“Even if the trait only evolved once, the fact that it has been retained across several species indicates that it confers a tremendous advantage,” Adriana Briscoe, an evolutionary biologist at the University of California, Irvine, tells The Atlantic.
What that advantage might be is unknown. The green comes from biliverdin, a byproduct of dying red blood cells. The lizards have it in such great abundance—20 times more than the highest concentration recorded in a human, according to The Atlantic—that such a level could kill other animals.
“There’s so much green pigment in the blood that it overshadows the brilliant crimson coloration of red blood cells,” coauthor Chris Austin, a biologist at Louisiana State University, tells NPR. “The bones are green, the muscles are green, the tissues are green, the tongue and mucosal lining is green.”
Austin tells NPR that the biliverdin likely didn’t give the animals an advantage in deterring predators, as animals are able to eat the skinks without untoward GI troubles (he himself ate them, saying it was like “bad sushi”). It’s possible the green blood protects the animals from malaria, as in vitro studies have shown that human blood with high levels of bilirubin, a close relative, can stave off infection.