In nature, every advantage increases an animal's chances of survival, and therefore its chances of reproducing. This simple fact has caused animal species to evolve a number of special camouflage adaptations that help them find food and keep them from becoming food. Often, animals blend in with their environment so that they might be overlooked. Other coloration or skin adaptations work specifically in relation to an animals behavioral patterns and only function correctly when used in combination with certain actions or behaviors. Other animals don't hide at all, but throw predators off by disguising themselves as something dangerous or uninteresting. All of these adaptations occur based on the processes inherent in the vision of and the vision capabilities of their specific predators/prey.
Concealing Colors
Most
animal species in the world have developed some sort of natural camouflage
using concealing colors that help them find food or avoid attack. This camouflage varies considerably
from species to species. An
animal's environment is often the most important factor in what the camouflage
looks like. The simplest
camouflage technique is for an animal to match the "background" of
its surroundings. This usually
does not simply mean a solid color that matches the color of; say a leaf it is
often found near. As the
Craik-OÕBrien illusion[1]
teaches us, we know that vision works by seeing outlines. We know a dog when we see one, not by
its color as much as by its shape.
An animal may have a variety of colors or just one color. This is less significant than the ultimate
goal of most camouflage. What
matters most is to break up the outline.
We see this in the example of the Narrow-Headed Frog. This animal is multi colored in a
similar pattern to the natural environment in which it is found. This causes the eye of its predators to
break up the outline of the shape of the frog, and instead see various shapes,
which have no relationship to a living creature. Instead this frog looks like a part of a rock with some mud
on it.
Photo by David
Parks
Paradoxophyla
palmata, a Narrow-Headed Frog native to Madagascar. The frog's brown
and yellow coloring, as well as its rough texture, allows it to blend in with
the mud and tree trunks in its environment.
Photo by Carl
Ressler
A
Tartan Hawkfish, photographed off the coast of Papua New Guinea - The fish's
striking coloration allows it to blend in with these bright gorgonian fans.
For most
animals, "blending in" is the most effective approach of camouflage.
You can see this everywhere. Deer,
squirrels, hedgehogs and many other animals have brownish, "earth
tone" colors that match the brown of the trees and soil at the forest
ground level. Unlike the
Narrow-Headed Frog, these animals have less of a specific color pattern against
which it is silhouetted; therefore its coloration must be less specific to work
in a variety of situations. Still,
there are often colorations, which help to break up their outline. Some examples include the white tailed
dear or Elk, both of which have light colorations on their backsides in order
to break up their outline. Sharks,
dolphins and many other sea creatures have a grayish-blue coloring, which helps
them blend in with the soft light underwater. As light penetrates the water the change in medium causes
refraction[2]
and dispersion.[3] The resulting light in the ocean is
somewhat bluer.
As we know
through studies of color mixing, color subtraction properties, within this type
of environment, make an animal of the same color difficult to see.[4] Also many of these animals have lighter
underbellies to compensate for the broadband spectrum of wavelengths seen when
looking upwards towards the surface and sunlight.
Another
trick of the camouflager is counter-shading. This occurs when the coloration of the upper parts of an
animal is darker than itÕs undersides.
Hence the effect of sunlight is counteracted. As more saturated broadband white light shines on the
animals back, its darker back absorbs more of the light so as not to be as
light in appearance. The
underbelly, which receives little direct sunlight, does not need as much compensation
to reduce its glare. Also the normal
casting of its shadow on the ground is distorted because more light is
reflected on the ground underneath the animal, due to its light colored
underside and it is less likely to be given away by its shadow. Other benefits to a lighter underbelly
in a hot climate are to absorb less light and allow the animal to remain
cooler, and to repel ticks and fleas, which tend to avoid light or white areas.
The means
of an animalsÕ coloration depends on its physiology. In most mammals, the camouflage coloration is in the fur,
since this is the outermost layer of the body. In reptiles, amphibians and fish, it is in the scales; in
birds it is in the feathers; and in insects it is part of the exoskeleton. The actual structure of the outer
covering may also evolve to create better camouflage. In squirrels, for example, the fur is fairly rough and
uneven, so it resembles the texture of tree bark. Many insects have a shell
that replicates the smooth texture of leaves.
Photo by David
Parks
A
cryptic frog - This species has developed a coloring, texture and form that are
similar to the leaves found in its environment.
Animals
produce their colors using chromatophores. There are two different types of chromatophores, biochromes
and schematochromes.
