Kermit the Frog used to sing that it wasn't easy being green, but that isn't the case for some real-life lizards.
A few lizards have a unusual secret: they have lime-green blood pumping through their arteries. This is the only species of green-blooded skink that lays eggs. The answer could provide new insights into human illnesses like jaundice and malaria.
Prasinohaema prehensicauda, a green-blooded skinkCHRISTOPHER AUSTINSeveral species of New Guinea skinks, a type of lizard, are just as colorful inside as they are outside-bright green blood runs through their veins, an oddity among animals.
"There's so much green pigment in the blood that it overshadows the brilliant crimson coloration of red blood cells", coauthor Chris Austin, a biologist at Louisiana State University, tells NPR. This green blood makes their muscles, bones, tongues and the insides of their mouths green.
All that green comes from high levels of biliverdin, a toxic waste product made during the body's normal breakdown of red blood cells. And scientists have been trying hard to figure out what benefit this characteristic - caused by high levels of an ordinarily toxic green bile pigment - may give them.
"I find it just absolutely remarkable that you've got this group of vertebrates, these lizards, that have a level of biliverdin that would kill a human being, and yet they're out catching insects and living lizard lives", says Susan Perkins, a researcher at the American Museum of Natural History in NY.
Although all the green-blooded species have been assigned to a single genus, Prasinohaema, there were signs that they are not that closely related.
To understand the evolutionary history of the green-colored blood, Rodriguez and his colleagues did a genetic analysis of 51 species of skink, including six species that have green blood (two of which were previously unknown to science). Researchers report in Science Advances today (May 16) that green blood likely arose in lizard lineages four different times. To their surprise, the results show that green blood evolved independently on at least four separate occasions.
"There really is a fundamental goal of this trait", says Perkins.
We already know the broad mechanism.
Austin initially thought that the buildup of biliverdin might deter predators by making the lizards distasteful.
However, the green-blooded lizards do still get malaria.
What's more, he's personally eaten raw red-blooded skinks and green-blooded skinks, and found that both tasted about the same-kind of like "bad sushi", says Austin. But Rodriguez says, that can't be it.
In humans, blood is red due to the presence of hemoglobin molecules.
Lately the scientists have been wondering if the lizards' green blood might protect them from parasites like malaria - although Austin admits that this is "pretty speculative".