The blood of most vertebrates—mammals, birds, fish, and typical reptiles—is deep red, a hue derived from the iron-containing protein hemoglobin. Hemoglobin is responsible for binding and transporting oxygen throughout the body, and the reddish color comes from the iron atom within the heme group. However, a small group of lizards defies this near-universal rule, having developed a completely different blood coloration. They survive with a system that would be fatal to almost any other animal.
The Unexpected Color of Lizard Blood
The blood of certain skinks, primarily those belonging to the genus Prasinohaema found in New Guinea and the Solomon Islands, is a striking lime green or sometimes blue-green. This intense coloration completely overpowers the natural crimson of the red blood cells. The high concentration of pigment stains the animal’s tissues, including the muscles, bones, tongue, and mucosal linings. This vibrant internal hue makes them unique among the world’s vertebrates.
The Chemical Explanation: Biliverdin Overload
The green blood results from an extreme accumulation of a bile pigment called biliverdin. Biliverdin is a normal byproduct of the breakdown of the heme component of hemoglobin. In most vertebrates, including humans, the liver converts this green biliverdin into a less-toxic, yellow pigment called bilirubin, which is then excreted. These specific lizards have evolved to halt or bypass this final conversion step, resulting in massive concentrations of biliverdin circulating in their bloodstream. Scientists have measured levels up to 40 times greater than what would be lethal in a human, a condition called green jaundice. The sheer volume of this compound overwhelms the red hemoglobin.
Living with Toxicity: Physiological Adaptation
These skinks remain healthy despite carrying toxic levels of biliverdin, which would cause severe jaundice, neurological damage, and death in nearly all other vertebrates. The lizards have evolved a resistance to this bile pigment toxicity, allowing them to live normally.
The repeated, independent evolution of green blood in four separate lineages of skinks suggests the trait provides a survival advantage. One leading hypothesis is that the high levels of biliverdin offer protection against blood parasites. The green-blooded skinks are susceptible to hundreds of malaria species, and the extreme biliverdin concentration may help suppress or fight these infections. Researchers are actively studying the genetics of these animals to understand the exact mechanism that grants them immunity to the pigment’s toxicity. Unlocking the secret to their survival could lead to new avenues for treating human diseases like malaria and severe jaundice.