[5] Biochromes, which are microscopic,
natural pigments in an animal's body, produce colors chemically. Their chemical makeup is such that they
absorb some wavelengths of light and reflect others. The apparent color of a pigment is a combination of all the
visible wavelengths of light that are reflected by that pigment. The other fascinating, but less common
way for an animal to produce colors is via microscopic physical structures,
called schematochromes.
Essentially,
these structures act like prisms, refracting and scattering visible light so
that a certain combination of colors are reflected.[6] Polar bears, for example, actually have
black skin but appear white because they have translucent hairs. When light shines on the hairs, each
hair bends the light rays a little bit.
This bounces the light around from hair to hair so that some light makes
it to the surface of the black skin, which absorbs the energy/heat (good for
conserving energy to keep warm in cold artic climates), and the rest of it is
deflected back out, producing the effect of white coloration. Actually a polar bear looks a little
yellowish compared to the white snow it is often around. An explanation could be that a slightly
larger percentage of wavelengths released are those in the mid-range hence the
yellowish (albeit very unsaturated) appearance.
Photos by
Feenicks
Polar
Bears can appear yellowish to a snow-white color, depending on the time of day
and the color of its background.[7]
In some
animals, the two types of coloration are combined. For example, reptiles, amphibians and fish with green
coloration typically have a layer of skin with yellow pigment and a layer of
skin that scatters and reflects light.
As discussed earlier most of the light in this environment is in the
blue and green area of the spectrum.
Through this strange combination of additive and subtractive color
mixing, these layers of skin often produce a grayish-green.
Of course,
an animal's surroundings may change from time to time. Many animals have developed special
adaptations that let them change their coloration as their surroundings
change. One of the biggest shifts
in an animal's surroundings occurs with the changing of seasons. In the spring and summer, a mammal's
habitat might be full of greens and browns, while in the fall and winter;
everything can be covered with snow.
While brown coloration is perfect for a summer-wooded environment, it
makes an animal an easy target against a white background. Many birds and mammals deal with this by
producing different colors of fur or feathers depending on the time of year. In
most cases, either changing amounts of daylight or shifts in temperature
trigger a hormonal reaction in the animal that causes it to produce different
biochromes.
Feathers and
fur in animals are like human hair and fingernails -- they are actually dead
tissue. They are attached to the
animal, but since they are not alive, the animal can do nothing to alter their
composition. Consequently, a bird
or mammal has to produce a whole new coat of fur or feathers in order to change
color. Many animals can switch the
coloring agent in the hair follicle on and off -- so in the summer, the hair is
pigmented brown with melanin[8],
but in the winter it is not pigmented, leaving the hair white.
Photo by David
Parks
As
the seasons change, the Arctic fox changes the color of its coat. In the spring
and summer, it has a dark coat, to match the brown dirt in its environment. In
the fall and winter, it turns white, to match the surrounding snow.
In many
reptiles, amphibians and fish, on the other hand, coloration is determined by
biochromes in living cells.
Biochromes may be in cells at the skin's surface or in cells at deeper
levels. These deeper-level cells
are called chromatophores. Some animals, such as various cuttlefish species, can
manipulate their chromatophores to change their overall skin color. These animals have a collection of
chromatophores, each of which contains a single pigment. An individual chromatophore is surrounded
by a circular muscle that can constrict and expand. When the cuttlefish constricts the muscle, all the pigment
is squeezed to the top of the chromatophore. At the top, the cell is flattened
out into a wide disc. When the muscle relaxes, the cell returns to its natural
shape of a relatively small blob. This blob is much harder to see than the wide
disc of the constricted cell. By
constricting all the chromatophores with a certain pigment and relaxing all the
ones with other pigments, the animal can change the overall color of its body.[9]
Using
partitive color mixing[10],
Cuttlefish with this ability can generate a wide range of colors and many
interesting patterns. By
perceiving the color of a backdrop and constricting the right combination of
chromatophores, the animal can blend in with all sorts of surroundings.
Cuttlefish may also use this ability to communicate with each other.
Another
animal that communicates through use of visual color cues is the most famous
color-changer, the chameleon. Chameleons
alter their skin color using a similar mechanism as Cuttlefish, but not usually
for camouflaging purposes. Chameleons tend to change their skin color when
their mood changes. This is
controlled by hormone balances, not intentional muscular control or as a direct
reaction to moving into different surroundings.
Photo by David
Parks
Chamaeleo
pardalis, a chameleon species found in the forests of Madagascar.
Chameleons can produce a wide range of colors and patterns on their skin, but
they do this primarily to express mood, not to blend in with different
environments.
