Why Are Frogs Green?
Ask most people to draw a frog, and invariably they color it green. In fact, frogs come in a variety of colors, but many are indeed green enough to be suitable mascots for a St. Patrick's Day parade. Being green when sitting on green leaves has obvious advantages if you want to avoid becoming an easy meal for some hungry predator.
The most vividly green ones tend to be treefrogs. Even the red-eyed treefrog of tropical Latin America, arguably the most photographed frog these days, is hard to spot when it is asleep on a leaf with its eyes closed and legs held tight against its body. But you can find lots of greenish frogs that spend their lives hopping along the ground or swimming in ponds. Presumably, as long as there is some green vegetation around, there will be some advantage to being green or being able to turn green at least some of the time.
Frogs are not green because they have green pigment in their skin. Instead, they use a complex arrangement of cells, a more complicated approach to be sure, but one that provides a tremendous potential for changing and adjusting their hue. In their skins they have three types of pigment cells (called chromatophores) stacked on top of each other. At the bottom are melanophores, containing a mostly dark pigment called melanin. These are the same cells that can make human skin various shades of brown. On top of the melanophores are iridophores, packed with highly reflective bundles of purine crystals, and on top of the iridophores are xanthophores, usually packed with yellowish pteridine pigments. In the typical green frog, light penetrates to the iridophores, which act like tiny mirrors to reflect mostly blue light back into the xanthophores above them. These cells act like yellow filters, so the light escaping the skin surface appears green to our eyes. Occasionally a frog is found that lacks the yellow xanthophore cells, and these are hard to miss because they are bright blue!
The real advantage to these stacks of pigment cells lies in the ability to use them to change color. All three types of cells can change shape and change the intensity and character of transmitted or reflected light by moving around the pigment within them. The melanophores at the bottom send tentacle-like projections around the iridophores and xanthophores. By dispersing their dark melanin pigment into these tentacles, these melanophores can darken an animal. Changes in the iridophores can produce changes in the nature of the light reflected into the xanthophores, and changes in the xanthophores can change their filtering effect.
By manipulating all three types of pigment cells, a wide range of colors can be produced, although usually the range extends from bright green to various shades of brown and gray. In frogs, all of these changes appear to be mediated by hormones circulating in the blood. The advantage of such color change is obvious. Imagine a frog leaping from a green leaf onto a brown tree branch. Melanin moves, reflective purine crystals shift position, yellow pteridine pigments cluster or disperse, and voila, that green frog that stood out like, well, like a green frog sitting on a brown branch is now a well camouflaged brown frog.
So your ordinary green frog has quite a few tricks when it comes to disguising himself. A frog that may be bright green on St. Patrick's Day just might be a dull brown or gray the next day, and it would have nothing to do with drinking too much beer, green or otherwise.