The colour of flowers in spectrally variable illumination and insect pollinator vision

The spectral reflectance of differently coloured Australian native plant flowers and foliage was measured and plotted in a colour triangle to represent the colour space of the honeybee. Spectral variations in illumination are shown to significantly change plant colours for bee vision without colour constancy. A model of chromatic adaptation based upon the von Kries coefficient law shows a reduction in plant colour shift, with the degree of correction depending upon position in colour space. A set of artificial reflectances is used to map relative colour shift caused by spectrally variable illumination for the entire colour space of the honeybee. The rarity of some flower colours in nature shows a correlation to a larger colour shift for these colours when illuminated by spectrally variable radiation. The model of chromatic adaptation is applied to illuminations used in a behavioural study on honeybee colour constancy by Neumeyer 1981. Surface colours used by Neumeyer are plotted in colour space for the various illuminations. The results show that an illumination-dependent colour shift correlates to a decrease in the frequency of bees correctly choosing a colour to which it was trained. Accepted: 23 February 1998     

Computation of color and brightness differences by neurons in the rabbit visual cortex

Changes in activity of 54 neurons in the rabbit visual cortex evoked by the replacement of eight color and eight achromatic stimuli in pairs were analyzed. The diffused stimuli generated by color SVGA monitor were used in the experiments. The earliest response of phasic neurons (50-90 ms after the replacement) was strongly correlated with differences between stimuli in color or intensity. This response ("the signal of differences") was used as a basis of a matrix (8 x 8) constructed for each neuron. Such matrices included mean numbers of spikes per second in responses to changes of different stimuli pairs. All matrices were subjected to factor analysis, and the basic axes (the main factors) of sensory spaces were revealed. It was found that 16 neurons (30%) detected only achromatic differences between stimuli. Perceptual spaces of these neurons were two-dimensional with brightness and darkness orthogonal axes. The spaces of 12 neurons (22%) were four-dimensional with two chromatic and two achromatic axes. The structure of the perceptual space reconstructed from neuronal spikes was similar to the space calculated from the early VEP components recorded under similar conditions and to another space reconstructed on the basis of rabbit's instrumental learning. The fundamental coincidence of color spaces revealed by different methods may reflect the general principle of vector coding in the visual system and suggests the coexistence of two independent cortical mechanisms of the detection of chromatic and achromatic differences.     

Innate colour preferences of flower visitors

Freshly emerged flower visitors exhibit colour preferences prior to individual experience with flowers. The understanding of innate colour preferences in flower visitors requires a detailed analysis, as, on the one hand, colour is a multiple-signal stimulus, and, on the other hand, flower visits include a sequence of behavioural reactions each of which can be driven by a preferential behaviour. Behavioural reactions, such as the distant approach, the close-range orientation, the landing, and the extension of mouthparts can be triggered by colour stimuli. The physiological limitations of spectral sensitivity, the neuro-sensory filters, and the animals' different abilities to make use of visual information such as brightness perception, wavelength-specific behaviour and colour vision shape colour preferences. Besides these receiverbased factors, there are restrictions of flower colouration due to sender-based factors such as the absorption properties of floral pigments and the dual function of flower colours triggering both innate and learned behaviour. Recordings of the spectral reflection of coloured objects, which trigger innate colour preferences, provide an objective measure of the colour stimuli. Weighting the spectral reflection of coloured objects by the spectral composition of the ambient light and the spectral sensitivity of the flower visitors' photoreceptors allows the calculation of the effective stimuli. Perceptual dimensions are known for only a few taxa of flower visitors.     

Colour categorization by domestic chicks

Spectral stimuli form a physical continuum, which humans divide into discrete non-overlapping regions or categories that are designated by colour names. Little is known about whether non-verbal animals form categories on stimulus continua, but work in psychology and artificial intelligence provides models for stimulus generalization and categorization. We compare predictions of such models to the way poultry chicks (Gallus gallus) generalize to novel stimuli following appetitive training to either one or two colours. If the two training colours are (to human eyes) red and greenish-yellow or green and blue, chicks prefer intermediates, i.e. orange rather than red or yellow and turquoise rather than green or blue. The level of preference for intermediate colours implies that the chicks interpolate between the training stimuli. However, they do not extrapolate beyond the limits set by the training stimuli, at least for red and yellow training colours. Similarly, chicks trained to red and blue generalize to purple, but they do not generalize across grey after training to the complementary colours yellow and blue. These results are consistent with a modified version of a Bayesian model of generalization from multiple examples that was proposed by Shepard and show similarities to human colour categorization.     

