Lurking quietly in the deep dark jungles of New Guinea are a group of lizards who share a rather striking feature: green blood. It's a rare trait for vertebrates to have, but new insights into this strange blood could lead to innovative medical treatments.
According to Star Trek lore, Spock's blood was green because it was copper-based and not iron-based, but as new research published today in Science Advances shows, green blood is a bit more complicated in real life. For Prasinohaema lizards, a group of skinks native to New Guinea, their green blood is due to a high concentration of green bile pigments, which also gives their muscles, bones, and tongue their bright, lime-green appearance.
The green bile produced by these lizards contains high levels of biliverdin — the same pigment that sometimes give bruises their bluish-green appearance (fun fact: it also gives some bird eggs their blue and green colour). In humans, an overabundance of biliverdin in the circulatory system and tissues triggers jaundice, a medical condition that causes yellowing of the skin and liver malfunction. But Prasinohaema lizards are packed with the stuff — and at levels 40 times the lethal dose for humans. It's also deadly when present in the circulatory system of other vertebrates. But for reasons that scientists can't quite explain, the green-blooded skinks remain perfectly healthy - no cellular damage, no jaundice, and no liver malfunction.
Another green-blooded lizard, this one from the lowlands of Papua New Guinea. Photo: Christopher Austin
"In addition to having the highest concentration of biliverdin recorded for any animal, these lizards have somehow evolved a resistance to bile pigment toxicity," Zachary Rodriguez, a researcher at Louisiana State University and the lead author of the new study, said in a statement. "Understanding the underlying physiological changes that have allowed these lizards to remain jaundice-free may translate to non-traditional approaches to specific health problems."
To learn more about these lizards and their freaky blood, Rodriguez's team studied 51 different species of New Guinea skinks, four of which are green-blooded (and yes, the biodiversity in New Guinea is intense). Genetic samples from 27 green-blooded lizards and 92 closely related red-blooded lizards revealed that all four of green-blooded lizards belonged to a distinct lineage. That's a surprising result, because it means the green pigment bile has evolved independently, at least four times, in these lizards. It's likely an excellent example of convergent evolution. These lizards are under similar environmental pressures, leading to the common adaptive trait.
The green blood, therefore, has some kind of beneficial value for the lizards. But what? In other animals, like some insects, fish, and frogs, elevated bile pigments act as antioxidants, which are good for cellular health. Biliverdin has also been associated with resistance to certain diseases. In humans, a related liver compound called bilirubin is known to be toxic to the malaria parasite.
For now, the role of the green bile remains a mystery, and future studies will now have to look deeper to figure this out. And in fact, research is already underway at Rodriguez's lab to study the effect of the green pigment on malaria and other parasites that infect these lizards. This research could also be applied to jaundice, which is caused by an excess of bile pigments.
Green blood may be weird, but it could unlock secrets to powerful new medicines.