Color Signals Research Articles

Deterring predator pursuit and attracting potential mates? The conspicuous melanized tail display of the zebra-tailed lizard

The evolutionary significance of color expression in animals is often framed within either an interspecific (e.g., antipredator display) or intraspecific (e.g., mate choice) context. In part, this dichotomy stems from assumptions that the expression of conspicuous sexually selected traits likely hinders survival in the wild. Here we address the potential for natural and sexual selection to reinforce expression of a conspicuous tail bar pattern in zebra-tailed lizards (Callisaurus draconoides). In C. draconoides, display of this tail bar pattern has historically been considered an interspecific signal to deter predator pursuit. However, a putative honest link between the tail bar pattern and escape likelihood (e.g., sprint speed) is unknown. Further, studies on related taxa also support the potential for this trait to be informative as a signal of individual quality during mating interactions as well. We assessed variation in the morphology of male and female C. draconoides, and then evaluated how variation in morphology (emphasizing the tail bar pattern) contributed to variation in performance (sprint capacity) and, for males, variation in female preference. Males were larger than females, but size was unrelated to variation in tail bar number. Males sprinted faster than females overall, but tail bar number only mattered for males: specifically, males with more tail bars sprinted faster. Females also preferred males with more tail bars. Overall, our findings suggest that natural (mediated via an honest link between bar number and sprint speed) and sexual (mediated via a preference for males with more tail bars) selection may mutually reinforce expression of a color signal. Thus, the potential adaptive benefits of a color signal may span intra- and interspecific contexts. For males at least, the tail bar pattern is useful for both deterring the pursuit of predators as well as attracting the attention of potential mates.

A biomimetic enzyme-linked immunosorbent assay (BELISA) for the analysis of gonadorelin by using molecularly imprinted polymer-coated microplates

An original biomimetic enzyme-linked immunoassay (BELISA) to target the small peptide hormone gonadorelin is presented. This peptide has been recently listed among the substances banned in sports by the World Antidoping Agency (WADA) since its misuse by male athletes triggers testosterone increase. Hence, in response to this emerging issue in anti-doping controls, we proposed BELISA which involves the growth of a polynorepinephrine (PNE)–based molecularly imprinted polymer (MIP) directly on microwells. PNE, a polydopamine (PDA) analog, has recently displayed impressive performances when it was exploited for MIP preparation, giving even better results than PDA. Gonadorelin quantification was accomplished via a colorimetric indirect competitive bioassay involving the competition between biotinylated gonadorelin linked to the signal reporter and the unlabeled analyte. These compete for the same MIP binding sites resulting in an inverse correlation between gonadorelin concentration and the output color signal (λ = 450 nm). A detection limit of 277 pmol L−1 was achieved with very good reproducibility in standard solutions (avCV% = 4.07%) and in urine samples (avCV% = 5.24%). The selectivity of the assay resulted adequate for biological specimens and non-specific control peptides. In addition, the analytical figures of merit were successfully validated by mass spectrometry, the reference anti-doping benchtop platform for the analyte. BELISA was aimed to open real perspectives for PNE-based MIPs as alternatives to antibodies, especially when the target analyte is a poorly or non-immunogenic small molecule, such as gonadorelin.Graphical abstractBiomimetic enzyme-linked immunosorbent assay (BELISA)

Tri-wavelength simultaneous ESPI for 3D micro-deformation field measurement.

Electronic speckle pattern interferometry (ESPI), a well-established technique for micro-deformation measurement, can be used to determine both in-plane and out-of-plane displacement components. Although many works in ESPI have been reported for three-dimensional (3D) displacement measurement, few works have focused on the simultaneous measurement of 3D deformation fields. Here we present an ESPI system that consists of three sub-interferometers for simultaneous measurement of all three displacement components and in-plane strain fields. A 3CCD color camera, a specially designed shifting stage, and three lasers with optimal wavelengths are used in this system. The lasers and 3CCD camera provide independent interferograms with different color signals, while the shifting stage allows the sub-interferometers to achieve simultaneous phase shifting. The results of color separation and experimental measurement demonstrate the utility of the system.