Biological significance of distinguishing between similar colours in spectrally variable illumination: bumblebees (<Emphasis Type="Italic">Bombus terrestris</Emphasis>) as a case study

Individual bumblebees were trained to choose between rewarded target flowers and non-rewarded distractor flowers in a controlled illumination laboratory. Bees learnt to discriminate similar colours, but with smaller colour distances the frequency of errors increased. This indicates that pollen transfer might occur between flowers with similar colours, even if these colours are distinguishable. The effect of similar colours on reducing foraging accuracy of bees is evident for colour distances high above discrimination threshold, which explains previous field observations showing that bees do not exhibit complete flower constancy unless flower colour between species is distinct. Bees tested in spectrally different illumination conditions experienced a significant decrease in their ability to discriminate between similar colours. The extent to which this happens differs in different areas of colour space, which is consistent with a von Kries-type model of colour constancy. We find that it would be beneficial for plant species to have highly distinctive colour signals to overcome limitations on the bees performance in reliably judging differences between similar colours. An exception to this finding was flowers that varied in shape, in which case bees used this cue to compensate for inaccuracies of colour vision.     

Learning and discrimination of coloured papers in the walking blowfly,Lucilia cuprina

Summary A new training and testing paradigm for walking sheep blowflies, Lucilia cuprina, is described. A fly is trained by presenting it with a droplet of sugar solution on a patch of coloured paper. After having consumed the sugar droplet, the fly starts a systematic search. While searching, it is confronted with an array of colour marks consisting of four colours displayed on the test cardboard (Fig. 1). Colours used for training and test include blue, green, yellow, orange, red, white and black.Before training, naive flies are tested for their spontaneous colour preferences on the test array. Yellow is visited most frequently, green least frequently (Table 2). Spontaneous colour preferences do not simply depend on subjective brightness (Table 1).The flies trained to one of the colours prefer this colour significantly (Figs. 5 and 9–11). This behaviour reflects true learning rather than sensitisation (Figs. 6–7). The blue and yellow marks are learned easily and discriminated well (Figs. 5, 9, 11). White is also discriminated well, although the response frequencies are lower than to blue and yellow (Fig. 11). Green is discriminated from blue but weakly from yellow and orange (Figs. 5, 9, 10). Red is a stimulus as weak as black (Figs. 8, 9). These features of colour discrimination reflect the spectral loci of colours in the colour triangle (Fig. 14).The coloured papers seem to be discriminated mainly by the hue of colours (Fig. 12), but brightness may also be used to discriminate colour stimuli (Fig. 13).     

Simultaneous and successive colour discrimination in the honeybee (Apis mellifera)

The colour discrimination of individual free-flying honeybees (Apis mellifera) was tested with simultaneous and successive viewing conditions for a variety of broadband reflectance stimuli. For simultaneous viewing bees used form vision to discriminate patterned target stimuli from homogeneous coloured distractor stimuli, and for successive discrimination bees were required to discriminate between homogeneously coloured stimuli. Bees were significantly better at a simultaneous discrimination task, and we suggest this is explained by the inefficiency with which the bees brain can code and retrieve colour information from memory when viewing stimuli successively. Using simultaneous viewing conditions bees discriminated between the test stimuli at a level equivalent to 1 just-noticeable-difference for human colour vision. Discrimination of colours by bees with simultaneous viewing conditions exceeded previous estimates of what is possible considering models of photoreceptor noise measured in bees, which suggests spatial and/or temporal summation of colour signals for fine discrimination tasks. The results show that when behavioural experiments are used to collect data about the mechanisms facilitating colour discrimination in animals, it is important to consider the effects of the stimulus viewing conditions on results.     

