Shell colour polymorphism associated with substrate colour in the intertidal snail Littorina saxatilis Olivi (Prosobranchia: Littorinidae)

Littorina saxatilis Olivi (1792), the rough winkle, is highly polymorphic in shell colour. Shell colour frequencies were studied at six locations in south-western Wales, U.K., each at a geological contact between red sandstone and grey limestone or volcanic rock. At each site shell colour frequencies were determined in samples from the contact zone and on red or grey rock on either side. Highly significant associations were found between shell colour frequencies and substrate colour. Grey shells were always more common on grey rock than on red rock, and brown shells were usually more common on red than on grey rock, suggesting selection for cryptic colouration. Shell colour frequency differences were also found between replicate samples taken only 5 m apart from the same kind of rock, and between samples from the same kind of rock at the six study sites. These latter differences suggest that selection for camouflage is not the only factor involved in maintaining shell colour polymorphism in this species.     

Using visual modelling to study the evolution of lizard coloration: sexual selection drives the evolution of sexual dichromatism in lacertids

Sexual selection has been invoked as a major force in the evolution of secondary sexual traits, including sexually dimorphic colourations. For example, previous studies have shown that display complexity and elaborate ornamentation in lizards are associated with variables that reflect the intensity of intrasexual selection. However, these studies have relied on techniques of colour analysis based on human – rather than lizard – visual perception. Here, we use reflectance spectrophotometry and visual modelling to quantify sexual dichromatism considering the overall colour patterns of lacertids, a lizard clade in which visual signalling has traditionally been underrated. These objective methods of colour analysis reveal a large, previously unreported, degree of sexual dichromatism in lacertids. Using a comparative phylogenetic approach, we further demonstrate that sexual dichromatism is positively associated with body size dimorphism (an index of intrasexual selection), suggesting that conspicuous coloration in male lacertids has evolved to improve opponent assessment under conditions of intense male–male competition. Our findings provide the first evidence for the covariation of sexual dichromatism and sexual size dimorphism in lacertids and suggest that the prevalent role of intrasexual selection in the evolution of ornamental coloration is not restricted to the iguanian lineage, but rather may be a general trend common to many diurnal lizards.     

What makes a feather shine? A nanostructural basis for glossy black colours in feathers

Colours in feathers are produced by pigments or by nanostructurally organized tissues that interact with light. One of the simplest nanostructures is a single layer of keratin overlying a linearly organized layer of melanosomes that create iridescent colours of feather barbules through thin-film interference. Recently, it has been hypothesized that glossy (i.e. high specular reflectance) black feathers may be evolutionarily intermediate between matte black and iridescent feathers, and thus have a smooth keratin layer that produces gloss, but not the layered organization of melanosomes needed for iridescence. However, the morphological bases of glossiness remain unknown. Here, we use a theoretical approach to generate predictions about morphological differences between matte and glossy feathers that we then empirically test. Thin-film models predicted that glossy spectra would result from a keratin layer 110-180 nm thick and a melanin layer greater than 115 nm thick. Transmission electron microscopy data show that nanostructure of glossy barbules falls well within that range, but that of matte barbules does not. Further, glossy barbules had a thinner and more regular keratin cortex, as well as a more continuous underlying melanin layer, than matte barbules. Thus, their quasi-ordered nanostructures are morphologically intermediate between matte black and iridescent feathers, and perceived gloss may be a form of weakly chromatic iridescence.     

A simple method for measuring colour in wild animals: validation and use on chest patch colour in geladas (Theropithecus gelada)

Adaptive hypotheses about colour variation are widespread in behavioural ecology, and several methods of objective colour assessment have been proposed and validated for use in a wide variety of taxa. However, to date, the most objective and reliable methods of assessing colour are not readily applied to wild animals. In the present study, we present a simple method for assessing colour in unrestrained, wild subjects using digital photography. The method we describe uses a digital camera, a colour standard, and colour analysis software, and can be used to measure any part of the visible colour spectrum. We demonstrate that the method: (1) is accurate and precise across different light conditions; (2) satisfies previous criteria regarding linearity and red, green, and blue equality; and (3) can be independently validated visually. In contrast with previous digital methods, this method can be used under natural light conditions and can be readily applied to subjects in their natural habitat. To illustrate this, we use the method to measure chest colour in wild geladas ( Theropithecus gelada ). Unique among primates, geladas have a red patch of skin on their chest and neck, which, for males, is thought to be a sexually selected signal. Offering some support to this hypothesis, we found differences in chest 'redness' for males across different age groups, with males in their reproductive prime exhibiting the reddest chests.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 231–240.     

