We Still Don't Know Why These Lizards Have Lime-Green Blood

Indian skink lying on a rock

The green blood is explained by the high concentration of biliverdin it contains

For the research, a team from LSU examined 51 types of skink lizards-including six species with green blood, two of which are new to science-by analyzing the DNA of various specimens. They concluded that the red-blood lizards are more prone to be found in the ancestral state as that of the green-blood lizards. We review the literature for fish species with high concentrations of plasma biliverdin and pathological biliverdin accumulation in humans; we find that Prasinohaema species have plasma biliverdin concentrations approximately 1.5-30 times greater than fish species with green blood plasma and 40 times greater than humans with green jaundice.

Biliverdin causes the lizards to have lime-green blood, muscles, bones, and tongues.

"In addition to having the highest concentration of biliverdin recorded for any animal, these lizards have somehow evolved a resistance to bile pigment toxicity", Rodriguez said.

So Austin, Perkins, and their colleague Zachary Rodriguez made a decision to create a kind of lizard family tree by studying the DNA of 51 Australasian skink species, including six species that have green blood. Because the compound that gives the blood its Nickelodeon-slime hue is highly toxic to humans, scientists have been perplexed about how these skinks manage to survive, let alone thrive. This happens as their entire bodies have high concentrations of biliverdin. "Understanding the underlying physiological changes that have allowed these lizards to remain jaundice-free may translate to non-traditional approaches to specific health problems".

Scientists previously believed that green-blooded skinks may be part of a closely related group but this wasn't the case.

Their analysis suggests the Prasinohaema genus consists of four different lineages, all tracing back to differed red-blooded ancestors.

"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", said Susan Perkins, curator and professor at the Sackler Institute for Comparative Genomics. But these lizard species don't look alike and have different lifestyles, with some laying eggs and others giving birth to live young. The green blood emerged in some of the lizards independently which means that the green blood might have come with an adaptive value. Having elevated levels of bile pigments was a positive advantage for those animals now scientists are trying to see how the green blood may help the skinks.

"It's rare in the animal kingdom", says Rodriguez, "but because it does appear, this suggests there has to be some beneficial properties to green blood".

"Our next goal is to identify the genes responsible for green blood", Rodriguez said. Still, they're continuing to explore what kind of relationship there might be between malaria and green blood. Bile pigment has been shown to act as an antioxidant scavenging free radicals and prevents diseases during in vitro fertilization.

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