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  • Writer's pictureSBE VIT

The Mystery of Camouflage

Camouflage: The natural disguise of animals, helping them to prey on others, or not become their predator’s next meal. Camouflage is a frequent sight in the animal kingdom, from the leopard’s spotted coat to the leaf mimic katydid’s wings. Most animals generally aim for crypsis, i.e. the ability to avoid observation by other animals. Some animals like chameleons change their colour to express their emotions. Others, like squid use camouflage to communicate their secrets in the middle of a large crowd.

Chameleons do camouflage to blend in with their environment, but as many of them can run at approximately 21 miles per hour, they can easily avoid their predators. The specialized cells that form the skin-the chromatophores, are how they change their colour. Dermis layer of the skin has different types of chromatophores. Xanthophores contain yellow pigment. Erythrophores contain red pigment, which is generally responsible for the stripes, and melanophores contain melanin, the pigment which is also found in human cells.


Chameleon


Although there are many green chameleons, they do not have any green pigment in their skin cells. This spiked the curiosity of Michel Milinkovitch, an evolutionary geneticist and biophysicist. He and his University of Geneva colleagues came across another layer of skin cell beneath the pigmentary cells. Those were found to be iridophores- cells with nanoscale guanine crystals arranged in triangular lattice. Under different conditions of pressure and chemicals, these crystals alter the spacing between them, which in turn affects the colour of light they reflect. Their colour changes due to changes in hormones in their body. In neutral state the crystals are tightly packed together and green or blue colour is observed. When the crystals move further apart, yellow, orange and red wavelengths are reflected. This implies a state of excitement- the chameleon is showing aggression or a desire to mate. To show that the chameleon is submissive, it disperses melanin into its upper skin layers giving it a dark appearance.


Cephalopods are often called chameleons of the sea. They include squids, octopuses, cuttlefish etc. Impressively, they change colour faster than chameleons since their chromatophores are controlled neurally. The chromatophores are present right under the skin and contain black, brown, red, orange or yellow pigments. In chameleons, cells change colour as pigments move around in them. In Cephalopods however, chromatophores are an elastic saccule that holds a pigment and they have 15-25 muscles attached to this saccule. When the muscles contract the saccule stretches, allowing the pigment inside to cover a large area. Fully expanded chromatophores are up to 1.5mm in diameter in squid and 0.3mm in cuttlefish. Each chromatophore is attached to a nerve ending. The animal can hence increase the size of one saccule while it decreases the size of another right next to it. This allows them to make complex patterns, like zebra stripes. Male cuttlefish display them to show aggressiveness.


Cephalopod - Male Cuttlefish


Under the layer of chromatophores, there is a layer of iridophore cells. These cephalopods can control the wavelength of light that is reflected by them. As a result the multiple rays add up in precise ways and reinforce each other producing intense bursts of colour, known as iridescence. This is how we see the metallic looking greens, blues and gold colours in some species. Iridophore cells not only produce iridescence, they also polarize the light. Squids and their relatives have eyes that are sensitive to polarized light, whereas their predators do not. This polarized light remains unaffected by chromatophores. Hence, these clever cephalopods can send signals to each other, while staying invisible to the nearby hunters.

Leucophores are the last layer of cells. They are flattened, branched cells that are believed to scatter and reflect incoming light. Hence, the color of the leucophores will reflect the predominant wavelength of light in the environment. For instance, if the animal is near the surface, there is an abundance of white light, the cells will be white. In deeper water, where blue light is abundant, they will be blue. Photophore are cells responsible for bioluminescence. They produce light with the help of chemical reactions. Some species have luminescent bacteria in sacs for the same purpose. Different cephalopods in diverse environments use various combinations of cells to produce color change. The displays can be spectacular sometimes, where they flash their entire body’s chromatophores at once, varying between pale and dark red as fast as four times per second.

The Golden Tortoise Beetle, found in Central America, is the first known insect to demonstrate rapid and controlled colour change. These smooth beetles have a transparent shell, made of thin plates, engraved with tiny grooves. The insect can fill these grooves with a red fluid, that makes the plates reflective, so that they take on a golden metallic shade. But if the beetle is agitated or mating, it drains the fluid, showing the red pigmentation below.


Golden Tortoise Beetle


The ability to change colour is essential for the survival of many animals. To hunt for food without being identified, to mimic another to avoid being eaten, or to just find a mate. Taking inspiration from these animals us humans have also incorporated camouflage in our lives. Military uniforms, aircrafts, ground vehicles and other equipment are successful in deceiving the eye with the help of camouflage, ensuring survival for our fellow humans at war too.


- written by Shikha Mimani




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