Not so colourful after all: eggshell pigments constrain avian eggshell colour space

Birds'' eggshells are renowned for their striking colours and varied patterns. Although often considered exceptionally diverse, we report that avian eggshell coloration, sampled here across the full phylogenetic diversity of birds, occupies only 0.08–0.10% of the avian perceivable colour space. The concentrations of the two known tetrapyrrole eggshell pigments (protoporphyrin and biliverdin) are generally poor predictors of colour, both intra- and interspecifically. Here, we show that the constrained diversity of eggshell coloration can be accurately predicted by colour mixing models based on the relative contribution of both pigments and we demonstrate that the models'' predictions can be improved by accounting for the reflectance of the eggshell''s calcium carbonate matrix. The establishment of these proximate links between pigmentation and colour will enable future tests of hypotheses on the functions of perceived avian eggshell colours that depend on eggshell chemistry. More generally, colour mixing models are not limited to avian eggshell colours but apply to any natural colour. Our approach illustrates how modelling can aid the understanding of constraints on phenotypic diversity.     

A new interpretation of flower guide colouration: Absorption of ultraviolet light enhances colour saturation

In melittophilous plants the colour pattern of the flowers, as perceived by bumblebees, is a gradient of centripetally increasing spectral purity. This pattern serves as a signal for innate flower recognition in naive bumblebees permitting orientation to flowers and landing on flowers. Structures which make up the total signal pattern can include the background (e.g., green leaves), corollas, and stamens or floral guides. How various colour parameters, such as dominant wavelength, intensity, and spectral purity influence the colour signal pattern of flowers is analyzed. The process of strong absorption of ultraviolet light is shown to be a mechanism for the enhancement of spectral purity in flower guides. The importance of other mechanisms is also demonstrated. The presence of a gradient of centripetally increasing spectral purity in floral colour patterns as perceived by a bumblebee's eyes is demonstrated by a comparison of the spectral reflectance in different parts of the flower and a representation of colour loci in the colour triangle.     

Reactive azo dye reduction by Shewanella strain J18 143

A bacterial isolate designated strain J18 143, originally isolated from soil contaminated with textile wastewater, was shown to reduce intensely coloured solutions of the reactive azo dye, Remazol Black B to colourless solutions. Phylogenetic placement based on 16S rRNA gene sequence homology identified the bacterium as a Shewanella species. Based on results from analyses of the end products of dye decoloration of Remazol Black B and the simpler molecule, Acid Orange 7, using capillary electrophoresis, UV-visible spectrophotometry and liquid chromatography-mass spectrometry, we suggest that colour removal by this organism was a result of microbially mediated reduction of the chromophore in the dye molecules. Anaerobic dye reduction by Shewanella strain J18 143 was 30 times more efficient than the reduction carried out by aerated cultures. Whole cells used a range of electron donors for dye reduction, including acetate, formate, lactate, and nicotinamide adenine dinucleotide (NADH), with formate being the optimal electron donor. The impact of a range of process variables was assessed (including nitrate, pH, temperature, substrate concentration, presence of an extracellular mediator) and results suggest that whole cells of Shewanella J18 143 offer several advantages over other biocatalysts with the potential to treat azo dyes.     

Cyclical training enhances the melanophore responses of zebrafish to background colours

Zebrafish respond to visual stimuli to adapt their body colour to the background. If, rather than being a simple on/off reaction to visual stimulation, the colour change involves cognitive and memory-related processes, training fish with cyclical changes of the background would be expected to increase its ability to change colour. To test this, we developed a standardized procedure for quantifying the responses of melanophores to background changes in living adult specimens of leopard, a zebrafish mutant with spotted stripes. After training with 2-day cyclical alternation of white and black backgrounds for over 20 days, the proportion of the melanosome-filled area of dorsal melanophores, which was 20% on the black background before the training, increased up to 97%. In these trained fish, a rapid melanosome aggregation occurred within 10 s of the background change from black to white. The results indicate that the zebrafish melanophore responses can be modulated by learning, in which areal and speed control of melanosome movement are important for dispersion and aggregation, respectively.     

