Iridescence: a functional perspective

Abstract

In animals, iridescence is generated by the interaction of light with biological tissues that are nanostructured to produce thin films or diffraction gratings. Uniquely among animal visual signals, the study of iridescent coloration contributes to biological and physical sciences by enhancing our understanding of the evolution of communication strategies, and by providing insights into physical optics and inspiring biomimetic technologies useful to humans. Iridescent colours are found in a broad diversity of animal taxa ranging from diminutive marine copepods to terrestrial insects and birds. Iridescent coloration has received a surge of research interest of late, and studies have focused on both characterizing the nanostructures responsible for producing iridescence and identifying the behavioural functions of iridescent colours. In this paper, we begin with a brief description of colour production mechanisms in animals and provide a general overview of the taxonomic distribution of iridescent colours. We then highlight unique properties of iridescent signals and review the proposed functions of iridescent coloration, focusing, in particular, on the ways in which iridescent colours allow animals to communicate with conspecifics and avoid predators. We conclude with a brief overview of noncommunicative functions of iridescence in animals. Despite the vast amount of recent work on animal iridescence, our review reveals that many proposed functions of iridescent coloration remain virtually unexplored, and this area is clearly ripe for future research.

Brilliant camouflage : photonic crystals in the diamond weevil, Entimus imperialis

“…With only minor changes of the viewing angle, the shiny spots feature strikingly different colours, which give them a diamond-like appearance. The strong directionality of the reflections indicates that the origin of these local, vivid colours is structural [2,3]. A closer look at the elytral pits shows that the diamond-like spots are assemblies of highly directionally reflective scales covering the walls of concave pits in the elytra (figure 1b).…”

“…Clearly, the scales are thus not vulnerable to mechanical wear. The concentration of the scales in pits, therefore, presumably serve three functions: the scales are protected from mechanical damage and accidental loss, the pattern of brilliant spots facilitates intraspecific recognition, and the integral green coloration favours camouflage in foliaceous environments [3,4].…”

“…The distance between the elytral pits is approximately 1 mm, so that, assuming a spatial resolution of the weevil eyes of approximately 18 [40], potential mates can discriminate adjacent spots from a distance as far as approximately 6 cm. The near-field brilliancy of the dotted pattern of the weevil's elytra may therefore be used as a mate recognition signal [3].…”

“…The resulting coloration mostly serves a biological function in intraand interspecific signalling [3,4], thus improving mating chances [5], but it can also be used for camouflage in the animal's native habitat [6 -8]. Generally, two types of coloration are recognized, pigmentary and structural.…”

Iridescent colour production in hairs of blind golden moles ( Chrysochloridae )

“…The simplest nanostructures are single or multiple layers of materials with different refractive indices (RI) that act as thin-film reflectors in clades as diverse as beetles and spikemosses [1]. While several different functions have been hypothesized for these flashy colours, most have focused on sexual display [2].…”