Technicolour deceit: a sensory basis for the study of colour-based lures

Genetically Encoded Lizard Color Divergence for Camouflage and Thermoregulation

Local adaptation is critical in speciation and evolution, yet comprehensive studies on proximate and ultimate causes of local adaptation are generally scarce. Here, we integrated field ecological experiments, genome sequencing, and genetic verification to demonstrate both driving forces and molecular mechanisms governing local adaptation of body coloration in a lizard from the Qinghai-Tibet Plateau. We found dark lizards from the cold meadow population had lower spectrum reflectance but higher melanin contents than light counterparts from the warm dune population. Additionally, the colorations of both dark and light lizards facilitated the camouflage and thermoregulation in their respective microhabitat simultaneously. More importantly, by genome resequencing analysis, we detected a novel mutation in Tyrp1 that underpinned this color adaptation. The allele frequencies at the site of SNP 459# in the gene of Tyrp1 are 22.22% G/C and 77.78% C/C in dark lizards and 100% G/G in light lizards. Model-predicted structure and catalytic activity showed that this mutation increased structure flexibility and catalytic activity in enzyme TYRP1, and thereby facilitated the generation of eumelanin in dark lizards. The function of the mutation in Tyrp1 was further verified by more melanin contents and darker coloration detected in the zebrafish injected with the genotype of Tyrp1 from dark lizards. Therefore, our study demonstrates that a novel mutation of a major melanin-generating gene underpins skin color variation co-selected by camouflage and thermoregulation in a lizard. The resulting strong selection may reinforce adaptive genetic divergence and enable the persistence of adjacent populations with distinct body coloration.

Captivating color: evidence for optimal stimulus design in a polymorphic prey lure

Many species – humans included – employ color as an instrument of deception. One intriguing example of this resides in the conspicuous abstract color patterns displayed on the bodies of female orb weaving spiders. These displays increase prey interception rates and thereby function at least as visual lures. Their chromatic properties however vary extensively, both across and within species, with discrete forms often co-existing in the manner of a stable polymorphism. Variation is principally expressed in terms of signal hue (color per se), but it is unclear how attractiveness scales with this property and if extant morphs are maximally attractive relative to a graded range of potential alternatives. We examined these questions by assessing catch rates among color-manipulated females of the dimorphic jeweled spider Gasteracantha fornicata in their natural webs. The manipulation altered dorsal appearance in a manner akin to adding six new variants of their existing white/yellow phenotypes. This magnified the natural variation in stimulus hue independently of chroma (saturation) across a range spanning most of the color spectrum. Catch rate varied across treatments in simple accordance with how greatly stimulus hue deviated from either of the two extant spider phenotypes. Predictions based on fly-perceived chromatic and achromatic background contrast were clearly unsupported despite dipterans constituting ~60 % of identifiable prey. This study supports the importance of signal coloration per se in G. fornicata and suggests that extant lure phenotypes reside in a broadly optimal spectral range for stimulating their aggregate prey community.

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.

Color lures in orb-weaving spiders: a meta-analysis

Lures are deceptive strategies that exploit sensory biases in prey, usually mimicking a prey’s mate or food item. Several predators exploit plant–pollinator systems, where visual signals are an essential part of interspecific interactions. Many diurnal, and even nocturnal, orb-web spiders present conspicuous body coloration or bright color patches. These bright colors are regarded as color-based lures that exploit biases present in insect visual systems, possibly mimicking flower colors. The prey attraction hypothesis was proposed more than 20 years ago to explain orb-web spider coloration. Although most data gathered so far has corroborated the predictions of the prey attraction hypothesis, there are several studies that refute these predictions. We conducted a multilevel phylogenetic meta-analysis to assess the magnitude of the effect of conspicuous orb-web spider body coloration on prey attraction. We found a positive effect in favor of the prey attraction hypothesis; however, there was substantial heterogeneity between studies. Experimental designs comparing conspicuous spiders to painted spiders or empty webs did not explain between-studies heterogeneity. The lack of theoretical explanation behind the prey attraction hypothesis makes it challenging to address which components influence prey attraction. Future studies could evaluate whether color is part of a multicomponent signal and test alternative hypotheses for the evolution of spider colors, such as predator avoidance and thermoregulation.

