Biologists are stumped by a peculiar group of lizards that have evolved to produce lime-green blood. A new study, published yesterday in the journal, Science Advances, suggests this adaptation could offer some sort of advantage from an evolutionary standpoint, but the reason as to why still remains unclear.
This curious group includes several species of skinks, a type of lizard belonging to the Scincidae family, and are commonly found in New Guinea and the Solomon Islands in the South Pacific, notes Smithsonian Magazine.
The green-blooded critters mainly live in lowland tropical forests and, according to Reuters, grow to be up to a foot (30 cm) long. Originally discovered in 1969, the unusual skinks were assigned to a genus called Prasinohaema, which literally means “green blood” in Greek.
Christopher Austin, from the Museum of Natural Science at Louisiana State University, has been studying these reptiles extensively and seems to be fascinated with them.
“Oh, these animals are gorgeous, truly some of the most beautiful and enigmatic lizards in the world, living on one of the most megadiverse islands on the planet.”
So, how did these skinks end up with such an extravagant blood color? As it turns out, their liver produces abnormally high levels of biliverdin, a green bile waste product that forms after hemoglobin cells expire and are depleted of iron.
“There’s so much green pigment in the blood that it overshadows the brilliant crimson coloration of red blood cells,” Austin pointed out.
In most animals, humans included, high amounts of biliverdin are toxic and can lead to jaundice. However, this large buildup of biliverdin doesn’t seem to affect Prasinohaema lizards at all. In fact, LiveScience reports that these odd skinks are “surprisingly healthy” despite having 40 times more biliverdin in their system than the lethal concentration in humans.
This story begins with the discovery of green-blooded lizards, and things get progressively weirder from there. https://t.co/xxqOUcZZTX— Ed Yong (@edyong209) May 16, 2018
“In addition to having the highest concentration of biliverdin recorded for any animal, these lizards have somehow evolved a resistance to bile pigment toxicity,” study lead author Zachary Rodriguez, from Louisiana State University (LSU), said in an LSU news release.
As an added bonus, the high levels of green pigment also influence the color of the creatures’ organs. Essentially, the Prasinohaema lizards are all green on the inside, explains Austin.
“The bones are green, the muscles are green, the tissues are green, the tongue and mucosal lining is green.”
Another striking thing about the green-blooded lizards is that they give birth to live offspring. Only one species, Prasinohaema virens, lays eggs, LiveScience notes.
In order to find out how these lizards evolved to have green blood, the researchers examined the DNA of 51 skink species of both the green-blooded and the red-blooded variety. The team looked at 27 lizards from six green-blooded species and 92 skinks with normal, red blood.
The research uncovered two puzzling facts, namely that the green-blooded skinks are more closely related to the red-blooded one than to each other, and that Prasinohaema lineages evolved to have green blood four different times, all independently of one another.
At this point, the reason for this strange adaptation is pure speculation, as researchers have yet to crack this mystery.
“There really is a fundamental purpose of this trait,” says study co-author Susan Perkins, also from the American Museum of Natural History. “We just don’t necessarily know exactly what it is right now.”
Possible explanations include some sort of protection mechanism against malaria, to which lizards are very susceptible. One theory is that the green blood protects the skinks against a host of parasites, like the many species of malaria that plague these reptiles. Another supposition is that biliverdin acts as an antioxidant, purging their system of free radicals known to cause a large number of diseases.
“The green-blooded skinks of New Guinea are fascinating to me as a parasitologist because a similar liver product, bilirubin, is known to be toxic to human malaria parasites. Ongoing work with the Austin lab examines the potential effect of the green blood pigment on malaria and other parasites that infect these lizards,” said Perkins.