THE OLYMPIAN (Olympia, Washington) 19 February 06 Frogs’ pigment gives them an advantage (Marty Becker)
Scientists recently reported finding a veritable Garden of Eden in an Indonesian mountain jungle, home to dozens of exotic new species including frogs.
While frogs come in a palate of colors, if you hand a kid or adult a drawing of a frog and ask them to color it in, people instinctively reach for the green crayon.
Why are most frogs green and for frogs that can change color, how does Mother Nature cue the hue?
As prey, frogs can be one hop short of escape from a hungry predator.
Being green with green vegetation around is a great advantage. Frogs aren’t green because they have green pigment in their skin. Rather, they have a complex arrangement of cells that allow them to adjust their hue (wouldn’t the Hollywood crowd love this gift!). Frog’s skin contains three types of pigment cells or chromatophores stacked on top of each other like a three-story apartment:
• Melanophores occupy the bottom floor and contain the darker pigment called melanin. Yup, these are the same type of cells that human sun worshippers cherish for various shades of brown.
• Iridophores are in the middle and are made up of highly reflective bundles of purine crystals.
• Xanthophores are on top, typically packed with yellowish pteridine pigments.
In the typical green frog, light penetrates to the iridophores where the crystals act like tiny mirrors to reflect mostly blue light back into the xanthophores above them where the cells act like yellow filters. The end result is the light that emanates from the skin surface appears green to the human eye.
The real advantage in having three types of pigment cells is that frogs can change their intensity and color when someone is craving frog legs for supper. Manipulated by hormones, the pigment cells typically give the frog a palate ranging from bright green to various shades of brown and gray.
A frog might choose green when sunning on a leaf, brown when it moves to a branch, or gray when it lands on a rock.
What about green frogs’ brightly colored froggy-friends? Is there a reason for the neon shades or are they just wanting to stand out from the crowd?
Yes to both.
“Green frogs typically don’t have the same selective strategy as their brightly colored cousins. Red or yellow coloration is a visual cue to predators that they are about to dine on a toxic treat. Because green frogs usually are not poisonous, they must instead rely on a camouflage as a survival strategy,” said Judy Wages, a teacher from Bonners Ferry Idaho.
“Kermit the Frog said, ‘It’s not easy being green.’ But in terms of natural selection, he probably wouldn’t want it any other way.”
Frogs’ pigment gives them an advantage