Why Do Mysterious Lizards Have Green Blood?

Roses are red, violets are blue … and these lizards’ blood is green.

It’s not a trick of the imagination or a penchant for food coloring—Prasinohaema skinks living on the island of New Guinea actually have green blood, bones, and tissue—and one scientist is trying to figure out why.

Green-blooded lizard.
Prasinohaema skinks are green all over. Photograph by Christopher Austin, Louisiana State University Museum of Natural Science

Louisiana State University biologist and National Geographic explorer Christopher Austin first became interested in these odd skinks in graduate school, when he read a 1969 paper in Science about a strange group of lizards with green blood.

“It immediately captivated my attention. I noticed that no one had really worked on these lizards since then to figure out the what and the why of the green blood,” said Austin, who studied the lizards for his Ph.D. at the University of Texas, Austin. (See pictures of new skinks found recently in the Caribbean.)

Why So Green?

The skinks are found almost exclusively on the island of Papua New Guinea, one of the world’s biodiversity hot spots. (One species can also be found in the Solomon Islands.)

When Austin began studying the animals in Papua New Guinea, he realized they have “incredibly high concentrations of the bile pigment biliverdin,” he said.

Green-blooded lizard's blood compared to typical red blood.
The skink’s blood (bottom) compared with red blood (top). Photograph by Christopher Austin, Louisiana State University Museum of Natural Science

In many species, including humans, oxygen in the blood is carried by hemoglobin, which gives blood its red hue. When hemoglobin starts to break down, it’s transported to the liver, where it’s broken down into molecules such as bilirubin and biliverdin.

These pigmented compounds are excreted with bile into the intestines. Biliverdin has a green hue (you can see the pigment in the greenish areas around a healing bruise), and very high levels of biliverdin in the blood of these skinks is what gives their blood its unusual color. (Also see “Blue Blood Helps Octopus Survive Brutally Cold Temperatures.”)

The green tint isn’t limited to their blood either. “The blood is green, the bones are green, the tissues are green—even their tongue is green,” Austin said.

No other vertebrate is known to have green blood.

Mystery Remains

The remaining mystery, Austin says, is that biliverdin is extremely toxic. If humans have even a tiny amount of biliverdin or bilirubin in their blood, we say that they are jaundiced (their skin takes on a yellowish tone).

Generally this only happens in people with liver damage or in newborns whose livers haven’t yet started to break down old hemoglobin. But if excess bilirubin or biliverdin in the bloodstream goes untreated, it can be deadly.

“It’s surprising because at these concentrations of bile pigments in the blood, [the skinks] should be completely jaundiced, if not dead,” Austin said.

Austin hypothesizes that the lizard evolved to tolerate the biliverdin because it may provide protection against a group of parasites called Plasmodium.

Best known for causing malaria in humans, Plasmodium also causes malaria in reptiles and birds. Austin believes that the presence of toxic biliverdin instead of hemoglobin may make it harder for Plasmodium to infect the skinks. (Also read: “Bedlam in the Blood: Malaria.”)

To answer this question, Austin is currently sequencing the genomes of the Prasinohaema skinks and comparing them with the genomes of closely related skinks that have red blood to identify any genetic changes that could provide resistance to Plasmodium.

Tell us: Why do you think the lizard has green blood?

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Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com