Not all blood is red. Some types of octopuses, mollusks and crustaceans have clear blood that turns blue in the presence of oxygen. Marine worms and brachiopods bleed violet. Some segmented worms have blood with a greenish hue. But for most vertebrates—a group that encompasses all animals with a backbone, such as mammals, fish, birds, reptiles and amphibians—their blood runs red due to the hemoglobin used to transport oxygen.
But that isn’t the case for all backboned critters: A group of skinks that live in New Guinea and the Solomon Islands have blood that is lime green. Now, researchers are beginning to figure out just how and why the little reptiles developed such an unusual and vibrant vital fluid, reports Ed Yong at The Atlantic.
The lizards, which are all classified in the genus Prasinohaema (meaning “green blood” in Greek), were discovered in 1969. But they weren’t studied in-depth until Christopher Austin of Louisiana State University became fascinated by them decades later.
As Austin tells NPR’s Nell Greenfieldboyce, the lizards' green coloring is not limited to their blood. “The bones are green, the muscles are green, the tissues are green, the tongue and mucosal lining is green,” he says.
That’s because they are stewed in a green pigment called biliverdin. “There’s so much green pigment in the blood that it overshadows the brilliant crimson coloration of red blood cells,” Austin says.
In most animals, explains Yong, hemoglobin cells die after about four months of service. The liver then gathers them and takes out the iron, creating the green waste product biliverdin, which is later transformed further into yellow bilirubin. If too much of these toxins build up in the blood, it can cause a yellowing of the skin called jaundice. If excessive amounts of the pigments accumulate, it can be fatal.
But not for Prasinohaema lizards.
They can keep going despite having 20 times the highest concentration of biliverdin ever found in a human. And for the person, the level was fatal.
By looking at the genetic relationships of these lizards, researchers determined how this strange adaptation evolved. The team examined the genomes of 51 skink species, including 27 individuals from six species of green-blooded skinks and 92 red-blooded lizards.
Surprisingly, the green-blooded skinks were not closely related. Instead, they were more closely related to red-blooded skinks, and the analysis suggests that the green-blood trait evolved at least four separate times. The research appears in the journal Science Advances.
Overall, the study suggests that there is some evolutionary advantage for having green blood that skinks from various habitats all developed over time. “There really is a fundamental purpose of this trait,” co-author Susan Perkins of the American Museum of Natural History tells Greenfieldboyce. “We just don't necessarily know exactly what it is right now.”
The team hypothesized that biliverdin might make the lizards unpalatable to predators, but birds aren’t deterred by the stuff. And, as Greenfieldboyce reports, Austin has eaten both red-blooded and green-blooded skinks. He says they both taste the same—disgusting.
The researchers also considered that the green might give the lizards extra camouflage. But not all of the skinks with green innards are green on the outside.
Their current, admittedly speculative, hypothesis is that the biliverdin-rich blood protects against parasites. Humans with elevated bilirubin, Greenfieldboyce reports, have some added protection against malaria parasites. Lizards, it turns out, are susceptible to hundreds of malaria species and the green blood might protect against some of them.
But it’s a tricky idea to test. “The naïve view is that if green blood evolved to prevent malaria, there would be no malaria in green-blooded lizards,” Austin tells Yong. But the lizards do get malaria. One explanation for this could be that a strain of parasite may also have evolved to overcome the defense and infect lizards with malaria in the constant evolutionary arms race.
Whatever the reason why the skinks have green blood, the fact that they can survive so much biliverdin is interesting and could provide biomedical insights, Adriana Briscoe of the University of California at Irvine, who was not involved in the study, tells Yong. Briscoe points out that studying the creatures could lead to new treatments for diseases like jaundice and malaria.
The researchers are now trying to figure out which of the lizard’s genes produce all the green running through their veins.