Photos by Wernher Krutein
Nudibranchs
are marine snails (Gastropod Molluscs) that have no shell at maturity.[11]
Some animal
species actually change which pigments are in their skin. Nudibranchs, a small sea creature change their coloration by altering
their diet. When a nudibranch
feeds from a particular sort of coral, its body deposits the pigments from that
coral in the skin and outer extensions of the intestines. The pigments show through, and the
animal becomes the same color as the coral. Since the coral is not only the
creature's food, but also its habitat, the coloration is perfect
camouflage. When the creature
moves on to a differently colored piece of coral, its body color changes with
the new food source. Similarly,
some parasite species, such as the fluke, will take on the color of their host,
which is also their home.
The
FlamingoÕs coloration is effected by its food intake as well, but not for
camouflage. A Flamingo eats
plankton, using baleen. Much like
a whale, it uses barbs on itÕs long tongue to strain plankton through the
baleen and down its throat. The
amount of pink coloration of a Flamingo is directly related to the amount of
plankton it is able to eat. The more
Plankton it eats, the pinker it becomes, and the more likely it will be
selected to reproduce.
This type
of cultural appearance effecting behavior is widely known to humans. For example, would-be heroes of ancient
Greece used harsh soaps and bleaches to lighten and redden their hair to the
color that was identified with honor and courage. First-century Romans
preferred dark hair, which was made so by a dye concocted from boiled walnuts
and leeks. Today, hair coloring
remains an important part of our culture, with a booming 75 percent of American
women reportedly coloring their hair.
Women have also decided that blondes don't necessarily "Have more
fun!" Red is currently the
most requested color at beauty salons.
And women aren't alone, men increasingly cover gray or, completely
change their look. Men's home
hair-color sales reached $113.5 million last year, a 50 percent increase in
just five years.
In addition
to background-matching coloration, many animals have distinctive designs on
their bodies that serve to conceal them. These designs, which might be spots,
stripes or a group of patches, can help the animal in a couple of ways. First,
they may match the pattern of "the model," the background of the
animal's surroundings. For example, animals that inhabit areas with tall,
vertical grass often have long, vertical stripes. Second, they may serve as
visual disruptions. Usually, the patterns are positioned "out-of-lineÓ
with the body's contours. That is, the pattern seems to be a separate design
superimposed on top of the animal. This makes it hard for the predator to get a
clear sense of where the animal begins and ends -- the pattern on the body
seems to run off in every direction.
The pattern
of contrasting shapes and forms that make up the distinctive stripes on the
coat of the zebra serve this purpose.
When looking from a distance the eye has difficulty in fitting a broken
color form into one solid form.
Rather it will have a tendency to view the light patches between the
dark stripes as the patches of light visible between grass and trees. So it is possible for the zebra to
merge into the grasslands and become invisible.
This
disruptive coloration is particularly effective when animals in a species are
grouped together. To a lion, a
herd of zebras doesn't look like a whole bunch of individual animals, but more
like a big, striped mass. The
vertical stripes all seem to run together, making it hard for a lion to stalk
and attack one specific zebra. The
stripes may also help a single zebra hide in areas of tall grass. To humans, a zebra's stripes stick out
like a sore thumb, so it's hard to imagine that the stripes act as camouflage,
but because lions are colorblind, it doesn't matter that the zebra and
surrounding environment are completely different colors.
Zoologists
believe stripes offer zebras protection from predators in a couple of different
ways. The first is as simple
pattern-camouflage, much like the type the military uses in its fatigue design. The wavy lines of a zebra blend in with
the wavy lines of the tall grass around it. It doesn't matter that the zebra's
stripes are black and white and the lines of the grass are yellow, brown or
green, because the zebra's main predator, is colorblind. The pattern of the camouflage is much
more important than its color, when hiding from these predators. If a zebra is standing still in
matching surroundings, a lion may overlook it completely.
This
benefit may help an individual zebra in some situations, but the more
significant means of protection has to do with zebra herds. Zebras usually travel in large groups,
in which they stay very close to one another. The pattern of each zebra's stripes blends in with the
stripes of the zebras around it.
This is confusing to the lion, which sees a large, moving, striped mass
instead of many individual zebras. The lion has trouble picking out any one
zebra, and so it doesn't have a very good plan of attack. It's hard for the lion to even
recognize which way each zebra is moving.
The lion's inability to distinguish zebras also makes it more difficult
for it to target and track weaker zebras in the herd.
Photo by
Steve Silber
When
Zebras stand together, their stripes tend to blend into one another, making it
difficult to distinguish one from the other.