A deep learning approach for remote heart rate estimation

Remote monitoring of elderly people or patients in home isolation is an essential part of modern telemedicine. Videoplethysmography (VPG) is a method of noncontact assessment of heart rate and other cardiovascular parameters. Many algorithms have been developed to extract and improve the quality of the VPG signal. The main objective of this study is to design a method that replaces existing multistage algorithms and provides continuous monitoring of the user’s pulse. The article presents a method of heart rate measurement based on the Long Short Term Memory (LSTM) Deep Neural Network. The proposed method outperforms the algorithm based on the analysis of the green component (G) and provides comparable results to the state-of-the-art methods such as Independent Component Analysis (ICA) and Plane Orthogonal to the Skin (POS). The best result for G was 6.49 bpm (beats per minute), ICA = 3.02 bpm, POS = 2.61 bpm, and for the proposed method was 3.26 bpm. While maintaining the accuracy comparable to ICA and POS algorithms, the LSTM network works well also beyond the visible spectrum, e.g., with infrared lighting when the color signal is not available and is easily adaptable to telemedicine applications.

Dark-eyed females: sexually dimorphic prespawning coloration results from sex-specific physiological response to hormone exposure in the sand gobyPomatoschistus minutus(Gobiiformes: Gobiidae)

AbstractThe function and regulation of female nuptial colour signals are poorly understood. In fish, colour is often mediated by chromatophores, allowing for rapid and versatile signalling. Here, we examine a distinct but temporary black line around the eyes and snout (‘dark eyes’) displayed by female sand gobies before spawning and never observed in males. We investigate the regulatory mechanism of the display by analysing the number of melanophores in both sexes in vitro and their response to hormonal exposure. We also test the hypothesis that dark eyes serve an anti-glare function and focus the line of sight, by analysing the frequency, intensity and duration of the display in bright and dim light, with and without males present. We show that the sexes do not differ in terms of the number of melanophores, but that males and females respond in different ways to exposure to melanocyte-stimulating hormone, which has a stronger dilatory effect in females and results in a darker line. However, the darkness of the iris is not affected. Neither light levels nor the presence of potential mates affect the frequency of the dark eye display, but the display is longer lasting and more intense in the presence of smaller nest-holding males.

Thioglycolic acid-functionalized gold nanoparticles: Capping agent-affected color perception stability towards nitrate sensing purpose

In comparison with colorimetric detection of metal ions, inorganic anions sensing on gold nanoparticles (AuNPs) seems to be more challenging and less explored due to their weak affinity towards the surface of such metal nanoparticles. Early efforts to detect nitrate in water mainly focused on amino thiol-functionalized AuNPs, thanks to the gold-sulfur interactions, sometimes adjusting the pH was required. In the present work, thioglycolic acid was preliminarily introduced for functionalizing purpose, in combination with the stability of colloidal AuNPs suspensions capped by either sodium citrate or ascorbic acid, which have not been previously reported. In contrast to the precipitation observed on ascorbic acid-capped AuNPs, the stability of color perception of freshly citrate-capped AuNPs and corresponding functionalized samples during 10 days extended their realistic nitrate sensing applications. A straightforward protocol involving three successive stages (synthesis of AuNPs, functionalizing and colorimetric sensing) exposed the color variations from red to purple and then blue, with the detection limit of nitrate of 50 ppm, qualifying the nitrate limit of drinking water authorized by the World Health Organization (WHO). The color signals were strictly relevant to surface plasmon resonance and particle size, evidenced by ultraviolet – visible spectroscopy (UV–vis) and transmission electron microscopy (TEM), respectively.

A Compensation Network With Error Mapping for Robust Remote Photoplethysmography in Noise-Heavy Conditions

Remote photoplethysmography (rPPG) measurement has received much attention due to its attractive applications in contactless physiological monitoring. It calculates the blood volume pulse from color signals with a normal camera. Artifacts caused by motion and illumination changes must be accounted for, especially in real-world usage. In this article, we propose a novel framework that includes an error compensation neural network after conventional signal-based rPPG extraction to improve the measuring results. Our main idea is to leverage facial image factors and frequency spectrum features to characterize the relevance between the current measurement conditions and measurement error; given inaccurate measurements, the compensation network computes the corresponding correction term to improve performance. The implementation of this compensation network includes a feature extractor based mainly on 1-D convolution kernels and a recurrent structure to account for temporal interference. Results of experiments conducted on two open datasets and two self-constructed datasets recorded for compact car drivers and passengers with driving disturbance demonstrate significant improvements over state-of-the-art methods, especially in challenging cases. The percentage of the mean absolute error with respect to that based on conventional signal-based methods improves by up to 72.3% for drivers and 66.7% for passengers.