Colour preferences and colour vision in poultry chicks

The dramatic colours of biological communication signals raise questions about how animals perceive suprathreshold colour differences, and there are long-standing questions about colour preferences and colour categorization by non-human species. This study investigates preferences of foraging poultry chicks (Gallus gallus) as they peck at coloured objects. Work on colour recognition often deals with responses to monochromatic lights and how animals divide the spectrum. We used complementary colours, where the intermediate is grey, and related the chicks' choices to three models of the factors that may affect the attractiveness. Two models assume that attractiveness is determined by a metric based on the colour discrimination threshold either (i) by chromatic contrast against the background or (ii) relative to an internal standard. An alternative third model is that categorization is important. We tested newly hatched and 9-day-old chicks with four pairs of (avian) complementary colours, which were orange, blue, red and green for humans. Chromatic contrast was more relevant to newly hatched chicks than to 9-day-old birds, but in neither case could contrast alone account for preferences; especially for orange over blue. For older chicks, there is evidence for categorization of complementary colours, with a boundary at grey.     

Colour changes of Octopus rubescens during attacks on unconditioned and conditioned stimuli

Octopus rubescens exhibits a sequence of skin colour changes when it attacks and captures prey. These displays were observed during attacks on (1) free-swimming crabs, (2) crabs of different sizes impaled on thin rods, and (3) positive and negative discriminative stimuli. Animals were housed separately in light-blue aquaria which also served as experimental apparatuses. Animals in each condition showed the same typical sequence of colour displays. This sequence is (1) before detection of crab: various colours, (2) on detection and during a free-swimming attack: colours ranging from light orange to grey, (3) on landing: colourless and nearly transparent, (4) on seizing the crab: spotted or mottled, and (5) afterward: various colours. The colour changes may be tied to locomotor acts and postural adjustments.     

No rainbow for grey bamboo sharks: evidence for the absence of colour vision in sharks from behavioural discrimination experiments

Despite convincing data collected by microspectrophotometry and molecular biology, rendering sharks colourblind cone monochromats, the question of whether sharks can perceive colour had not been finally resolved in the absence of any behavioural experiments compensating for the confounding factor of brightness. The present study tested the ability of juvenile grey bamboo sharks to perceive colour in an experimental design based on a paradigm established by Karl von Frisch using colours in combination with grey distractor stimuli of equal brightness. Results showed that contrasts but no colours could be discriminated. Blue and yellow stimuli were not distinguished from a grey distractor stimulus of equal brightness but could be distinguished from distractor stimuli of varying brightness. In addition, different grey stimuli were distinguished significantly above chance level from one another. In conclusion, the behavioural results support the previously collected physiological data on bamboo sharks, which mutually show that the grey bamboo shark, like several marine mammals, is a cone monochromate and colourblind.     

Contrast versus colour in aposematic signals

Conspicuousness is an important feature of warning coloration. One hypothesis for its function is that it increases signal efficacy by facilitating avoidance learning. An alternative, based on the handicap hypothesis, suggests that the degree of conspicuousness holds information directly about the quality of the prey, and that predators associate and learn about the conspicuousness of the coloration, and not the actual colour pattern. We studied the relative importance of signal contrast and the colours of signals for predator attention during discrimination. We used young chicks, Gallus gallus domesticus, as predators and small blue or red paper cones on either matching or contrasting paper backgrounds as stimuli associated with palatable or unpalatable chick crumbs. In four treatment groups, birds could use either cone and/or background colour, cone colour only, background colour only or cone-to-background contrast as cues for discrimination. Only birds in the contrast treatment failed to learn their discrimination task. Birds that had a choice between cone and background colour as cues used the cone colour and they learned the task faster than did birds that had to use background colour as a cue. The results suggest that birds primarily attend to the colours of signals and disregard contrast in discrimination tasks; they thus fail to support a handicap function of conspicuous aposematic coloration. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Colorimetry and prime colours – a theorem

Human colour vision is the result of a complex process involving topics ranging from physics of light to perception. Whereas the diversity of light entering the eye in principle span an infinite-dimensional vector space in terms of the spectral power distributions, the space of human colour perceptions is three dimensional. One important consequence of this is that a variety of colours can be visually matched by a mixture of only three adequately chosen reference lights. It has been observed that there exists one particular set of monochromatic reference lights that, according to a certain definition, is optimal for producing colour matches. These reference lights are commonly denoted prime colours. In the present paper, we intend to rigorously show that the existence of prime colours is not particular to the human visual system as sometimes stated, but rather an algebraic consequence of the manner in which a kind of colorimetric functions called colour-matching functions are defined and transformed. The solution is based on maximisation of a determinant determining the gamut size of the colour space spanned by the prime colours. Cramer’s rule for solving a set of linear equations is an essential part of the proof. By means of examples, it is shown that mathematically the optimal set of reference lights is not unique in general, and that the existence of a maximum determinant is not a necessary condition for the existence of prime colours.     