Generally,
this sort of camouflage doesn't hide an animal's presence, but merely
misrepresents it. Many fish
species are similarly camouflaged.
Their vertical stripes may be brightly colored, which makes them stand
out to predators, but when they swim in large schools, their stripes all meld
together. This confusing spectacle
gives predators the impression of one big, swimming blob. Another example of misrepresentation is
the common trait of many animals, particularly fish, to have dark spot like
markings on their tails. A
predator, expecting the fish to swim away in a forward direction, will move at
a trajectory to intercept the fish in front of it. If the fishÕs tail looks like its head, it can often easily
get away.
Photo by Prizm
Many
tropical fish use their coloration in combination with evasive tactics to avoid
predators.[12]
A related
camouflage tactic is for an animal to take on the appearance of some other
object. One of the most famous
examples of this sort of impressionist is the walking stick, an insect that
looks like an ordinary twig. A
predator can easily distinguish a walking stick from its surroundings, but the
predator thinks itÕs only a stick, and ignores it. You can also see this sort of camouflage in some Katydid
species, which have evolved so that they look just like tree leaves. Many leaf insects so closely resemble
the green leaves of a tree, with their shape, markings and movements that other
insects have been so deceived that they have actually nibbled on the
insect. Many of such animals also
have an uncanny ability to remain motionless for extended periods of time,
adding to the impression that they are a part of the backdrop.
Photo courtesy
Scott Camazine
Walking
sticks have adapted to resemble their surroundings. Most of the time, their
predators pass them by as they would a real twig.
Photo courtesy
Carl Rossler
A
leafy sea dragon, photographed off the coast of Australia. Leafy sea dragons
have developed flowing appendages and vivid coloration that lets them blend in
with the undersea plant life in their environment.
Other
animals use a more aggressive sort of mimicry. Several moth species have developed striking designs on
their wings that resemble the eyes of a larger animal. The back of the Hawk Moth Caterpillar
actually
looks like a snakeÕs head, a frightening visage for most predators the moth
would come across. A simpler
variation on this adaptation is simple color mimicry. In many ecosystems, smaller poisonous animals develop a
bright coloration. Predators learn
to steer clear of these colors, lest they get a mouthful of venom. Over time, non-poisonous species may
develop the same coloration, cashing in on the nasty reputation of the
poisonous species.
Mimicry
is a different approach than ordinary camouflage, but it works toward the same
end. By developing a certain appearance, an animal species makes itself a
harder target for predators and a sneakier hunter for prey. In different areas
around the world, you'll see all sorts of variations and combinations on the
basic elements of camouflage. As animal species evolve, they become more and
more in tune with their environment. Often, these sorts of adaptations are more
effective survival tools than an animal's more aggressive weapons of defense
(teeth, claws, beaks). After all, being entirely overlooked by a predator is
preferable to having to put up a fight.
There
are a variety of means for animals to use their environment, the abilities of
the senses of other animals in their eco-system, and physical properties of
light, to produce an effective means of camouflage. Some types of camouflage need very specific situations
occurring in their specific niche to be effective. Others are much more flexible and can work well in a variety
of situations. Camouflage is an
essential part of every animalÕs existence and a fascinating subject of study.
[1] Page 189-190
Seeing The Light; Falk, Brill, Stork 
[2] SnellÕs Law – Page 53 Seeing The Light; Falk, Brill, Stork
[3] Page 62-63 Seeing The Light; Falk, Brill, Stork
[4] Imagine trying to see red lines on a map while looking at it using a red light bulb. Although the red lines may not appear different, if the background of the map was white, they would seem to have disappeared because the area surrounding the red lines would too be red.
[5]http://www.clarku.edu/departments/biology/biol201/CSantos/Chromatophores.htm
[6] Page 63 Seeing The Light; Falk, Brill, Stork
[7] http://members.ozemail.com.au/~feen/pb_old.html
[8] Info on melanin: http://www.britannica.com/eb/article?tocId=9051870
[9] Info on cuttlefish:
http://www.tonmo.com/articles/basiccuttlefish.php
http://www.heptune.com/cutfish.html
[10] A process of color mixing whereby small separate sources of various colors are placed close to each other. If the eye cannot see them separately, the colors mix additively. Page 63 Seeing The Light; Falk, Brill, Stork
[11] http://www.photovault.com/Link/Animals/Aquatic/lGastropods/Nudibranch.html
[12] http://www.heavenly-creatures.net/xinher/archive/003765.html