Color me green!

When I went out to pick raspberries this morning, I found something much more delightful than a bunch of mating Japanese beetles (the scourge of the berry patch!) — a couple of 1-inch Gray Treefrogs hiding in plain sight on the green leaves of the raspberries.

From the side, this little one with its dark facial markings and dark lateral stripe was more obvious.

Although this species is named the Gray Treefrog, because they are quite gray with a dark blotchy pattern sometimes, in bright sunlight on a green background, they are well camouflaged as they match their background. In fact, this frog even matches the particular shade of green of the raspberry leaf on which it rests.

I wonder if they eat Japanese beetles? There are plenty of other insects resting on the raspberry leaves for these little guys to dine on. But these frogs are really only active at night, and usually seek shaded vegetation for their daytime rest.
The color matching camouflage is impressive in both the shade (this photo) and the sun (photo above).
How exactly does a gray treefrog become green?
Looking more like the gray treefrog, I photographed this maxi-sized (2.5-inch) adult in the early morning while it was sitting under an evergreen in the leaf litter.

Frog skin contains a stack of color-producing cells called chromatophores, and many frog species like the Gray Treefrog, have 3 sets of them: a deep layer called melanophores that contain a black/brown pigment called melanin, an intermediate layer called iridophores that lack pigment but contain particles that can reflect blue light, and an upper (most superficial) layer called xanthophores that contain yellow pigment.

Now, it should be more obvious how a Gray Treefrog can transform quickly from its gray color that is produced by the dispersion of deep-lying melanin pigment to a bright green color, produced by the interaction of blue-reflected light from the iridophores passing through the yellow pigment of the xanthiphores (i.e., blue plus yellow equals green to our eyes).

The dispersion of pigment in frog skin is controlled by nerves and hormones, which act on the chromatophores to aggregate (condense) or disperse pigment. Physiologically, in a matter of seconds, when melanophores aggregate their pigment to uncover the iridophores and xanthophores disperse their pigment, a gray frog turns green!

Color changes can even happen while frogs are sitting in the dark in my covered water tank. It just depends on their physiological state, the temperature of their environment, and the amount of hormonal or nerve stimulus they are experiencing.

“Eye” wonder

Last year I wrote about the possible basis for the many variations of eye color in birds (click here to read that post).  Unlike most mammals which sport a limited variety of pale blue, gray, green, to dark brown shades, I’ve noticed that frogs and toads, like birds, also exhibit a rich variety of eye colors.  And so, “eye” wonder why?


Leopard frogs have a golden-colored iris with some dark speckles running through it.


Similarly, Gray Treefrogs also exhibit that golden-colored iris with the darks streaks running through it.

But these highly colorful eyes are tame, compared to those of a couple of South American frogs.

ghost glass frog

Those are some mesmerizing eyeballs in this Ghost Glass Frog. Glass frogs have see-through skin, especially on their ventral surface, which is as transparent as glass permitting a view of their internal organs.  Are those wild purple lines etched on the silver background of its iris to captivate female glass frogs, or frighten potential predators?


Red-eyed Treefrogs sit quietly on the underside of leaves during the day. But if disturbed by a predator or nosy human, they flash their big red eyes, orange toes, and yellow and purple legs to startle and gain time to escape.  [Photo from Wikipedia, By Carey James Balboa via Wikimedia Commons]

The variety of eye color in frogs and toads is astounding, as captured in this collage by Jodi Rowley.

eye color in frogs-Jodi Rowley

Just a sampling of the normal variation among amphibian species… From

Surely all this ocular advertisement has purpose — beyond frightening would-be predators?  Any speculation from you, dear readers?

In addition to the flash of color provided by the wide-open frog eye, you may have noticed that frogs have the ability to project their eyes outward from their head, or retract the eyes inward level with their skull.


This leopard frog has staked out a calling site, hoping to attract females. But it’s broad daylight and he needs to be able to see approaching predators (and photographers). 

Extrinsic eye muscles that elevate the eye above the level of the head actually give the frog a 360 degree view of its environment.  Movement of the eyes downward presses on the roof of the frog’s mouth, helping to propel food down the back of the throat toward the stomach.  Eyes — the multiple use organ!

Color me green, or gray, or brown…

Gray Treefrog, Hyla versicolor

The ever-changeable, Gray Treefrog.  How does it do it?

Green is a popular color for frogs, and birds too, but that lovely green color doesn’t come from a green pigment as you might expect, but from the interaction of multiple layers of specialized color and light-reflecting cells in the upper layers of their skin.

Frogs have three layers of chromatophores (color-producing cells) in their skin. The deepest layer are melanophores that produce melanin pigment giving skin a brown to black color. The middle layer are iridiphores which contain no pigment but instead have mirror like plates capable of producing iridescence, or when viewed from a certain angle, reflect blue light. The most superficial layer of chromatophores are xanthophores that contain a yellow pigment.  When the middle layer of iridophores interact with the top layer of yellow-pigment containing xanthophores, you get what elementary school students have learned:  yellow paint + blue paint = green color.

gray treefrog color

A closeup look at the skin of Gray Treefrogs reveals a mottled pattern of color — the result of different stimuli to those color producing cells in the skin.

Animals that change their skin color (like some frogs and reptiles) not only can change the shape and size of the chromatophore but the dispersion of the pigment within the cells.  Thus, tightly contracted chromatophores with no dispersion of pigment might appear white, and the opposite pattern would look black. Moderate dispersion of melanin pigment in the deepest layer of chromatophores looks gray. Dispersion of pigment in the xanthophores coupled with dispersion of light-reflecting particles in the iridophores produces variation of yellow, blue, and green colors in the skin.  Combination of all of the above produces the mottled pattern of coloration the animal uses to blend in with its background — a protective camouflage.

gray treefrog color-closeup

An even closer, close-up look at the skin of the Gray Treefrog.

Color change seems to be temperature dependent:  warmer frogs are lighter in color to reflect incident light; darker frogs are generally cooler and the darker skin helps them absorb heat.  Skin color becomes lighter when these frogs are placed on a brighter background, and darker when placed on a dark background.  This color matching is part of the effective camouflage protection.  And lastly color change may reflect the mood of the animal — e.g., sexual display, territorial display, etc.  The stimulus for all of this change begins in the brain, is transmitted by hormones, and carried out by the actions of the chromatophores — all in a matter of seconds to minutes.

A really detailed and fascinating discussion of how color change is achieved in animals (in this case a Chameleon) is shown below.