In our mammal-oriented existence, as mammals ourselves, it’s easy to get lazy and assume that all animals function like some version of ourselves. A visit to the Zoo should instantly deter those thoughts, but the differences can get weirder if we go inside some animals. A key function of blood, or hemolymph as the similar substance in many invertebrates, is to molecularly hold oxygen and get it distributed to the cells. When your day maybe isn’t the best one ever, you may be reminded that human blood is red. This is because the molecule we use to transport oxygen (hemoglobin, as in all mammals) contains an iron component that, when attached to a molecule of oxygen, takes on a red color. Oxygen + iron = oxidized iron (= rust!)—you get the picture. But octopus, horseshoe crabs, and some other creatures use hemocyanin to hold oxygen. It contains copper, so their blood is blue. And there are other varieties and colors out there among animals.
Well, today I learned something new and wanted to share with you all. I’ve known for years that some frogs, including glass frogs, have greenish muscles and bones. This is because they have a higher level of biliverdin in their tissues. I never knew the function of this form of physiology, however. Today I learned about a few species of lizards from New Guinea that have green blood, and I mean really green blood! These lizards have phenomenally high levels of biliverdin in their blood. That certainly is odd, but what’s even more interesting is that biliverdin is really toxic. For example, when our blood cells naturally die or get crushed, they produce bilirubin (yellow in color) and biliverdin (green) as they decompose. These are those lovely yellow and green marks around a bad bruise. Your liver cleans that out of your system as quickly as possible, because of its toxicity. Jaundice is a condition where the liver fails to, or cannot keep up with, absorption of these toxins and results in those sickly shades of skin. But back to the lizards … how they tolerate their own toxic blood is one question, but another question is why? Dr. Chris Austin and colleagues have been studying this system, and they made the really insightful connection between biliverdin and malaria parasites. The short version is that these lizards only suffer one of the many reptile-associated malarial parasites. Their blood is simply too toxic for the parasites to survive. The one species of parasite that they do harbor, evidently, has evolved tolerance to high levels of biliverdin. I should add, in closing, that the lizards’ blood is actually red (i.e., hemoglobin) but that red coloration is masked by these high levels of green-colored biliverdins.
I learned something at work today! That always makes me happy, and I am glad to share my “discoveries” with our web audience.
Citation: Austin, C.C., and K. W. Jessing 1994. Green-blood pigmentation in lizards. Comparative Biochemistry and Physiology Part A: Physiology 109:619–1122.
Rodriguez, Z.B., S.L. Perkins, and C.C. Austin. 2018. Multiple origins of green blood in New Guinea lizards. Science Advances 4: eaao5017
You should be able to find copies of these studies on ResearchGate or a GoogleScholar search.
Hope to see you soon at the Zoo.
Joe Mendelson, PhD
Director of Research