These lizards bleed green

Blood chemistry unknown in mammals or other reptiles still needs a good explanation

P. prehensicauda

NO DENTIST NEEDED  Green blood gives the mouth tissue and tongue its unusual but perfectly healthy color in Prasinohaema lizards (female P. prehensicauda shown). Some fish and insects have greenish blood but no known mammals or other reptiles do.  

Christopher C. Austin/LSU

“Dark lime green” is how biologist Zachary Rodriguez describes the blood of the Prasinohaema lizards of New Guinea and surrounding islands. “Vivid,” he adds.

With green blood comes Granny Smith-colored muscles and bones and a blue-green mouth, exposed during defensive posturing. But the strangest thing about the five species of Prasinohaema lizard is that they can live like that.

Lime, apple and avocado can be risky blood colors. They indicate that these lizard species build up a toxic substance called biliverdin. The lizards’ red blood cells still depend on hemoglobin, the stuff that ferries oxygen and makes most animal blood red, but any lizard-blood redness is overwhelmed by massive concentrations of the green biliverdin. A breakdown product of hemoglobin, biliverdin gives the greenish edge to bruised human flesh. Most animal bodies quickly whisk it away.

High concentrations of biliverdin, say over 50 micromoles per liter, make humans sick with jaundice. The lizards, however, do just fine with 714 to 1,020 µM/L.

It’s tempting to wonder if evolution has favored green blood because toxic biliverdin might make predators spit out any lizard they start to bite. Not so, based on current evidence, Rodriguez says. An old test found that a predatory bird and a snake relished green-blooded lizards; he’s heard that cats love them, too.

Plenty of other ideas are still in play, among them: The biliverdin may reduce susceptibility to malaria or to cell damage from the sun’s ultraviolet rays, or even add some extra camouflage for life in trees. To look for clues to how evolution drove the death-defying color, Rodriguez and his adviser, Christopher Austin at Louisiana State University in Baton Rouge, are using genetics to create a genealogical tree of the lizards and their relatives.

Figuring out where among the ancestors the blood color arose could give clues to what kind of lifestyles or environments favored toxic green. So far, oddly enough, the biliverdin-tinted lizards don’t seem to be each other’s closest known relatives. Some Prasinohaema species look as if they have red-blooded sister species (classified in other genera) that are evolutionarily closer than any other Prasinohaema.

However the final tree turns out, this evolutionary tale will be lively, with red-and-green, stop-and-go lizard history.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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