A modified thermal convection drying technique for herbarium preservation

A simple and efficient installation for drying plants that is both rapid and preserves colour, and which works on the principle of autonomous thermal convection, is presented. The described apparatus offers significant advantages including: uniform heat distribution and short drying times; a system for applying pressure to prevent shrinkage, which uses polyurethane foam sheets and elastic straps; a compact, space-saving system suitable for field or laboratory applications; absorbent materials which can be re-used immediately; ability to run off differing power supplies (110/220 V) and to use infra-red lamps of varying specifications; low cost and ease of use.     

A modified thermal convection drying technique for herbarium preservation

A simple and efficient installation for drying plants that is both rapid and preserves colour, and which works on the principle of autonomous thermal convection, is presented. The described apparatus offers significant advantages including: uniform heat distribution and short drying times; a system for applying pressure to prevent shrinkage, which uses polyurethane foam sheets and elastic straps; a compact, space-saving system suitable for field or laboratory applications; absorbent materials which can be re-used immediately; ability to run off differing power supplies (110/220 V) and to use infra-red lamps of varying specifications; low cost and ease of use.     

The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: Chromodoris epicuria, C. aureopurpurea, C. annulata, C. coi and Risbecia tryoni colour groups

Twenty-five species of chromodorid nudibranchs from the Indo-West Pacific are described including nine which are new to science. There are six new species of Chromodoris , two of Nounea and a new species which forms the type of a new monotypic genus. Of the other species discussed in this paper, ten are species of Chromodoris , five Risbecia and one Digidentis. Species with the following colour patterns are discussed: white with orange or yellow spots; white with yellow (or orange) and purple spots; white with purple spots or patches and mantle border; white with a sinuous submarginal brown band. A full review of the relevant literature is included as is a discussion on Indian Ocean and West Pacific species pairs.     

Colour choice behaviour in the pollen beetle Meligethes aeneus (Coleoptera: Nitidulidae)

The pollen beetle Meligethes aeneus Fabricius (Coleoptera, Nitidulidae), a pest of oilseed rape (Brassica napus), is known to respond to coloured stimuli; however, current understanding of the underlying mechanisms of colour choice in this species is limited. In the present study, physiological and behavioural experiments are conducted to determine the response of the pollen beetle to colours in the field. Spectral sensitivity is measured in 10 animals using the electroretinogram technique. Light flashes (100 ms) at varied wavelengths (340–650 nm, 10‐nm steps) and at different light intensities are applied to the eye after dark adaptation. In behavioural experiments in the field, 100 water traps of varying colours (from yellow to green to blue with varying amounts of white and black added, and with known spectral reflectance) are set out on a bare soil field in May 2008. The mean spectral sensitivity curve of M. aeneus peaks at 520 nm; however, a model template fitted to the long wavelength tail of the observed curve reveals a peak at approximately 540 nm (green). A secondary sensitivity peak is observed in the ultraviolet (UV) range (370 nm). A total of 2482 pollen beetles are captured in the coloured traps. The results show that the pollen beetles' preference for yellow over other colours can be modelled as a colour opponent mechanism (green versus blue); however, further experiments are needed to specify responses to colours with higher UV reflectance. These findings may be used to optimize trap colours for monitoring to help develop integrated pest management strategies for pollen beetle control.     

Morph‐dependent fitness and directional change of morph frequencies over time in a Dutch population of Common buzzards Buteo buteo

How genetic polymorphisms are maintained in a population is a key question in evolutionary ecology. Previous work on a plumage colour polymorphism in the common buzzard Buteo buteo suggested heterozygote advantage as the mechanism maintaining the co‐existence of three morphs (light, intermediate and dark). We took advantage of 20 years of life‐history data collected in a Dutch population to replicate earlier studies on the relationship between colour morph and fitness in this species. We examined differences between morphs in adult apparent survival, breeding success, annual number of fledglings produced and cumulative reproductive success. We found that cumulative reproductive success differed among morphs, with the intermediate morph having highest fitness. We also found assortative mating for colour morph, whereby assortative pairs were more likely to produce offspring and had longer‐lasting pair bonds than disassortative pairs. Over the 20‐year study period, the proportion of individuals with an intermediate morph increased. This apparent evolutionary change did not just arise from selection on individual phenotypes, but also from fitness benefits of assortative mating. The increased frequency of intermediates might also be due to immigration or drift. We hypothesize that genetic variation is maintained through spatial variation in selection pressures. Further studies should investigate morph‐dependent dispersal behaviour and habitat choice.     

