Nature's 'true' colours
From how it is perceived to how it can be manipulated, human interaction with colour pales in comparison to that of other creatures in the animal kingdom. Katy Storch reports.
In the mid-1900s, the first lens replacement surgery in the human eye was carried out. This launched a 50-year period of growth and experimentation that would establish the most commonly-used eye surgery procedure in the world today. But when the surgery was first developed, scientists didn’t realise the complexity of the human eye.
Scientists weren’t aware, for example, that the human eye lens had evolved the ability to absorb UV light (ultraviolet) and protect the retina from its damaging effects.
This evolutionary achievement certainly warrants merit – imagine the state of our eyes without that basic, natural protection from UV rays – but one can’t help wondering how different our world would look if we had the ability to see UV light.
For some people in the world, this fantasy is actually their reality. Due to an absence of the natural crystalline lens (a condition known as aphakia), or the replacement of the lens with a synthetic intraocular lens that does not absorb UV light, there are people living in the world today who are able to see in the UV spectrum.
“The most striking experience [for a person who could see UV light], I think, would be all the new patterns that [he or she] could see in the world around … on flowers, butterflies, birds,” says Nathan Morehouse, a biologist at the University of Pittsburgh in the U.S. “In many of these cases, the UV colours are very bright, produced by structures that reflect UV light like a mirror.”
Many animals have evolved the ability to see colours we could hardly dream of.
In the animal kingdom, iridescent belles and bioluminescent beasts roam the world, communicating through their fine-tuned evolutionary genius in ways that humans are incapable of mimicking. Numerous discoveries by colour communication specialists have amounted to an impressive and ever-growing understanding of the evolutionary significance of colour among these creatures.
As humans, we are trichromats, which means we have three cones inside the eye to perceive colour. Whatever the type of light that may be shining, we can only perceive it through these three cones.
But for dogs, cats, and many other mammals, two cones of colour vision are enough to survive, allowing them to distinguish between colours in the blue to yellow spectral area. Humans have evolved the ability to see in the green to red spectral area that these animals lack.
In a lecture he gave for the Queensland Brain Institute at the University of Queensland in Brisbane in August 2010, researcher Justin Marshall suggested why this might be: “The sorts of tasks that probably drove this addition [of a third spectral sensitivity] are foraging tasks where the need to detect red against green (leaves or fruit) became important for survival,” he said.
However, humans are far from exceptional in the colour vision arena of the animal world. Many animals have evolved the ability to see colours we could hardly dream of.
Take the mantis shrimp for example. This stomatopod – or marine crustacean – has actually evolved the ability to see a grand total of 12 colour channels. Many butterflies and dragonflies possess the power of five cones to perceive colour. And overall, most of the animals around us, at the very least, have the ability to see ultraviolet light in the 300-400 nanometre (nm) spectrum. Our vision cuts off at 400 nm and higher, according to Marshall.
There are a number of tricks used by different organisms to create
these myriad colours.
Even more astonishing are the colours that radiate from some animals’ bodies. Milky coral and beige crustaceans glow, deep-sea critters stage bioluminescent disco shows thousands of metres below the surface, and iridescent butterflies shimmer for the mates they fancy.
There are a number of tricks used by different organisms to create these myriad colours. The most basic and common mode of colour production involves pigments, which are chemical compounds designed to selectively absorb and reflect certain wavelengths of light.
The ability of many animals to use the pigment in their skin for camouflage is well established and long-studied. The various mechanisms of camouflage can be grouped into six different categories. Cephalopods – a class of molluscs that includes octopi, squid and cuttlefish – are the kings of deception. Some of them use all six to hide from predators.
The secret of their concealment lies in pigment-containing chromatophore organs dispersed across the skin. Through muscular contraction and relaxation, cephalopods can control the pigment activity within the chromatophores and thereby change color on demand.
In other cases, the need for camouflage has little or no weight
in propelling evolution.
In other animals, we see the production of camouflage occurring in a slightly more permanent way. Hopi Hoekstra, a biologist from Harvard University in the U.S., conducted a study on Florida beach mice and found that the rodents had evolved a new coat colour to blend in with the white sandy beaches of their environment in a period of just four to six thousand years.
The species exists around the southeastern U.S. and are known as ‘oldfield mice’ in other areas. The mice are extremely variable in coat colour, suggesting that migration onto darker and lighter soils affects the colour of fur in the mice.
Hoekstra identified a mutation in the gene responsible for lighter and variable coat colour among the beach mice and drew a parallel to ancient Neanderthals and woolly mammoths. It turns out that the same gene mutation was found in mammoth populations and the Neanderthals years ago, suggesting that there may have been dark and light forms of wooly mammoths, as well as red-headed and pale-skinned Neanderthals.
Scientists in this field try to understand the story behind these evolutionary adaptations. In the case of beach mice, Hoekstra and colleagues carried out tests involving various coloured plastic mice to validate their hypothesis that the mice were evolving a lighter colour to blend in with their environments and avoid predatory threats.
In other cases, however, the need for camouflage has little or no weight in propelling evolution. Often times, sexual selection plays a much greater role.
Beyond his love for relaying fascinating anecdotes, Morehouse is also a specialist on colour communication and sexual selection in butterflies. He spends his days pondering the meaning of bright colours among male butterflies, and digging deep to understand exactly why the females are so enamoured by their flashy hues.
Some butterflies mix two separate modes of colour production. They use the traditional pigment mode, as well as what is known as the structural technique. Structural colour differs from pigment colouration due to the presence of many minute patterns of scales running along the surface of the animal.
