A few lizards have a strange secret: they have lime-green blood pumping through their arteries. This green blood makes their muscles, bones, tongues and the insides of their mouths green. And it seems this bright green blood is so advantageous, it has evolved at least four times.
Quite what the advantage might be is unclear, because the green colour is due to high levels of a toxin called biliverdin. “The lizards should be dead,” says Christopher Austin at Louisiana State University in Baton Rouge, who has studied them 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.
But no. Austin’s team has now analysed the DNA of two new species they have just discovered, as well as the five species already known. To their surprise, the results show that green blood evolved independently on at least four separate occasions.