Colour-changing animals, explained

Animals from chameleons to cuttlefish can manipulate their colours for a variety of reasons – to hide from predators, intimidate rivals, or woo mates.Wednesday, 19 February 2020

In nature, the ability to change colour can be key to survival. Vision is a very important sense in much of the animal kingdom, and many animals have come up with unique ways to use this sense to enhance their own survival. They may use this superpower to vanish into their environments—or to boldly assert their dominance.

Seasonal change

For animals that live in environments that change dramatically between seasons, it's useful to shift colours to blend in with the background as habitats transform from lush in summer to snowy in winter.

The rock ptarmigan, a grouse found in the far north of Eurasia and North America, sports brown plumage in the summer. But as autumn progresses, the ptarmigan molts. New, pure white feathers replace its earth-toned ones. By winter, the bird is snow white, allowing it to avoid detection by predators in its Arctic habitat. Stoats undergo a similar transformation, with the weasel-like mammals gradually shedding their rich, brown fur for a white coat. While camouflage helps the grouse hide, stoats are predators. Their camouflaged white coats help them blend into snow, making it easier to hunt small mammals and birds.

More than 20 species of birds and mammals in the northern hemisphere undergo total colour transformations from brown to white between summer and winter. As days shorten in autumn and lengthen again in spring, these animals get hormonal signals that trigger the turnover of fur or feathers.

Seasonal changes in colouration also happen in habitats you might not expect, including underwater. Chameleon shrimp, for example, can be green or red. Research published in 2019 revealed that while the crustaceans hang out on red or green algae that matches their own colour, they can also slowly shift to the other form as the dominant algae type in their rock pool habitat fluctuates with the seasons.

Meanwhile, goldenrod crab spiders can flip their camouflage if they change locations. They lie in wait for prey on white or yellow flowers, changing their colours to match the flower they live on. If a yellow spider moves to a white flower, it can move its yellow pigments underneath cells that contain white pigments, a process that takes several days.

Quick changers

Some colour changers can transform their hues in a matter of seconds.

Chameleons, for instance, can induce colour change in less than half a minute with the help of special cells in their skin. Some of these are “dermal chromatophores” that contain pigment and are shaped like an asterisk adorned with long extensions.

“You can think of these as the highways where pigments can move around,” says Russell Ligon, a biologist at Cornell University.

These pigments then interact with another type of cell called an iridophore, which contains reflective crystals. Chameleons can stretch their iridophores to change the wavelength—and therefore the colour—of the light they reflect. The reflected light from iridophores works in concert with the pigment in the chromatophores to produce the suite of brilliant blues, reds, and oranges seen in many chameleons. (Read more about how chameleons change colour.)

This colour-changing prowess can be useful as camouflage, but chameleons primarily use their abilities for other purposes. While calmly resting on a branch, a chameleon’s colour may be relatively green and dim. But in tense social situations, chameleons’ skin tightens, shifting their iridophores and changing colours accordingly. Rapid colour displays are especially useful among males quarrelling over territory. Male veiled chameleons signal their temperament to would-be rivals by changing tone, turning brighter to signal aggression and darker to convey submission. A brief colour “battle” can stop a costly physical altercation before it starts, Ligon says.

Some crustaceans and fish also can change colors rapidly. Australian fiddler crabs can darken their blue colour patterns in about 20 minutes when courting or stressed, and small fish called rock gobies can darken or lighten their colours in less than a minute to better blend into their tidepool surroundings, avoiding the gaze of hungry shorebirds.

Super quick changers

Other animals have evolved colour-changing mastery that can take place in the blink of an eye.

Chameleon sand tilefish, for instance, can flash from blue-green to red in half a second thanks to a special type of iridophore in their scales that’s activated by an adrenaline response.

But cephalopods like octopuses, squids, and cuttlefish are the uncontested masters of rapid colour change. Their pigment-containing chromatophores are directly controlled by their central nervous system, says William Gilly, a biologist at Stanford University’s Hopkins Marine Station. In chameleons, these cells change colour as pigment moves around in them, but cephalopods can actually change the shape of the cells themselves, triggering lightning-fast colour shifts.

Squid and cuttlefish can flash bands of rippling pigment to signal that they’re ready to mate, Gilly says. The displays can be particularly spectacular in Humboldt squid, which flash their entire body’s chromatophores at once, strobing between pale and dark red as fast as four times per second.

Golden tortoise beetles may have the most striking rapid colour-changing system of all. The Central American beetles—smooth, egg-shaped insects about the width of a pencil eraser—have a transparent shells that houses thin, stacked layers of plates etched with extremely small grooves. The beetle can fill these grooves with red fluid that makes the plates appear perfectly smooth so that they become reflective, taking on a metallic golden hue. But if agitated or mating, the beetle quickly drains the fluid from the grooves, breaking the illusion and showing the bright red pigmentation below.

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