Blood is commonly associated with a bright red hue, rooted in the shared biology of almost all mammals, birds, fish, and reptiles. This crimson color comes from the protein hemoglobin, which relies on iron atoms to bind and transport oxygen throughout the body. However, the color of blood is strictly determined by the oxygen transport molecule. In the animal kingdom, this vital function is not always performed by iron-based compounds, leading some animals to possess circulatory fluids that appear blue, violet, or bright green.
The Specific Animal
The most striking example of green-colored blood is found in certain species of skinks belonging to the genus Prasinohaema. These small reptiles are endemic to the forests of New Guinea and the Solomon Islands. Upon injury, their blood appears a vivid lime-green, a color so intense it completely overwhelms the normal red tint of hemoglobin. The green pigment is highly concentrated throughout the animal’s tissues, resulting in the muscles, bones, tongue, and mucous membranes all appearing bright green.
The Chemical Mechanism of Green Blood
The intense green coloration of these skinks is caused by an extremely high concentration of the bile pigment known as biliverdin. Biliverdin is a common waste product in nearly all vertebrates, created during the natural breakdown of the heme component of hemoglobin. In most animals, the body quickly processes this biliverdin, converting it into a second bile pigment called bilirubin for excretion. The skinks, however, halt this metabolic pathway, causing massive quantities of unconverted biliverdin to accumulate in their plasma.
The concentration of biliverdin in these skinks can be up to 40 times higher than levels considered lethal in humans. In people, such a buildup of bile pigments would cause a severe condition called jaundice. The skinks have uniquely evolved a mechanism to tolerate these hyper-concentrations of the toxic pigment without suffering any obvious ill effects. This physiological tolerance suggests the trait confers an evolutionary advantage despite the inherent toxicity.
Scientists theorize that the retention of biliverdin serves as a defense against blood parasites, such as lizard-specific strains of malaria. High levels of bile pigments are known to be toxic to Plasmodium parasites, which cause malaria. The green blood trait has been observed to evolve independently at least four separate times within this lineage of lizards, indicating a strong selective pressure favoring the trait. This repeated emergence suggests that the green blood provides some form of protection, likely related to combating parasitic infection.
Other Blood Colors in the Animal Kingdom
Other animals exhibit different blood colors due to the presence of specialized oxygen-transport proteins. The vast majority of vertebrates, including humans, rely on hemoglobin, which uses iron atoms to bind oxygen, giving the blood its characteristic red color.
In contrast, many invertebrates, such as octopuses, squid, and horseshoe crabs, circulate blood that appears blue. This color is due to the respiratory pigment hemocyanin, which uses copper atoms to bind oxygen molecules instead of iron.
Another variation is found in certain marine worms and leeches, which utilize an iron-containing protein called chlorocruorin, giving their blood a light green color. The difference between these green-blooded worms and the skinks is fundamental, however, as the worms use a dedicated, iron-based respiratory pigment for oxygen transport. The skinks, conversely, employ the standard red hemoglobin for oxygen transport, with the green color being a secondary result of the enormous concentration of biliverdin.