The bile pigments, known as biliverdin and bilirubin, are toxic by-products of red blood cell catabolism - the process by which enzymes breakdown large molecules into smaller components.
Why do some lizards have green blood? Six species bleed lime green! It turned out that numerous species with green blood aren't closely related to one another, and have more in common phylogenetically with red-blooded skinks.
"Oh, these animals are gorgeous, truly some of the most lovely and enigmatic lizards in the world, living on one of the most mega-diverse islands on the planet", Austin said.
Prasinohaema prehensicauda is a green-blooded lizard with high concentrations of biliverdin, or a toxic green bile pigment, found in New Guinea. "Ongoing work with the Austin lab examines the potential effect of the green blood pigment on malaria and other parasites that infect these lizards", said co-author Susan Perkins, curator and professor at the Sackler Institute for Comparative Genomics and the Division of Invertebrate Zoology, American Museum of Natural History. They discovered that there are four separate lineages of green-blooded lizards, and each likely shared a red-blooded ancestor. The excess of green bile pigment essentially eclipses the normal ruddy hue of their red blood cells. "The problem is that there's green-blooded lizards that aren't green, and there's red-blooded lizards that are green", he explains. "The most probable explanation is that green blood, though rare, evolved independently multiple times", says Austin. "Evolution can do incredible things given enough time", Austin said. The pigment might have caused them to have green blood and green bones, but they became immune to developing adverse reactions to it. We only have low levels of biliverdin in our bodies, and we can spot it at the surface whenever we get hurt and develop bruises.
"Our current hypothesis is that this novel and toxic physiology might have evolved to reduce or preclude the infection of blood parasites such as malaria", Austin said. This suggests that green blood might have been an adaptive reaction, but researchers can not tell why the process had occurred. Slightly elevated levels of bile pigments in other animals, including insects, fish and frogs, have played potentially positive roles in these animals.