N ature’s J ewels All photos (c) naturepl.com

For many, the word iridescence conjures images of shiny jewellery, glittering seas, or sparkling diamonds. But it’s also a term that belongs in the animal kingdom. Creatures as diverse as insects, reptiles, birds and fish employ iridescence in a variety of ways, but we still don’t know for sure what benefits this resplendence affords the animals themselves. Iridescence, defined as a see are called structural colours lustrous, rainbow-like play of (as opposed to colours that are Fig.1 CONSTRUCTIVE INTERFERENCE colour that tends to change produced by chemical pigments - I with the angle of view, is such as the chlorophyll that makes something that most people are plants appear green). familiar with, whether they know When talking about iridescence, it or not. You can find it on a soap it helps to remember that light is bubble, in an oil spill in a wet car made up of waves. If two waves are park, or on the back of a CD. It is out of synch - that is, the crest of the reason why, when holding an one wave meets the trough of ano- abalone shell, you might turn it ther wave - then they cancel each Waves in similar phase Waves amplify each other in your hand, marvelling at the other out (a phenomenon known colours that skip across its inner as ‘destructive interference’). But if surface. the crests and troughs of the waves DESTRUCTIVE INTERFERENCE These arresting displays of colour line up (or have the same ‘phase’) all have one thing in common: they then they amplify each other. This arise from the interaction of light so-called ‘constructive interference’ with the physical structure of the is what causes iridescence. (See object in question. The colours we Fig.1). Waves out of phase Waves cancel each other

A lustrous, To understand how light waves interfere with each other, The interference-based colours seen in a soap we have to think about the surface of an object on a mi- bubble are produced by a single thin film. But the croscopic level. A soap bubble, for instance, is a very thin surface structures of biological organisms are rainbow-like sheet of water sandwiched between two layers of soap typically more complex. The more surface layers molecules, which form outer and inner surfaces. As light something has, the more chances there are for play of colour passes through the outer surface, some of it is reflected the reflections to magnify each other, and the As well as spectral colours, iridescence is back, while some penetrates to the deeper inner surface more pronounced the iridescence will be. Such sometimes used to describe something with where, again, it is reflected back (see Fig.2). Depending on structures are called multilayer reflectors. a radiant or attractive quality including me- the extra distance the second wave travels before rejoi- ning the first, the two waves will either amplify or cancel tallic colours and opal-like effects. The effect Fig.2 Interference of light waves reflected changes with perspective,as the light reflec- each other out. If the extra distance matches a specific from the outer and inner surfaces of ted from the iridescent surface enters the wavelength of light then constructive reinforcement a thin soap film. eye from different angles. can occur. But if the distance is the equivalent of half a

White light wavelength, then destructive cancellation occurs instead. V

Because white light is made up of all the colours of the rainbow (where each colour corresponds to a different V

wavelength), the reinforcement of some hues and the V

suppression of others creates a rainbow-like optical effect, Colour dependsV on interference

with different wavelengths of light producing different V V

colours. What’s more, the colours change because light AIR V V strikes the bubble at different points and from different angles, altering the distances between the externally and SOAP FILM internally reflected waves. Iridescent colouration is broadly distributed in the animal kingdom and appears to have evolved independently in a number of different groups.

All animal colours are caused by either pigmen- tary colouration, structural colouration, or a combination of both.

Iridescence has evolved multiple times At the optimal angle of view, iridescent in different groups of organisms and can colours can appear highly pronounced, be produced by different optical pheno- whereas at others, they can disappear enti- mena. Some species possess ‘diffraction rely, leaving only the pigmented colours gratings’: reflective surfaces with regularly such as the black melanins in the feathers ordered parallel ridges or grooves that of the Costa’s Hummingbird (left). The disperse different wavelengths of light in directionality of iridescent colours might different directions. Others have ‘photonic allow animals to orient themselves so crystals’: crystalline lattice structures that that they appear conspicuous to certain allow some wavelengths of light to pass receivers (such as mates) but dull to others through, while reflecting others. Because (such as predators). Directionality also the resulting iridescent colours are so allows animals to produce rapid flashes of bright and conspicuous when seen from colour, which may have evolved as an anti- the optimum viewing angle, they provide predator defence. Sudden shifts in colour many opportunities for visual communica- and brightness may dazzle or confuse tion, perhaps helping animals to recognise predators, making it more difficult to strike members of their own species, distinguish a target. sexual identity or maturity, co-ordinate In beetles, iridescence may also play a role group movements (such as in bird flocks or in warning colouration, indicating strong fish schools) or perform courtship displays toxicity, or - in species with no chemical to potential mates and aggressive displays defences - as a means mimicking other, to rivals. more toxic species. [Left] Even though oils are colourless, a thin film of oil on water produces a rainbow of hues due to light interference effects.

