Prasinohaema skinks, residents of New Guinea, have green bones, green muscles and green tongues. Even their blood is green. And it's also a deadly poison. But how their blood got so green, and so toxic, remains something of a mystery.
To be fair, the hue itself isn't a complete mystery. According to Louisiana State University biologist Christopher Austin, who began studying the skinks for his PhD work in the late eighties, the bright, lime-green colored plasma that permeates the skinks' "muscles, bones, tongue and mucosal tissues" can be attributed to extremely high concentrations of the bile pigment biliverdin, pictured here.
National Geographic's Carrie Arnold explains:
In many species, including humans, oxygen in the blood is carried by hemoglobin, which gives blood its red hue. When hemoglobin starts to break down, it’s transported to the liver, where it’s broken down into molecules such as bilirubin and biliverdin.
These pigmented compounds are excreted with bile into the intestines. Biliverdin has a green hue (you can see the pigment in the greenish areas around a healing bruise), and very high levels of biliverdin in the blood of these skinks is what gives their blood its unusual color.
The remaining mystery, Austin says, is that biliverdin is extremely toxic. If humans have even a tiny amount of biliverdin or bilirubin in their blood, we say that they are jaundiced (their skin takes on a yellowish tone).
You really only see jaundice in newborns, or adults with severely impaired livers. The excess bilirubin, if left untreated, can prove fatal. But then that's what's SO weird about these skinks.
In the top capillary tube is your typical, hemoglobin-red blood; in the bottom tube is blood from a Prasinohaema skink.
The NatGeo article cited above claims that Prasinohaema skinks are the only vertebrates known to have green blood. Not true. Green-colored plasma has been documented in plenty of vertebrates, including several families of fish and a number of frog species. At least two things, then, make Prasinohaema truly unique.
One: the skinks are the only known amniotes (/pedantry) with green blood. Two: The concentration of biliverdin coursing through their veins is just obscenely high – "approximately 1.5–30 times higher than fish species with green blood plasma, and 40 times greater than humans with green jaundice," according to measurements published by Austin in 1994. “At these concentrations," he says, the skinks "should be completely jaundiced, if not dead."
And yet, the skinks are very much alive. Intriguingly, they may be alive not in spite of their biliverdin-laced blood, but because of it.
Above: P. virens, one of the five identified species of green-blooded skinks belonging to Prasinoheima.
Austin thinks Prasinoheima skinks may have evolved a tolerance for the bile pigment as a defense mechanism against Plasmodium parasites – the microorganisms responsible for malaria infections. The presence of biliverdin instead of hemoglobin, he reasons, may block Plasmodium's usual route of infection.
To recap: Prasinoheima is a genus of skinks in New Guinea with green blood. Blood that is green because of biliverdin – a pigment that, in humans, is generally associated with blunt-force trauma and jaundice. A pigment which, were it found in the blood of a human at the same concentrations seen in a Prasinoheima skink, would almost certainly be found in a very dead human. And finally that these skinks may have evolved to shrug off this otherwise lethal vital fluid in order to protect themselves against malaria. Why? Well... almost ceratinly because evolution in remote regions of the world (like the island of New Guinea, for example) is very weird – as we've seen before.