Prey and predators perceive orb-web spider conspicuousness differently: evaluating alternative hypotheses for color polymorphism evolution

Color polymorphisms have been traditionally attributed to apostatic selection. The perception of color depends on the visual system of the observer. Theoretical models predict that differently perceived degrees of conspicuousness by two predator and prey species may cause the evolution of polymorphisms in the presence of anti-apostatic and apostatic selection. The spider Gasteracantha cancriformis (Araneidae) possesses several conspicuous color morphs. In orb-web spiders, the prey attraction hypothesis states that conspicuous colors are prey lures that increase spider foraging success via flower mimicry. Therefore, polymorphism could be maintained if each morph attracted a different prey species (multiple prey hypothesis) and each spider mimicked a different flower color (flower mimicry hypothesis). Conspicuous colors could be a warning signal to predators because of the spider’s hard abdomen and spines. Multiple predators could perceive morphs differently and exert different degrees of selective pressures (multiple predator hypothesis). We explored these 3 hypotheses using reflectance data and color vision modeling to estimate the chromatic and achromatic contrast of G. cancriformis morphs as perceived by several potential prey and predator taxa. Our results revealed that individual taxa perceive the conspicuousness of morphs differently. Therefore, the multiple prey hypothesis and, in part, the multiple predator hypothesis may explain the evolution of color polymorphism in G. cancriformis, even in the presence of anti-apostatic selection. The flower mimicry hypothesis received support by color metrics, but not by color vision models. Other parameters not evaluated by color vision models could also affect the perception of morphs and influence morph survival and polymorphism stability.

The evolution of anti-bat sensory illusions in moths

Prey transmit sensory illusions to redirect predatory strikes, creating a discrepancy between what a predator perceives and reality. We use the acoustic arms race between bats and moths to investigate the evolution and function of a sensory illusion. The spinning hindwing tails of silk moths (Saturniidae) divert bat attack by reflecting sonar to create a misleading echoic target. We characterized geometric morphometrics of moth hindwings across silk moths, mapped these traits onto a new, robust phylogeny, and found that elaborated hindwing structures have converged on four adaptive shape peaks. To test the mechanism underlying these anti-bat traits, we pit bats against three species of silk moths with experimentally altered hindwings that created a representative gradient of ancestral and extant hindwing shapes. High-speed videography of battles reveals that moths with longer hindwings and tails more successfully divert bat attack. We postulate that sensory illusions are widespread and are underappreciated drivers of diversity across systems.

Interspecific visual signalling in animals and plants: a functional classification

Organisms frequently gain advantages when they engage in signalling with individuals of other species. Here, we provide a functionally structured framework of the great variety of interspecific visual signals seen in nature, and then describe the different signalling mechanisms that have evolved in response to each of these functional requirements. We propose that interspecific visual signalling can be divided into six major functional categories: anti-predator, food acquisition, anti-parasite, host acquisition, reproductive and agonistic signalling, with each function enabled by several distinct mechanisms. We support our classification by reviewing the ecological and behavioural drivers of interspecific signalling in animals and plants, principally focusing on comparative studies that address large-scale patterns of diversity. Collating diverse examples of interspecific signalling into an organized set of functional and mechanistic categories places anachronistic behavioural and morphological labels in fresh context, clarifies terminology and redirects research effort towards understanding environmental influences driving interspecific signalling in nature.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Jewelled spiders manipulate colour-lure geometry to deceive prey