[Right] White light splits into its component colours as it passes through the feathers of some birds.

Spider webs [below] and sugar crystals [below right] can also produce structu- ral hues under the right conditions.

Could iridescence be accidental? [1] [2] Despite the theory that iridescence that the moles can move more effi- The eyes of many nocturnal ani- evolved for communication, some ciently through dirt or sand. Further mals contain a multilayer reflec- research suggests that it may be evidence suggests that iridescence tive structure called the tapetum a by-product of biological tissues may be a by-product of properties lucidum. This structure, which that are nanostructured to provide designed primarily for strength. improves night vision by reflecting non-communicative benefits, and For example, iridescent feathers [2] light back through the retina a that pretty visuals are merely a side often contain the pigment melanin, second time for maximal stimula- effect. For evidence of this, look which can provide better structu- tion of photoreceptors, also pro- at the golden mole [1] - the only ral integrity than feathers without duces an iridescent metallic-like known mammal to exhibit irides- melanin. Similarly, the diffraction [3] [4] ‘eyeshine’ [3]. Additionally, some cent fur. Like other mole species, gratings that create iridescent fishes and cephalopods (e.g. octopi golden moles are blind and spend coloration in some beetles also and squid) have iridescent struc- most of their lives in near-darkness, afford them stronger shells - to the tures in their cornea [4], which may so why should their fur shimmer? extent that some fossilised beetles function to reduce the amount of Researchers believe it is an evo- have been found with their irides- downwelling light entering the eye, lutionary accident, arising from cent structures still intact. without reducing the amount of flattened hairs that are designed to It’s also possible that iridescence sidewelling light (in essence, redu- repel water and reduce friction so is a side-effect of better eyesight. cing unnecessary solar glare). Blues and greens Birds are among the most common structural Iridescence is hues, as seen in the observed widely dazzling plumage in birds, and pro- of peacocks - but bably evolved inde- iridescence in birds pendently in several covers a wide groups. chromatic spectrum.

Although it has been recorded Opposite: The shimmering feathers of in many different species, the fiery-throated hummingbird scientists are yet to fully understand (Panterpe insignis) refract light like a the role of iridescence in birds. The most prism, splitting it into its component intuitive explanation is that it evolved colours. Many hummingbird species as a means of communication, such as exhibit iridescent plumage, which some courtship displays. But other explanations scientists believe has evolved as a result include camouflage and even temperature of nectar competition and the need to regulation. aggressively defend feeding territory. Birds

Iridescence is broadly distributed in birds, and can be seen in groups as diverse as hummingbirds, ducks, pheasants, pigeons, kingfishers and the aptly- named birds of paradise. Iridescent colouration is frequently concentrated in the head and throat regions, but structural hues can also be seen in the wings and tail feathers of some species. Feathers

Iridescence in birds is produced by light-scattering nanostructures in the feathers, such as keratin, melanin or air bubbles. The exact position of these elements is very precise, and the diffe- rence between one colour and another may be no more than a few nanometres (one billionth of a metre). Absorption of background light by pigments is also In peacocks, a ‘photonic crystal’ structure made up of melanin granules (or melanosomes) important in some cases, as it makes the connected by keratin, gives rise to their shimmering feathers. Incoming light waves are structural colours more vivid. broken up and then recombined and reflected back as vibrant colours. Fish The neon tetra [below] has a bright iridescent stripe that runs laterally along the body and includes the iris of the eye.

The goldfish gets its name from the glittering, metallic sheen of its scales, when viewed from certain angles.

Iridescence is common among fish and colourful corals as a result). Iridophores is produced by iridophores in the skin or are also responsible for the silvery appea- scales. Iridophore cells deposit ‘photonic rence characteristic of many marine and crystals’ made of guanine and assemble freshwater fish. The bright, shiny scales them into layers of light-reflecting are thought to function as vertical mirrors, ‘platelets’. The reflectance can result in a providing perfect camouflage in the water vast array of colours, as demostrated most column from all directions (except directly strikingly in iridescent reef species (which above or below). The metallic sheen may may be better camouflaged against also serve to dazzle or confuse predators. Reptiles & Amphibians

