards had red blood, and that green blood developed independently four times independently in different lines.
Data strongly support this scenario of four independent origins of green blood, "says Austin, adding that the team is doing sequential genetic work to confirm this.
If that turns out to be true, the question is why some lizards would find it easier to be green.
"There is really a fundamental purpose to this feature," says Perkins. "We just do not know exactly what it is."
Austin initially thought that but he has offered the green-headed lizards to the captive birds, and the birds have devoured them.
He also personally ate and discovered raw, red-blooded skinks and green-skinned skinks that both tasted the same taste of similar "bad sushi," says Austin.
Another potential benefit the researchers considered was that that all the extra green provided better camouflage in green leaves. But Rodriguez says that can not be. "The problem is that there are green-flowered lizards that are not green, and there are red-flowered lizards that are green," he explains.
Recently, scientists are wondering if lizard green blood could protect them from parasites like malaria – though Austin admits that this is "pretty speculative."
He notes that test tube experiments have shown that moderately elevated bilirubin levels protect against human malaria infection – and hundreds of malaria species have been known to infect lizards.
If the green blood protects against malaria, it is not perfect because they have found a malaria parasite that lives within a green-flowered lizard, says Perkins. Nevertheless, they continue to explore the relationship between malaria and green blood.
This basic research could one day lead to a better understanding of both this disease and jaundice, says Rodriguez. He notes that some fish have been found to have elevated biliverdin levels, and they may also explain the green blood of some frogs.
"It's rare in the animal world," says Rodriguez, "but it seems as if this suggests there must be some positive features for green blood."
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