A new metal-free near-infrared fluorescent probe based on nitrofuran for the detection and bioimaging of carbon monoxide releasing molecule-2 in vivo.

As a stable donor for releasing controlled amounts of CO, carbon monoxide releasing molecule-2 (CORM-2) is a new type of therapeutic drug that contributes to exploring the pathophysiological effects of CO. The accurate detection of CORM-2 in biological systems is of great significance for controlling its dosage as a therapeutic drug and elucidating the reaction mechanisms of CO, but currently there is a lack of metal-free near-infrared fluorescent probes. Herein, a new metal-free near-infrared fluorescent probe based on nitrofuran which could selectively identify CORM-2 was designed and it has been successfully applied in living cells, zebrafish and mice. After reacting with CORM-2, both the color and fluorescence signal of the solution are restored, which is ascribed to the reduction of the nitro group. The spectroscopic probe DXPN shows high sensitivity to CORM-2 with a low detection limit of 87 nM and near-infrared fluorescence emission of 712 nm. Notably, this is the first time that paper chips are being used as a carrier to detect CORM-2 through fluorescence signals instead of the traditional liquid phase detection mode of fluorescent probes. These superior properties of the probe make it a promising and reliable tool for exploring the role played by CORM-2 in biological systems.

Rapid measurement of waterborne bacterial viability based on difunctional gold nanoprobe.

Rapid measurement of waterborne bacterial viability is crucial for ensuring the safety of public health. Herein, we proposed a colorimetric assay for rapid measurement of waterborne bacterial viability based on a difunctional gold nanoprobe (dGNP). This versatile dGNP is composed of bacteria recognizing parts and signal indicating parts, and can generate color signals while recognizing bacterial suspensions of different viabilities. This dGNP-based colorimetric assay has a fast response and can be accomplished within 10 min. Moreover, the proposed colorimetric method is able to measure bacterial viability between 0% and 100%. The method can also measure the viability of other bacteria including Staphylococcus aureus, Shewanella oneidensis, and Escherichia coli O157H7. Furthermore, the proposed method has acceptable recovery (95.5–104.5%) in measuring bacteria-spiked real samples. This study offers a simple and effective method for the rapid measurement of bacterial viability and therefore should have application potential in medical diagnosis, food safety, and environmental monitoring.

Hydrogel-integrated microfluidic platform for separation-free colorimetric and distance-based detection of biomarkers in whole blood

Rapid and reliable detection of disease-related biomarkers in whole blood remains a great challenge due to the complex composition of blood. Here, we report a novel point-of-care test (POCT) platform for direct detection of biomarkers in blood sample, in which horseradish peroxidase (HRP) and 3,3′,5,5′-tetramethylbenzidine (TMB) embedded poly(ethylene) glycol diacrylate (PEG-DA) hydrogel were integrated in a specific designed microfluidic chip. In the presence of the biomarkers triggered enzyme-catalyzed product H2O2, a distinct TMB-oxTMB color signal can be captured effectively by smartphone. Meanwhile, ingeniously utilized hydrogel and photothermal oxTMB as “hotplate”, a visual quantitative distance signal can be attained by colored liquid moving driven through the increase in air pressure inside the chip due to heating of the “hotplate”. Besides, the porous hydrogel can serve as a molecular sieve via size exclusion and avoid interference of co-existed biomacromolecules for directly sensing glucose and cholesterol in blood samples. Furthermore, the consistency of the results obtained by colorimetric and distance-based microfluidic assays demonstrates that the platform offered various analytical pathways for biomarkers determination. The detection strategy for biomarkers capable of producing H2O2 indicated it also has great potential in disease diagnosis as a versatile platform.