Although iridescence is rare in reptiles and amphibians is produced by iridophores, amphibians, there are some notable which create iridescence by diffracting light exceptions. The aptly-named rainbow boa within crystal platelets. The results are (Epicrates cenchria) and sunbeam snake sometimes influenced by overlying (Xenopeltis unicolor) are conspicuously pigments. For example, the green iridescent, while fence lizards and some colouration seen in a wide variety of skinks are also known to change colour reptiles and amphibians is commonly the depending on the angle of view. Like product of blue structural colouration and a fish, structural colouration in reptiles and yellow pigmentary colouration. Invertebrates

Many invertebrate species exhibit Opposite: The iridescent bands on the prominent iridescence, with perhaps the cornea of horse and deer flies are caused most notable examples being beetles, by reflecting multilayers, which may help butterflies and molluscs (see pages 12-15) to enhance the discrimination of colour but the phenomeon is observed across an by serving as ‘spectral filters’ for the extensive range of invertebrates, from the underlying photoreceptors. The effect, sparkling bodies of jewel wasps to the rain- when seen up close, is both mesmerising bow-like shimmer of bobbit worms. and bizarre. [1] [2]

[3] [4]

In insects [1-2], iridescence commonly [5] [6] occurs on the wings and bodies of flies, wasps, bees, dragonflies and damselflies. Among the arachnids, jumping spiders [3] are particularly colourful, with several species exhibiting pronounced iridescence. Some marine worms, such as bobbits [4] are known to produce iridescent coloura- tion through diffraction gratings and arrays of photonic crystals, while comb jellies [5] have microscopic hair-like structures called cilia, which flash with different colours as they beat. In crustaceans, iridescent mul- tilayer reflectors are found in sapphrinid copepods [6]. The most common Beetles iridescent colour in beetles is green, probably because it aids in camouflage. Although it may look highly conspi- cuous in a museum, a vivid green beetle may be harder to see in a rainforest.

Several mechanisms for iridescence have photonic crystals. arisen in beetles. The most common is Most insect structural colours are in the multilayer reflectors (thin, parallel layers green-blue-violet range, but red, gold, of stacked chitin - a common scaffolding and copper colours may also be observed. material among insects). Other mecha- The shade of colour and its intensity are nisms include diffraction gratings (nanos- determined by several factors, including cale arrays of paralell ridges or slits that the thickness and structure of the shell, the scatter white light into its constituent number of microscopic layers it has, and wavelengths), and three-dimensional the angle of the incoming light. The elytra (or shells) of many beetle species are prized for their iridescence. In some cultures, jewel beetles and scarabs are used in human ornamentation such as headdresses and jewellery, but we still don’t know what purpose iridescence may afford the beetles themselves. Camouflage is a likely candidate, but iridescence may also serve as warning colouration or play a role in locating or detecting mates. For example, in the case of Phanaeus dung beetles [top left], bright iridescent bodies contrast starkly with a darkly-coloured horn. Since the size of the horn is important in mate choice, the iridescence may serve to emphasise this uniquely male trait. Butterflies & Moths

Butterfly wings have been the focus of scales have evolved a transparent surface much scientific research, dating back to the so that light can easily penetrate the inter- 1800s when Victorian naturalists suggested nal layers. These create manifold surfaces that the bright iridescent markings in some so that, when light hits the wings, it is species might be designed to dazzle and reflected back numerous times. The confuse predators, or even camouflage combined reflections and the precise them in forest habitats. The wings are spacing of the layers produces constructive covered with thousands of microscopic interference - and therefore iridescence. scales, which are arranged like overlapping Microscopic ridges on the scales can also roof tiles. In many iridescent species, the create or enhance the effect. Molluscs

Squid, octopus and cuttlefish are well The role that color-shifting iridescence many of which have iridescent ‘nacreous known for their dynamic colouration. Irido- plays in squid is not yet fully understood, layers’ on the inner surface of their shells. phores in the skin underlie chromatophores but it could be a signal of aggression, much Also known as mother of pearl, this nacre (pigmented organs), and their combined like the blue-ringed octopus, which flashes is made up of tiny overlapping plates of effects produce reflectance patterns that iridescent blue rings as a warning signal calcium carbonate, laid down in a brick encompass the entire range of the visible whenever it feels threatned. The rest of wall-like structure and held together with spectrum. Some squid are even capable of the time, the octopus is brownish-yellow an organic glue. The uniform stacking of adjusting and fine-tuning their own irides- to blend in with its surroundings. Squid layers below the surface of the shell causes cence (unlike other animal groups, where and octopi are molluscs - a group that also interference effects that contribute to the structural colouration is a fixed quality). encompass oysters, clams and scallops, iridescent colours. Contact: [email protected]

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