by: Tom Hetherington
petsplace.com
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.
