Why Do These Lizards Have Toxic Lime-green Blood?

Prasinohaema prehensicauda is a green-blooded lizard with high concentrations of biliverdin, or a toxic green bile pigment, found in New Guinea. Chris Austin (LSU)

Why do some lizards have green blood?

This question was the focus of a recent research expedition to New Guinea, a large island in the continent of Australia, which is home to a group of unusual green-blooded lizards known as Prasinohaema.

These reptiles have muscles, bones and tongues that appear to be bright lime green, due to high levels of a toxic green bile pigment in their blood known as biliverdin. Biliverdin is toxic to humans and causes jaundice. In fact, the concentration of biliverdin in the blood of the lizards is 40 times higher than the lethal concentration in humans—but this doesn't seem to affect the lizards. (Bile pigments are colored compounds found in many organisms that result from the breakdown of haemoglobin—a protein that's responsible for transporting oxygen in many animals).

"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 biologist from Louisiana State University (LSU), 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".

Rodriguez is lead author of a new study, to be published in the journal Science Advances, looking into the evolutionary history of green blood. For the research, a team from LSU examined 51 types of skink lizards—including six species with green blood, two of which are new to science—by analyzing the DNA of various specimens.

They found that green blood in these lizards likely evolved four different times, and that all of the green-blooded lizards were probably descended from a red-blooded ancestor.

The fact that green blood emerged independently on numerous occasions suggests it may be evolutionarily beneficial, according to the researchers. While the function of the green bile in the lizards remains uncertain, several studies have shown that that bile pigment can have antioxidant properties and also helps to prevent disease during in vitro fertilization. Numerous insect, fish and frog species have elevated levels of bile pigment

Co-author of the study Susan Perkins, a curator and professor at the Sackler Institute for Comparative Genomics and the Division of Invertebrate Zoology, American Museum of Natural History, thinks that the new findings could have important medical implications.

"The green-blooded skinks of New Guinea are fascinating to me as a parasitologist because a similar liver product, bilirubin, is known to be toxic to human malaria parasites," she said.

Ongoing work at LSU is examining the potential effect of the pigment responsible for green blood on the malaria parasite and others that infect the lizards, Perkins added. The researchers say the next step is to identify the specific genes responsible for green blood.