The effect of cholinergic substances on the mechanisms of visual recognition in monkeys

In monkeys (Macaca mulatta) instrumental reflex was elaborated with differentiation of black-and-white and colour visual stimuli in condition of systemic administration of pharmacological preparations selectively influencing the functional state of cholinergic brain structures. Differentiation of black-and-white and colour stimuli is not disturbed by atropine (0.1 mg/kg) and amizile (up to 1.5 mg/kg) injections; at greater doses frustration of the instrumental reflex takes place. Differentiation of black-and-white and colour stimuli is disturbed at injection of various doses of antidepressant phthoracizine: 5 mg/kg and 7 mg/kg, respectively. These disturbances are restored by the injection of definite doses of galantamine; for correction of colour differentiation a greater dose is required. The obtained data point to differences in neurophysiological and neurochemical processes responsible for black-and-white and colour vision.     

Vertical lek placement of forest-dwelling manakin species (Aves, Pipridae) is associated with vertical gradients of ambient light

We studied the interaction of ambient light and plumage colour in four sympatric neotropical manakin species which differ in signal colours and vertical lek placement. We estimated bird conspicuousness by modelling chromatic and achromatic background contrast of signal colours in tetrachromatic colour space. Spectral composition of ambient light varies gradually from the understory to the canopy and may affect where manakins lek. Under the given spatial requirements for their horizontal display flights, manakins place their leks at that position along the vertical gradient where ambient light increases chromatic and/or achromatic contrast of their colour signals against the background and/or within their coloration patterns. This suggests that physical factors can be important for placement of display sites, since they may critically influence the effectiveness and efficiency of conspecific communication. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 647–658.     

WWW design code - a new tool for colour estimation in animal studies

BACKGROUND: The colour of animals' skin, fur, feathers or cuticula has been estimated in a large number of studies. The methods used to do so are diverse, with some being costly and not available to all researchers. In a study to measure plumage colour in a bird species, a new method of creating a colour chart was developed. While colour-charts have their own limitations, these can be minimised when they have the following properties: 1) being readily available to the majority of biologists, 2) containing a large array of colours to allow accurate recording and differentiation of subtle colour differences, 3) low cost, 4) adhering to a world-wide standard, and 5) being available in both hard-copy and digital formats to allow for various analytical methods. The method described below satisfies all of these requirements. RESULTS: Colour charts estimated to fit the range of the species' plumage colours were created on the computer screen using web software that allowed for HTML-coding (in this case Dreamweavertrade mark). The charts were adjusted using feathers from dead specimens until a satisfying range of darker and lighter colours were found. The resulting chart was printed out and was successfully used in the field to determine the plumage colour of hand-held birds. CONCLUSION: Access to a computer and printer, and the software to enable the creation of a chart, is within the reach of the vast majority of biologists. The numbers of colours that can be generated should suit most studies, with the advantage of the method being that the chart can be individually tailored to the species under study. HTML colour coding is a worldwide standard, thus the colours used in studies can be described in the methods section of journal articles using the six-digit alphanumeric code. We believe this method is very useful as a low-tech method for future estimation of individual colour.     

On the measurement and classification of colour in studies of animal colour patterns

In studies of animal colouration it is no longer necessary to rely on subjective assessments of colour and conspicuousness, nor on methods which rely upon human vision. This is important because animals vary greatly in colour vision and colour is context-dependent. New methods make it practical to measure the colour spectrum of pattern elements (patches) of animals and their visual backgrounds for the conditions under which patch spectra reach the conspecific's, predator's or prey's eyes. These methods can be used in both terrestrial and aquatic habitats. A patch's colour is dependent not only upon its reflectance spectrum, but also upon the ambient light spectrum, the transmission properties of air or water, and the veiling light spectrum. These factors change with time of day, weather, season and microhabitat, so colours must be measured under the conditions prevalent when colour patterns are normally used. Methods of measuring, classifying and comparing colours are presented, as well as techniques for assessing the conspicuousness of colour patterns as a whole. Some implications of the effect of environmental light and vision are also discussed.     

