The other day I stumbled upon an intriguing picture of a red-eyed tree frog (Agalychnis callidryas) on Wikipedia:
The caption states that the frog is in “cryptic water conservation posture,” while the photo’s file name suggests it is in “camouflage mode.” What puzzled me is why a frog would turn ghostly pale on a green leaf. Not exactly inconspicuous.
So I asked Twitter:
@rowhoop, News Editor at New Scientist, said: “@ferrisjabr maybe the leaf looks like that in the UV part of spectrum, which birds (predators) are sensitive too. or maybe not.”“@ferrisjabr Imitating bird droppings?” suggested writer, artist and editor @StanCarey
I liked these ideas so I decided to follow up with some herpetologists (scientists who study amphibians and reptiles).
Karen Warkentin, a biologist as Boston University who studies the ecology, evolution and development of frogs – including the red-eyed tree frog – responded as follows:
I don’t know that I’d trust what the image looks like all that much. The frog looks pretty washed out, maybe from flash? The frog is asleep (daytime). There’s lots of speculation but we really don’t know why these (or many) frogs change color. Red-eyed treefrogs can be darker or lighter green, but they can’t be dramatically different colors.
Ohhh…right. Flash. That does make sense.
A fellow herpetologist, James Hanken of Harvard University, independently agreed with Professor Warkentin:
I can offer two possible explanations of the strong contrast between the frog and its leafy background:
- The contrast is an artifact of the photographer’s camera setup. The photographer used a close-up flash, and the bright light reflected right back at the camera lens and “washed out” the frog’s green color. (The leaf remained dark green because it is oriented at an oblique angle to the camera lens and didn’t reflect the flash directly back at the camera.)
- If the strong contrast is real (which I doubt), then the frog may be better camouflaged against the leafy background when viewed in the ultraviolet or infrared spectrum, which some predators can see.I have collected this species in the field (Costa Rica), and to me “camouflage mode” refers to the fact that the frog, by adjusting its body posture, is concealing its brightly colored flanks, thighs, toe webbing, and eyes. By only showing its green color, these frogs truly are difficult to see when they’re sitting on a green leaf.
So it seems when red-eyed tree frogs camouflage themselves, they hunker down on a leaf and tuck away all the strikingly inked bits that make them so photogenic. They may even tune their body’s shade of green, but they probably do not turn albino on the spot. Perhaps staying so still during the hot day also helps them conserve water, as suggested in the original caption.
I guess this is yet another reason to be wary of what you read and see on Wikipedia (although I will defend its usefulness to the death! Seriously, some articles have the most amazing references and external sources: links to amazing primary sources).
I also noticed that the photographer behind this picture, one John J. Mosesso, has many nature photographs all over the web, especially in the National Biological Information Infrastructure’s Library of Images. John, are you out there? Any thoughts about this photo?