Green blood is one of the most unusual characteristics in the animal kingdom—and the hallmark of a group of lizards in New Guinea. The rare coloration comes from exceptionally high concentrations of biliverdin, or green bile pigment. In mammals, it's converted to the toxic bilirubin, which causes jaundice in human newborns. So why do these lizards remain healthy with levels of green bile that are 10 to 15 times higher than the lethal concentration in humans?
A team of researchers, including Museum Curator Susan Perkins and colleagues from Professor Chris Austin’s laboratory in Louisiana State University (LSU), are trying to answer the question. This week, they’ve published a study in the journal Science Advances that investigates the evolutionary history of this rare characteristic.
“In addition to having the highest concentration of biliverdin recorded for any animal, these lizards have somehow evolved a resistance to the toxicity,” said Zachary Rodriguez, a doctoral candidate in Louisiana State University’s Department of Biological Sciences. “Understanding the underlying physiological changes that have allowed these lizards to remain jaundice-free may translate to non-traditional approaches to specific health problems.”
While most vertebrates have bright red blood derived from heme, green-blooded skinks have lime-green colored blood—which in turn results in bright green muscles, bones, tongue, and mucous membranes.
For the last 27 years, LSU’s Austin has led expeditions to the megadiverse island of New Guinea to discover and document reptile and amphibian diversity. In the latest study, Austin and Rodriguez teamed up with Perkins to examine DNA from 27 green-blooded skinks and 92 closely related red-blooded lizards. They discovered that there are four separate lineages of green-blooded lizards, and all likely shared a red-blooded ancestor.
The study finds that green blood likely emerged independently in various lizards, which suggests that green blood has an adaptive value. Previous studies in other animals have shown that bile pigment can act as an antioxidant, scavenging free radicals as well as preventing disease during in vitro fertilization. However, the function of green bile pigment in these lizards is still uncertain.
“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,” Perkins said. “Ongoing work with the Austin lab examines the potential effect of the green blood pigment on malaria and other parasites that infect these lizards.”