Looking like mother makes mallard ducklings dominant over their siblings

Colour variation in time and space among animals may affect social relationships such as pairing and dominance interactions. For instance, some birds are naturally sensitive to leg colour, with some colours being more visible or attractive than others. The colour of the leg-rings used to mark birds may thus be related to behavioural and reproductive variables. Most studies have investigated this effect for adults during reproduction, but leg-ring colour may also affect the behaviour of young birds. We tested the potential effect of leg-ring colours on the within-brood dominance hierarchy of mallard (Anas platyrhynchos) ducklings while each brood formed a stable and exclusive family unit with its mother. Ducklings did not acquire a within-brood dominance rank according to the colour of their own ring. This result suggests that mallards may not have a sensory bias for a given colouration. However, ducklings wearing a ring of the same colour as one of the two rings of their mother were dominant over their siblings. We discuss the potential behavioural and methodological implications of this result.     

Expertomica Fishgui: comparison of fish skin colour

The colour hue and saturation of the fish skin are important factors in fish breeding, reflecting the momentary physiological status of the fish. Algorithms evaluating fish colour were created in the MATLAB environment and compiled in the stand‐alone application, Expertomica Fishgui. Fish groups fed on several diets (varying in carotenoid supplements) were compared as concerns image analysis methods on colour changes. Images were processed and evaluated to locate body positions, areas and colour saturation. Conversion from the Red‐Green‐Blue (RGB) colour space to chromatic colours was carried out to reduce the colour space. The threshold of skin chromatic colour was set on basis of the chromatic histograms using Otsu automatic bimodal segmentation. The average colour was calculated through all pixels of the selected area in the original image and through all images of the group. Colour conversion to the Hue‐Saturation‐Value (HSV) colour space was also carried out. Relative deviation between the saturation of samples in the control and the experimental groups provided a means of judgement. The dominant wavelength was also determined.     

Perception or preference? Mediation of gene effects in the colour choices of naive quail chicks (C. coturnix japonica)

The purpose of this study was to examine gene effects in the peripheral and central neural mediation of colour preferences in artificially selected Japanese quail chicks (Coturnix coturnix japonica). Behavioural data indicated preferences of blue over red and red over blue in the respectively selected genetic lines, and general preferences for white in both lines. Preferences were influenced by luminance variations of testing stimuli. Choices between segmented stimuli that combined blue, red, and white, in various patterns of systematically varied hues and luminances, indicated perceptual discrimination of colours in each line. Preference values combined within stimuli summated additively in choice responses, suggesting no genetic association between colour preference and colour perception. Electrophysiological data indicated systematic differences between waveshapes of averaged evoked potentials according to hues of eliciting stimuli. These data also counterindicated the involvement of colour perception in the genetic variations of colour preference. However, no significant differences between waveshapes were found in relation to genetic variations in preferences.     

WWW design code – a new tool for colour estimation in animal studies

Background

The colour of animals' skin, fur, feathers or cuticula has been estimated in a large number of studies. The methods used to do so are diverse, with some being costly and not available to all researchers. In a study to measure plumage colour in a bird species, a new method of creating a colour chart was developed. While colour-charts have their own limitations, these can be minimised when they have the following properties: 1) being readily available to the majority of biologists, 2) containing a large array of colours to allow accurate recording and differentiation of subtle colour differences, 3) low cost, 4) adhering to a world-wide standard, and 5) being available in both hard-copy and digital formats to allow for various analytical methods. The method described below satisfies all of these requirements.

Results

Colour charts estimated to fit the range of the species' plumage colours were created on the computer screen using web software that allowed for HTML-coding (in this case Dreamweaver?). The charts were adjusted using feathers from dead specimens until a satisfying range of darker and lighter colours were found. The resulting chart was printed out and was successfully used in the field to determine the plumage colour of hand-held birds.

Conclusion

Access to a computer and printer, and the software to enable the creation of a chart, is within the reach of the vast majority of biologists. The numbers of colours that can be generated should suit most studies, with the advantage of the method being that the chart can be individually tailored to the species under study. HTML colour coding is a worldwide standard, thus the colours used in studies can be described in the methods section of journal articles using the six-digit alphanumeric code. We believe this method is very useful as a low-tech method for future estimation of individual colour.