Evaluating bulk velocities and dispersion effect in a steep gravel-bed channel by detecting tracer clouds

The measurement of flow velocity in steep mountainous channels is extremely difficult due to the shallow flow environment. One promising method is capturing the path of tracers and establishing the relation between bulk velocity and tracer transport processes. In this study, two types of dye and salt tracers were utilized for bulk velocity measurement in a steep gravel-bed rectangular channel by analyzing color and electrical conductivity signals. Characteristics velocities gained from the electrical conductivity approach are close to the true values and those from the color space approach need to be corrected. The HSV space signals are more effective to indicate those characteristic boundaries of the dye solution. By analyzing the tracer residence time, longitudinal dispersion characteristics are essentially associated with turbulence negatively and flow resistance linearly and affect the velocity estimation. This study provides new insights to estimate bulk velocity and turbulence induced dispersion in gravel-bed streams.

Hormonal correlates of male dominance rank, age, and genital colouration in vervet monkey (Chlorocebus pygerythrus)

Primates are the most colourful members of the Mammalian clade. In vervet monkeys (Chlorocebus pygerythrus), males are characterized by their red penis and blue scrotum. Such colour signals are often used in conspecific communication, and thus could be used to convey signaller condition. We quantified scrotal and penile colour characteristics using digital photographs between May-June 2016 from males in two neighboring groups along the shores of Lake Nabugabo, Uganda. We examined the relationship between fecal hormones, male dominance rank, age (adult vs. immature), and colour. Adult males were higher ranking than immatures, but there were no rank or age differences in fecal hormone levels. Glucocorticoids and androgens were positively correlated in immature, but not adult males. All scrotal characteristics were predicted by age, with adult males having more teal (i.e., less blue, more green) and more luminant scrota. Within adult males, those with higher androgens levels had more saturated blue scrotal colouration and higher-ranking males were more luminant. Penile colouration was also associated with age and rank. High-ranking males had a more saturated red penis, and adult male penile colour was more luminant and bluer than in immature males. Our findings are consistent with previous reports that scrotal colouration advertises sexual or reproductive maturity (i.e., age), but we also find that within adult males, colour also advertises dominance rank and may be mediated by androgen levels. Penile colouration also appears to signal information about male age and dominance rank but does not appear to be mediated by hormones.

Development of one-step isothermal methods to detect RNAs using hairpin-loop signal converters and proximity proteolysis reaction

Ribonucleic acids (RNAs) provide valuable information for biological systems and act as important indicators of disease states. RNAs are diverse in size and structure, and various strategies have been proposed for the detection of nucleic acids; however, developing them into point-of-care (POC) tests has been challenging as most of them consist of complex time-consuming steps. Here, we propose a strategy to assay RNAs using a hairpin-loop (HP) converter and proximity proteolysis reaction (PPR). Interaction between the loop part of HP and its target exposes a single strand of nucleotides, which acts as the template for PPR. A pair of protease and zymogen-conjugated nucleic acids associates with the adjacent regions of the template, resulting in an enhanced proteolysis reaction between protease and zymogen. The activated zymogen then generates a color signal through the hydrolysis of a chromogenic substrate. The combination of HP converter and PPR allowed the same pair of protease- and zymogen-nucleic acids to be used for different RNAs. Guidelines were provided for designing HP converters based on computational analyses and experimental characterizations. This strategy using an HP converter and PPR has been successfully applied to develop simple isothermal methods for the detection of various RNAs, including several microRNAs and KRAS mRNA, in the picomolar range in 1 h. The simplicity of designing HP converters and the beneficial properties of PPR as POC tests would enable the development of novel methods to detect RNAs under low-resource conditions.

Hydrogel Paper-Based Analytical Devices: Separation-Free In Situ Assay of Small-Molecule Targets in Whole Blood.

While colorimetric-based assays are very convenient to determine biomarkers in point-of-care testing (POCT), they often suffer from pretreatment procedures for separation of plasma or serum from whole blood samples. Here, we report a simple colorimetric paper-based analytical device (c-PAD) that is capable of performing sample-to-answer analysis by directly dropping the whole blood sample on paper. This is accomplished by utilizing sodium alginate hydrogel, which exhibits a nanometer-scale porous structure to effectively prevent the passage of large red blood cells and hemoglobin molecules, to encapsulate enzymes and chromogenic reagents. As the small targets in the blood sample enter the sensing region to trigger a chromogenic reaction, the resulting color signal is recorded by a smartphone. The interference from the red blood to the color signal can be completely avoided without the requirement of any separation process. The analytical performance of the method is evaluated by assaying glucose in real blood samples. The results show that rapid and accurate analysis can be achieved with the limit of detection as low as 0.12 mM. In addition, simultaneous detection of different targets (glucose, cholesterol, and triglycerides) in whole blood can be achieved by fabricating c-PAD with multiple sensing regions. Owing to its several essential advantages including an extremely simple procedure for fabrication, sample-to-answer analysis without tedious pretreatment, and capability to perform high-throughput analysis, the proposed c-PAD will be of great value in POCT applications of whole blood samples.