When light hits these small scales it is reflected back in varying wavelengths. Unlike pigment colouration, animals that have evolved the ability for structural colour production emit different wavelengths, or colours, of light depending on the angle of view. In other words, they are iridescent.
One of Morehouse’s subjects is the cabbage white butterfly, a small and somewhat ubiquitous species whose wings are white with black markings. These butterflies may seem ordinary, but as the saying goes, what you see isn’t always what you get.
Not only can these butterflies see in the UV range of the spectrum, but through a combination of pigment and structural colour, they can also produce colour that falls in the UV range. Small amounts of pigment are packaged in structured scales along the male butterfly wings, empowering them with the allure of iridescence in order to attract the ladies.
The past decade has seen great strides in revealing the role of fluorescence and bioluminescence in the natural world.
When a cabbage white female butterfly is lusting over her male counterpart, she is actually seeing a “brilliant royal purple” emanating from the wings, says Morehouse.
But why should the colour of these butterflies matter to one another? As it turns out, males with the most panache also possess superior genes for acquiring food, because pigment production is costly and requires sufficient protein consumption. The amount of “bling on the wing” tells females how much protein these males are capable of procuring, and thus informs them of their genetic qualifications, says Morehouse.
So the most dazzling males are sexually selected, and are most likely to spread their genes throughout the species. Although used most often for sexual selection, structural colouring is also used for inter-species identification and other mating procedures.
The past decade has seen great strides in revealing the role of fluorescence and bioluminescence in the natural world. From predator repulsion to injury response, these stunning colours have evolved to accomplish a wide array of functions.
The third type of colour production mechanism – fluorescence – involves a manipulation of light. A pigment on an organism absorbs light at certain wavelengths (often in the UV spectrum) and then re-emits it in a different wavelength that fluoresces.
Research has shown that certain species of parrots possess fluorescent feathers for the same reason that the cabbage white butterfly shines purple – the more bright the feather, the more appealing the genes for the opposite sex. But the purpose of fluorescence is unknown in many other creatures, such as the jellyfish species Aequorea Victoria, of which the green fluorescent protein (GFP) molecule was extracted from for use in biomedical research.
Injured coral were more likely to fluoresce than those coral that were healthy.
Research is currently being done to better understand the purpose of fluorescence among underwater corals. A study lead by Caroline Palmer from the Australian Research Council’s Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, and published in the journal PLoS One in 2009, identified a possible function for the fluorescing capabilities of coral in the Caribbean. Palmer found that injured coral were more likely to fluoresce than those coral that were healthy.
The coral release free radicals, such as hydrogen peroxide, to heal injuries; however, too much hydrogen peroxide can do damage to the healthy areas of the coral. Palmer found that the coral had evolved a defence mechanism against possible damage from free radicals by releasing fluorescent proteins to act as antioxidants.
Other theories regarding the purpose of coral fluorescence suggest that it serves as a mechanism to provide the algae symbiotically inhabiting the coral with an enhanced light source for photosynthesis when sunlight is limited. Conversely, it may serve as a way of lessening the amount of UV light that shines on the algae, resulting in a protection of the algae and lower levels of coral bleaching.
Fluorescence is a source of glowing light that is triggered by an external light source, so those animals that can fluoresce first need light to do so. It seems almost unfathomable that an organism could have evolved the ability to produce a similar glowing light internally, but that is exactly what bioluminescence, the fourth and final colour production mechanism, is.
Bioluminescence is produced through a chemical reaction in the biological system of an organism. Two chemicals within an organism are mixed together, but rather than heating up, as is often the result of a chemical reaction, light is emitted. Fireflies and other animals on land, and marine animals in the deep-sea, create an internal glow using this type of radiation.
A bit of mystery still remains in all areas of
the colour communication research realm.
Fireflies and beetles produce lights in patterns of flashes, explains Peter Vikusic, a physicist at the University of Exeter in Britain and an expert on the physics of colour production. There is even a kind of firefly bug that can mimic the flash frequency of other species, thereby attracting close-related specimens and making a meal out of them, he says. Bioluminescence likely evolved for communication at the bottom of the ocean. It is a costly process for an organism to produce this light, so the adaptation must have been very useful.
Edith Widder, a marine biologist with Ocean Research & Conservation Association (ORCA) in Florida, is one of the foremost experts on bioluminescence in the deep-sea. Her research revealed strong evidence that animals in the deep ocean use bioluminescence to ward off predators, and paved the way for other pundits in the field.
One such bioluminescententhusiast is Sönke Johnsen, a biologist at Duke University in the U.S., who studies crabs and other crustaceans existing on the ocean floor. On the sea floor, animal vision is different: eye size diminishes as the depth increases, but once you go as low as the sea floor, they suddenly grow large again, according to Johnsen. At that depth, everything one touches seems to glow, he adds.
Johnsen and his team have identified a greater amount of green bioluminescence on the ocean floor, in contrast to the predominately blue bioluminescence found in most regions of the sea. It is likely that the green bioluminescence is characteristic of coral on the ocean floor. Crabs that are able to see this bioluminescence may have evolved these capabilities in order to sort good food from toxic food.
In truth, a bit of mystery still remains in all areas of the colour communication research realm. Even the most basic style of colour production (pigment) is still being investigated and understood. We remember Darwin as the first well-known scientist to liken human origins with those of animals. He once noted, “Animals, whom we have made our slaves, we do not like to consider our equal.”
Marshall expounds on Darwin’s reflections, “as arrogant humans we tend to assume we are the pinnacle of evolution, however, certainly in sensory terms this is far from true.” It seems we do indeed have much to learn from our more ‘primitive’ animal counterparts after all.