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 adaptations that help them find food and keep them
from becoming food. One of the most widespread and varied
adaptations is natural camouflage, an animal's ability to hide
itself from predator and prey.
Photo courtesy David
Parks
Paradoxophyla
palmata, a narrow-headed frog native to Madagascar.
The frog's brown and yellow coloring, as well as its
rough texture, allow it to blend in with the mud and
tree trunks in its
environment.
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In this edition of HowStuffWorks,
we'll see how animals blend in with their environment so that
others might overlook them. We'll look at a few sophisticated
hiders who can change their camouflage in accordance with a
change in their surroundings. In addition to these expert
hiders, we'll look at some animals who don't hide at all, but
throw predators off by disguising themselves as something
dangerous or uninteresting.
Concealing Colors
Most animal species in the
world have developed some sort of natural camouflage that
helps them find food and avoid attack. The specific nature of
this camouflage varies considerably from species to species.
Photo courtesy Carl
Roessler
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.
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There are several factors that determine what sort of
camouflage a species develops:
- Camouflage develops differently depending on the
physiology and behavior of an animal. For example, an animal
with fur will develop a different sort of camouflage than an
animal with scales, and an animal that swims in large
schools underwater will develop different camouflage than
one that swings alone through the trees.
- 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. In this case, the various
elements of the natural habitat may be referred to as the
model for the camouflage.
- Since the ultimate goal of camouflage is to hide from
other animals, the physiology and behavior of an animal's
predators or prey is highly significant. An animal will not
develop any camouflage that does not help it survive, so not
all animals blend in with their environment the same way.
For example, there's no point in an animal replicating the
color of its surroundings if its main predator is
color-blind.
For most animals, "blending in" is the most effective
approach. You can see this sort of camouflage 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. Sharks, dolphins
and many other sea creatures have a grayish-blue coloring,
which helps them blend in with the soft light underwater.
Photo courtesy David
Parks
A cryptic frog.
This species has developed a coloring, texture and form
that are similar to the leaves found in its
environment.
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There are two ways in which animals produce different
colors.
- Biochromes, which are microscopic, natural
pigments in an animal's body, produce colors chemically.
Their chemical makeup is such that they absorb some colors
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.
- Animals may also produce colors via microscopic physical
structures. Essentially, these structures act like prisms,
refracting and scattering visible light so that a certain
combination of colors are reflected. 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 it a little bit. This bounces the light
around so that some of it makes it to the surface of the
skin and the rest of it is deflected back out, producing
white coloration. 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
light to reflect a blue color. Combined, these layers of
skin produce green. To learn more about coloration and
light, check out How Light
Works.
Both physical and chemical coloration is determined
genetically; they are passed on from parent to offspring. A
species develops camouflage coloration gradually, through the
process of natural selection. In the wild, an
individual animal that more closely matches its surroundings
is more likely to be overlooked by predators, and so lives
longer. Consequently, the animal that matches its surroundings
is more likely to produce offspring than an animal that does
not match. The camouflager's offspring will likely inherit the
same coloration, and they will also live long enough to pass
it on. In this way, the species as a whole develops ideal
coloration for survival in their environment.
The means of coloration depends on an animal's
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.
Camouflaging coloration is very common in nature -- you see
it to some degree in the majority of species. But it is much
less common for an animal to be able to change its coloration
to match a changing environment. In the next section, we'll
look at a few of the animals that use this sort of adaptive
camouflage.
Color Change
In the last section, we saw
that the most basic form of camouflage is a coloration that
matches an animal's surroundings. 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 the 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.
Reproduced with permission of the Minister of
Public Works & Government Services Canada,
2001
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.
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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. 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.
Photo courtesy 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.
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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.
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. The most famous color-changer,
the chameleon, alters its skin color using a similar
mechanism, but not usually for camouflaging purposes.
Chameleons tend to change their skin color when their mood
changes, not when they move into different surroundings.
Some animal species actually change which pigments are in
their skin. Nudibranches (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.
Many fish species gradually produce different pigments
without changing their diet. This works something like
seasonal molting in mammals and birds. When the fish
changes environments, it receives visual cues of a new
surrounding model. Based on this stimulus, it begins to
release hormones that change how its body produces pigments.
Over time, the fish's coloring changes to match the new
surroundings.
The Element of Disguise
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-line" 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.
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. Since
lions are colorblind, it doesn't matter that the zebra and
surrounding environment are completely different colors.
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.
Photo courtesy Carl
Roessler
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.
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Generally, this sort of camouflage doesn't hide an animal's
presence, it merely misrepresents it. 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
its only a stick, and so 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.
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.
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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 snake
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,
other, 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.
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