These Lizards Keep Evolving Toxic Bright-Green Blood And We Don’t Know Why

16

It might sound like something from science-fiction, but there are lizards whose blood runs green.

From green muscles to green bones, green mouths to green blood, the color can range from a deep blue-green to a bright lime-green. And yet despite the fact that this occurs totally naturally in half a dozen or so species, we have absolutely no idea why. It’s particularly weird considering this green blood is toxic, and by all accounts, they should be dead.

On the Pacific island of New Guinea, researchers from Louisiana State University have been studying the green-blooded lizards that live there. While all are grouped into Prasinohaema (literally Greek for “green-blood”), it was assumed that they were unique and that it has simply evolved once in their ancestors.

The green skink blood compared to ‘normal’ red blood. Dr. Christopher Austin/LSU Museum of Natural Science

But it turns out that these six species of virescent reptiles actually come from separate lineages, and that the verdant colored blood has evolved independently four separate times from red-blooded ancestors. Their genetic analysis of the reptiles has been published this week in Science Advances.

The red hue of our blood is caused by the iron-based hemoglobin pulsing through our veins, the blood of the skinks is colored instead by a molecule known as biliverdin. This is the pigment usually found in green bile, and in most cases is toxic in high concentrations, causing jaundice when it accumulates in the circulatory system. It is also responsible for that green tinge you sometimes get on bad bruises.

Considering that the pigment is usually associated with toxicity, the fact that any animal has it, let alone that it has evolved four times is something of a surprise. It suggests that the color gives some sort of advantage to the lizards.

The pigment has been found in higher than expected levels in some insects, fish, and frogs, and some experiments have suggested that it might help mop up free radicals that are circulating in the blood, potentially preventing disease.

“In addition to having the highest concentration of biliverdin recorded for any animal, these lizards have somehow evolved a resistance to bile pigment toxicity,” said Zachary Rodriguez, of Louisiana State University, who led the research. “Understanding the underlying physiological changes that have allowed these lizards to remain jaundice-free may translate to non-traditional approaches to specific health problems.”

The researchers are also interested in another curious property of biliverdin, in that in humans, it has been found to kill the malaria parasite. Understanding how and why the lizards make so much of it could, therefore, help us fight this disease.