Lizards Are Mysteriously Evolving Toxic Green Blood

If you had lime-green blood flowing through your arteries, how would you feel? Perhaps you’d be grossed out. Or maybe you’d feel like a glowing, green superhero.

It turns out that some lizards have this alien-like slime pumping through their scaly bodies, but scientists don’t know exactly why. In fact, by all rights, these lizards should be dead, since the green color is the result of high levels of biliverdin, a toxin. And yet—these critters have evolved the green blood at least four separate times, suggesting that it must be advantageous in some way.

An example of a green-blooded lizard, Prasinohaema prehensicauda

Here’s Michael Le Page, reporting for New Scientist:

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 analyzed 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.

What’s weirder still is that the species demonstrating this evolved trait aren’t all closely related. That makes figuring out the specific mechanism behind this strange feature even more challenging.

In most animals, the red pigment hemoglobin from old blood cells is broken down into biliverdin. Then it’s converted to a yellow pigment, bilurubin, and cleared from the bloodstream by the liver. However, in green-blooded lizards, the breakdown of biliverdin doesn’t happen. The question is: why?

So far, scientists have hypothesized that toxic green blood protects against malaria. They think this because high levels of biliverdin in human blood kill malaria parasites. But these lizards still get malaria, so the answer remains out of reach.