Green-blooded reptiles are on the edge of poisoning themselves
The greenest lizards in the world
On the island of Papua New Guinea, you can find the greenest lizards in the world.
They're mostly green on the outside, of course, but it's what's underneath that makes the difference. Their tongues are green, so are their bones, their muscles and every drop of blood in their bodies.
In fact, the name for this group of lizards is Prasinohaema — Greek for green-blood.
The mystery of green blood
Dr. Christopher Austin, Curator and Professor at the Museum of Natural Science and the Department of Biological Science at Louisiana State University, has spent two decades trying to solve the mystery of green lizard blood.
In a newly-published study in the journal Science Advances, he and his team look at the evolutionary origins of the lizards' green blood, and how it might help with jaundice, or yellow skin disease, in humans.
What sets the the green-blooded lizards apart from their red-blooded relatives, according to Austin, is the high concentration of a pigment called biliverdin in their bloodstream. Biliverdin is created from red blood cells that are broken down inside the body. In humans, biliverdin is further broken down into bilirubin, which has a yellowish colour instead of green, and causes jaundice when it builds up in high levels in humans.
The compound is toxic to most vertebrates, including humans, and is generally filtered by the liver and excreted. What's surprising is that the green-blooded lizards are perfectly healthy, with levels of biliverdin that are 10 to 20 times higher than the lethal concentration in humans. Austin thinks the lizards must have developed resistance to biliverdin at some point.
Could green blood be an evolutionary advantage?
"We don't know how this evolutionary trait transitioned from normal red blood to green blood," says Austin, "but it's plausible that small increases in biliverdin conferred some selective advantage, and over evolutionary time, even greater concentrations conferred an even greater advantage."
There's so much of this green pigment in the blood of these lizards that it overshadows the bright crimson coloration of red blood cells.- Dr. Christopher Austin
He speculates that high concentrations of biliverdin might have protected them from parasites like malaria that infect red blood cells.
It's been shown in lab experiments that when human blood is infected with malaria, a low concentrations of bilirubin kills the cells infected by malaria, says Austin. Given the hundreds of malaria parasites that infect lizards, the green blood could be protective for them.
Implications for human health
Austin's lab is doing more work to test their hypothesis. They're also sequencing the genome of the green-blooded lizards and their red-blooded relatives to look for the gene that's responsible for the trait.
Finding the gene for green blood may help find treatment for jaundice in humans.
"Jaundice is a common problem in newborn babies. It affects about 60 percent of full-term and 80 percent of premature babies." says Austin. "If we can identify those genes and understand how they operate, it might allow us to provide some sort of medical approach to treat jaundice in newborn babies or individuals with compromised liver function."