The colour line and the colour scale in the twentieth century

Some more recent evidence supports Du Bois’ prediction that the twentieth century would prove the century of the colour line. It indicates that men have always and everywhere shown a preference for fair complexioned women as sexual partners, whereas males seeking a mate are rarely disadvantaged by a dark complexion. In the employment market in the USA, a dark complexion is a significant disadvantage for both males and females. Though there is no properly comparable evidence from other countries, there appears to be a widespread tendency for any negative valuation of darker skin colour to be incorporated into a scale of socio-economic status. In some situations a colour scale is replacing the colour line.

Du Bois’ reference to differences of colour has been largely superseded in English-speaking countries by references to differences of race. From a policy standpoint, the switch from colour to race has had both positive and negative consequences. From a sociological standpoint, it has made it more difficult to disaggregate the dimensions of social difference and to dispel the confusions engendered by ideas of racial difference.     

Early rearing enrichments influenced nest use and egg quality in free-range laying hens

In Australia, free-range egg production pullets are typically reared indoors, but adult layers get outdoor access. This new environment may be challenging to adapt to, which could impair egg production and/or egg quality. Adaptation might be enhanced through rearing enrichments. We reared 1386 Hy-Line Brown® chicks indoors with three treatments across 16 weeks: (1) a control group with standard litter housing conditions, (2) a novelty group providing novel objects that changed weekly, and (3) a structural enrichment group with custom-designed structures to partially impair visibility across the pen and allow for vertical movement. Pullets were transferred to a free-range system at 16 weeks of age with daily outdoor access provided from 25 until 64 weeks. Daily egg production at different laying locations (large nests, small nests and floor), weekly egg weights and egg abnormalities were recorded from 18 to 64 weeks old. External and internal egg quality parameters of egg weight, shell reflectivity, albumen height, haugh unit, yolk colour score, shell weight and shell thickness were measured at 44, 52, 60 and 64 weeks. There was a significant interaction between rearing treatment and nest box use on hen-day production from weeks 18 to 25 (P < 0.0001) with the novelty hens laying the most eggs and the control hens the fewest eggs in the nest box. Similarly, from 26 to 64 weeks, the novelty hens laid more eggs in the large nest boxes and fewer eggs on the floor than both the structural and control hens (P < 0.0001). Egg weight and abnormalities increased with age (P < 0.0001), but rearing treatment had no effect on either measure (both P ≥ 0.19). Rearing treatment affected shell reflectivity and yolk colour with the control hens showing paler colours across time relative to the changes observed in the eggs from enriched hens. The novelty hens may have established nest box laying patterns as they were more accustomed to exploring new environments. The differences in egg quality could be related to stress adaptability or ranging behaviour. This study shows that enriching environments during rearing can have some impacts on production parameters in free-range hens.     

The contribution of structural-, psittacofulvin- and melanin-based colouration to sexual dichromatism in Australasian parrots

Colour ornamentation in animals is exceptionally diverse, but some colours may provide better signals of individual quality or more efficient visual stimuli and, thus, be more often used as sexual signals. This may depend on physiological costs, which depend on the mechanism of colour production (e.g. exogenously acquired colouration in passerine birds appears to be most sexually dichromatic). We studied sexual dichromatism in a sample of 27 Australasian parrot species with pigment- (melanin and psittacofulvin) and structural-based colouration, to test whether some of these types of colouration are more prominent in sexual ornamentation. Unlike passerines, in which long wavelength colouration (yellow to red) usually involves exogenous and costly carotenoid pigments, yellow to red colouration in parrots is based on endogenously synthesized psittacofulvin pigments. This allows us to assess whether costly exogenous pigments are necessary for these plumage colours to have a prominent role in sexual signalling. Structural blue colouration showed the largest and most consistent sexual dichromatism, both in area and perceptually relevant chromatic differences, indicating that it is often ornamental in parrots. By contrast, we found little evidence for consistent sexual dichromatism in melanin-based colouration. Unlike passerines, yellow to red colouration was not strongly sexually dichromatic: although the area of colouration was generally larger in males, colour differences between the sexes were on average imperceptible to parrots. This is consistent with the idea that the prominent yellow to red sexual dichromatism in passerines is related to the use of carotenoid pigments, rather than resulting from sensory bias for these colours.