Skin coloration is a culturally-specific cue for attractiveness, healthiness, and youthfulness in observers of Chinese and western European descent.

Facial skin coloration signals information about an individual and plays an important role in social interactions and mate choice, due its putative association with health, attractiveness, and age. Whether skin coloration as an evolutionary significant cue is universal or specific to a particular culture is unclear and current evidence on the universality of skin color as a cue to health and attractiveness are equivocal. The current study used 80 calibrated, high-resolution, non-manipulated images of real human faces, either of Chinese or western European descent, which were rated in terms of attractiveness, healthiness, and perceived age by 44 observers, 22 western European (13 male; mean age ± SD = 24.27 ± 5.30) and 22 Chinese (7 male; mean age ± SD = 26.05 ± 3.96) observers. To elucidate the associations between skin coloration and these perceptual ratings and whether these associations are modulated by observer or image ethnicity, a linear mixed-effect model was setup with skin lightness (L*; CIELAB), redness (a*) and yellowness (b*), observer and image ethnicity as independent variables and perceived attractiveness, healthiness, and estimated age as dependent variables. We found robust positive associations between facial skin lightness (L*) and attractiveness, healthiness, and youthfulness, but only when Chinese observers judge facial images of their own ethnicity. Observers of European descent, on the other hand, associated an increase in yellowness(b*) with greater attractiveness and healthiness in Chinese facial images. We find no evidence that facial redness is positively associated with these attributes; instead, an increase in redness (a*) is associated with an increase in the estimated age of European facial images. We conclude that observers of both ethnicities make use of skin color and lightness to rate attractiveness, healthiness, and perceived age, but to a lesser degree than previously thought. Furthermore, these coloration cues are not universal and are utilized differently within the Chinese and western European ethnic groups. Our study adds to the growing body of work demonstrating the importance of skin color manipulations within an evolutionary meaningful parameter space, ideally using realistic skin models based on physical parameters.

Context-dependent coloration of prey and predator decision making in contrasting light environments.

A big question in behavioral ecology is what drives diversity of color signals. One possible explanation is that environmental conditions, such as light environment, may alter visual signaling of prey, which could affect predator decision-making. Here, we tested the context-dependent predator selection on prey coloration. In the first experiment, we tested detectability of artificial visual stimuli to blue tits (Cyanistes caeruleus) by manipulating stimulus luminance and chromatic context of the background. We expected the presence of the chromatic context to facilitate faster target detection. As expected, blue tits found targets on chromatic yellow background faster than on achromatic grey background whereas in the latter, targets were found with smaller contrast differences to the background. In the second experiment, we tested the effect of two light environments on the survival of aposematic, color polymorphic wood tiger moth (Arctia plantaginis). As luminance contrast should be more detectable than chromatic contrast in low light intensities, we expected birds, if they find the moths aversive, to avoid the white morph which is more conspicuous than the yellow morph in low light (and vice versa in bright light). Alternatively, birds may attack first moths that are more detectable. We found birds to attack yellow moths first in low light conditions, whereas white moths were attacked first more frequently in bright light conditions. Our results show that light environments affect predator foraging decisions, which may facilitate context-dependent selection on visual signals and diversity of prey phenotypes in the wild.

Fly pollination drives convergence of flower coloration.