Selection is expected to favour the evolution of efficacy in visual communication. This extends to deceptive systems, and predicts functional links between the structure of visual signals and their behavioural presentation. Work to date has primarily focused on colour, however, thereby understating the multicomponent nature of visual signals. Here I examined the relationship between signal structure, presentation behaviour, and efficacy in the context of colour-based prey luring. I used the polymorphic orb-web spider Gasteracantha fornicata, whose yellow- or white-and-black striped dorsal colours have been broadly implicated in prey attraction. In a manipulative assay, I found that spiders actively control the orientation of their conspicuous banded signals in the web, with a distinct preference for near-diagonal bearings. Further field-based study identified a predictive relationship between pattern orientation and prey interception rates, with a local maximum at the spiders' preferred orientation. There were no morph-specific effects on capture success, either singularly or via an interaction with pattern orientation. These results reveal a dynamic element in a traditionally 'static' signalling context, and imply differential functions for chromatic and geometric signal components across visual contexts. More broadly, they underscore how multicomponent signal designs and display behaviours may coevolve to enhance efficacy in visual deception.

Crab Spider Lures Prey In Flowerless Neighborhoods

One fundamental question in prey luring systems is to understand how visual signals are interpreted by the receiver. Predators lure prey by falsely imitating the signal of a model, or may exploit sensory preferences of the receivers, which search for rewarding signals. Crab spiders reflect ultraviolet (UV) light, ambush pollinators on flowers, and manipulate flower UV signals altering the behavior and response of prey. Whereas crab spiders typically depend on flowers to forage, adult Epicadus heterogaster departs from this standard behavior by preying on pollinators upon green leaves, even in the absence of flowers nearby. This species has a conspicuous abdomen resembling the shape of a flower, which may reflect UV signals similar to that of flowers, and thus attract pollinators. Nevertheless, no empirical evidence is available that E. heterogaster foraging on leaves mimics flowers, nor how this crab spider interacts with its prey. Field and laboratory experiments demonstrated that UV reflection of adult E. heterogaster is the main signal responsible for the attraction of pollinators. This is the first study to demonstrate that a crab spider attracts pollinators regardless of flower UV signal, which may represent an evolutionary pathway beyond the dependence of flowers.

Colour polymorphic lures exploit innate preferences for spectral versus luminance cues in dipteran prey

BACKGROUND

Theory predicts that colour polymorphism may be favored by variation in the visual context under which signals are perceived. The context encompasses all environmental determinants of light availability and propagation, but also the dynamics of perception in receivers. Color vision involves the neural separation of information into spectral versus luminance channels, which often differentially guide specific tasks. Here we explicitly tested whether this discrete perceptual basis contributes to the maintenance of polymorphism in a prey-luring system. The orb-weaving spider Gasteracantha fornicata is known to attract a broad community of primarily dipteran prey due to their conspicuous banded dorsal signal. They occur in two morphs ("white" and "yellow") which should, respectively, generate greater luminance and color contrast in the dipteran eye. Given that arthropods often rely upon luminance-versus-spectral cues for relatively small-versus-large stimulus detection, we predicted a switch in relative attractiveness among morphs according to apparent spider size.

RESULTS

Our experimental tests used colour-naïve individuals of two known prey species (Drosophila hydei and Musca domestica) in replicate Y-maze choice trials designed to manipulate the apparent size of spider models via the distance at which they are viewed. Initial trials confirmed that flies were attracted to each G. fornicata morph in single presentations. When given a simultaneous choice between morphs against a viewing background typical of those encountered in nature, flies exhibited no preference regardless of the visual angle subtended by models. However, when backgrounds were adjusted to nearer the extremes of those of each morph in the wild, flies were more attracted by white morphs when presented at longer range (consistent with a reliance on achromatic cues), yet were unbiased in their close-range choice.

CONCLUSION

While not fully consistent with predictions (given the absence of a differential preference for stimuli at close range), our results demonstrate an effect of apparent stimulus size upon relative morph attractiveness in the direction anticipated from present knowledge of fly visual ecology. This implies the potential tuning of G. fornicata morph signal structure according to a perceptual feature that is likely common across their breadth of arthropod prey, and complements recent observational work in suggesting a candidate mechanism for the maintenance of deceptive polymorphism through the exploitation of different visual channels in prey.

Luring

Bulbert and Wignall introduce the ways animals lure their prey.