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.
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.
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.