Plant-pollinator interactions provide a natural experiment in signal evolution. Flowers are known to have evolved colour signals that maximise their ease of detection by the visual systems of important pollinators such as bees. Whilst most angiosperms are bee pollinated, our understanding on how the second largest group of pollinating insects, flies, may influence flower colour evolution is limited to the use of categorical models of colour discrimination that do not reflect the small colour differences commonly observed between and within flower species. Here we show by comparing flower signals that occur in different environments including total absence of bees, a mixture of bee and fly pollination within one plant family (Orchidaceae) from a single community, and typical flowers from a broad taxonomic sampling of the same geographic region, that perceptually different colours, empirically measured, do evolve in response to different types of insect pollinators. We show evidence of both convergence among fly-pollinated floral colours but also of divergence and displacement of colour signals in the absence of bee pollinators. Our findings give an insight into how both ecological and agricultural systems may be affected by changes in pollinator distributions around the world.

Major Flower Pigments Originate Different Colour Signals to Pollinators

Flower colour is mainly due to the presence and type of pigments. Pollinator preferences impose selection on flower colour that ultimately acts on flower pigments. Knowing how pollinators perceive flowers with different pigments becomes crucial for a comprehensive understanding of plant-pollinator communication and flower colour evolution. Based on colour space models, we studied whether main groups of pollinators, specifically hymenopterans, dipterans, lepidopterans and birds, differentially perceive flower colours generated by major pigment groups. We obtain reflectance data and conspicuousness to pollinators of flowers containing one of the pigment groups more frequent in flowers: chlorophylls, carotenoids and flavonoids. Flavonoids were subsequently classified in UV-absorbing flavonoids, aurones-chalcones and the anthocyanins cyanidin, pelargonidin, delphinidin, and malvidin derivatives. We found that flower colour loci of chlorophylls, carotenoids, UV-absorbing flavonoids, aurones-chalcones, and anthocyanins occupied different regions of the colour space models of these pollinators. The four groups of anthocyanins produced a unique cluster of colour loci. Interestingly, differences in colour conspicuousness among the pigment groups were almost similar in the bee, fly, butterfly, and bird visual space models. Aurones-chalcones showed the highest chromatic contrast values, carotenoids displayed intermediate values, and chlorophylls, UV-absorbing flavonoids and anthocyanins presented the lowest values. In the visual model of bees, flowers with UV-absorbing flavonoids (i.e., white flowers) generated the highest achromatic contrasts. Ours findings suggest that in spite of the almost omnipresence of floral anthocyanins in angiosperms, carotenoids and aurones-chalcones generates higher colour conspicuousness for main functional groups of pollinators.

A Simple Minimized System Based on Moving Drops for Antioxidant Analysis Using a Smartphone.

In this paper, a novel antioxidant analysis is proposed using a simple minimized device based on moving drops as solution handling and a smartphone as a detector. This approach is based on the colorimetric determination of the scavenging activity against 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•), expressed as the half-maximal inhibitory concentration (IC50), vitamin C equivalent antioxidant capacity (VCEAC), and Trolox equivalent antioxidant capacity (TEAC). A small drop of the positive control or the samples moves by eluting an ethanol drop down by the force of gravity to react with a DPPH• drop in the detection zone. The color change of DPPH• is monitored by a smartphone camera, and the color signals are processed using Adobe Photoshop software. The magenta-to-yellow ratio was successfully applied to evaluate the percentage of DPPH• inhibition with no significant difference compared with the reference spectrophotometric method at a confidence level of 95%. The total phenolic content (TPC) was measured using the Folin–Ciocalteu assay. An application to Miang (fermented tea leaf extract) showed the consonant relationship between the scavenging activity of DPPH• and TPC.

Flower colour and size signals differ depending on geographical location and altitude region

Bees are major pollinators of angiosperms and have phylogenetically conserved colour vision but differ in how various key species use achromatic information that is vital for both flower detection and size processing. We modelled green contrast and colour contrast signals from flowers of different countries where there are well established differences in availability of model bee species along altitudinal gradients. We tested for consistency in visual signals as expected from generalization in pollination principles using phylogenetically informed linear models. Patterns of chromatic contrast, achromatic green contrast and flower size differed among the three floras we examined. In Nepal there is a significant positive correlation between flower size and colour contrast in the subalpine region, but a negative correlation at the lower altitudes. At high elevations in Norway, where pollinators other than bees are common, flower size was positively correlated with colour contrast. At low and medium altitudes in Norway and in Australia, we did not observe a significant relationship between size and colour contrast. We thus find that the relationship between size, green and colour contrast cannot be generalized across communities, thus suggesting that flower visual signal adaptations to local pollinators are not limited to chromatic contrast.