Sunday, 21 August 2016

These lizards bleed green - Blood chemistry unknown in mammals or other reptiles still needs a good explanation – via Herp Digest

By Susan Millius, Science Magazine issue: Article will appear in Vol. 190, No. 4, August 20, 2016, p. 4

“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.

C.C. Austin and K.W. Jessing. Green-blood pigmentation in lizards. Comparative Biochemistry and Physiology Part A. Vol. 109, November 1994, p. 619. doi: 10.1016/0300-9629(94)90201-1.

Further Reading
S. Milius. Animals’ jaundice pigment found in plants. Science News Online. Published February 20, 2009.

R. Zhao et al. A study on eggshell pigmentation: biliverdin in blue-shelled chickens. Poultry Science. Vol. 85, March 2006, p. 546. doi: 10.1093/ps/85.3.546.

R.H. Hackman. Green pigments of the hemolymph of insects. Archives of Biochemistry and Biophysics. Vol. 41, November 1952, p. 166. doi: 10.1016/0003-9861(52)90517-1.

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