Scientists are now trying to understand how these lizards might benefit from blood that's green.
If you ever examine the innards of a green-blooded skink, you might take a second (or even a third) look: The muscles, bones and even the tongues of these lizards have a bright, lime-green color - not from their diet, but because of the copious amount of green bile that's in their blood. Researchers said on Wednesday a DNA study resolved their family tree, finding that green-bloodedness evolved four different times among lizards called skinks on New Guinea. "There's so much green pigment in the blood that it overshadows the brilliant crimson coloration of red blood cells", says Chris Austin, a biologist at Louisiana State University who has studied these lizards for decades.
Since green blood is so unusual and found only in a few species of lizards living on the island of New Guinea, it was assumed it evolved just once. All the living green-blooded lizards were thought to derive from just one ancestral species. To their surprise, the results show that green blood evolved independently on at least four separate occasions.
Although all the green-blooded species have been assigned to a single genus, Prasinohaema, there were signs that they are not that closely related. Some species lay eggs while others give birth to live young, Austin says, and they are found everywhere from lowland forest to 3000-metre mountains. This particular group of lizards might somehow be predisposed to evolving green blood, says Austin.
We already know the broad mechanism. These skinks' green blood comes from high levels of biliverdin, a green bile pigment that, when converted to bilirubin, causes jaundice.
It turns out that the green-blooded lizards are not each other's closest relatives, according to a report in the journal Science Advances.
"I find it just absolutely remarkable that you've got this group of vertebrates, these lizards, that have a level of biliverdin that would kill a human being, and yet they're out catching insects and living lizard lives", says Susan Perkins, a researcher at the American Museum of Natural History in NY. "There must be some selective advantage".
Lately the scientists have been wondering if the lizards' green blood might protect them from parasites like malaria - although Austin admits that this is "pretty speculative". There may have been an evolutionary arms race, during which the lizards kept increasing biliverdin levels while the parasites became more resistant. He notes that elevated levels of biliverdin have been found in some fish, and they may also explain the green blood in some frogs.