The detectability of the colour pattern in the aposematic firebug, Pyrrhocoris apterus: an image-based experiment with human ‘predators’

Ontogenetic change in effectiveness of chemical defence against different predators in Oxycarenus true bugs

Many prey species change their antipredator defence during ontogeny, which may be connected to different potential predators over the life cycle of the prey. To test this hypothesis, we compared reactions of two predator taxa - spiders and birds - to larvae and adults of two invasive true bug species, Oxycarenus hyalinipennis and Oxycarenus lavaterae (Heteroptera: Oxycarenidae) with life-stage-specific chemical defence mechanisms. The reactions to larvae and adults of both true bug species strikingly differed between the two predator taxa. The spiders were deterred by the defences of adult bugs, but the larval defences were ineffective against them. By contrast, birds attacked the larvae considerably less often than the adult bugs. The results indicate a predator-specific ontogenetic change in defence effectiveness of both Oxycarenus species. The change in defence is likely linked to the life-stage-specific composition of secretions in both species: whereas secretions of larvae are dominated by unsaturated aldehydes, secretions of adults are rich in terpenoids, which probably serve dual function of defensive chemicals and pheromones. Our results highlight the variation in defence between different life stages and the importance of testing responses of different types of predators.

A field experiment characterizing variable detection rates during plant surveys

Surveys aimed at finding threatened and invasive species can be challenging due to individual rarity and low and variable individual detection rates. Detection rate in plant surveys typically varies due to differences among observers, among the individual plants being surveyed (targets), and across background environments. Interactions among these 3 components may occur but are rarely estimated due to limited replication and control during data collection. We conducted an experiment to investigate sources of variation in detection of 2 Pilosella species that are invasive and sparsely distributed in the Alpine National Park, Australia. These species are superficially similar in appearance to other yellow-flowered plants occurring in this landscape. We controlled the presence and color of flowers on target Pilosella plants and controlled their placement in plots, which were selected for their variation in cover of non-target yellow flowers and dominant vegetation type. Observers mimicked Pilosella surveys in the plots and reported 1 categorical and 4 quantitative indicators of their survey experience level. We applied survival analysis to detection data to model the influence of both controlled and uncontrolled variables on detection rate. Orange- and yellow-flowering Pilosella in grass- and heath-dominated vegetation were detected at a higher rate than nonflowering Pilosella. However, this detection gain diminished as the cover of other co-occurring yellow-flowering species increased. Recent experience with Pilosella surveys improved detection rate. Detection experiments are a direct and accessible means of understanding detection processes and interpreting survey data for threatened and invasive species. Our detection findings have been used for survey planning and can inform progress toward eradication. Interaction of target and background characteristics determined detection rate, which enhanced predictions in the Pilosella eradication program and demonstrated the difficulty of transferring detection findings into untested environments.

Seasonal variations in bird selection pressure on prey colouration

The direction and strength of selection for prey colouration by predators vary in space and time and depend on the composition of the predator community. We tested the hypothesis that bird selection pressure on prey colouration changes through the season due to changes in the proportion of naïve juvenile individuals in the bird community, because naïve and educated birds differ in their responses to prey colours. Bird predation on caterpillar-shaped plasticine models in two boreal forest sites increased sevenfold from early summer to mid-summer, and the time of this increase coincides with the fledging of juvenile birds. In early summer, cryptic (black and green) models were attacked at fivefold higher rates compared with conspicuous (red and yellow) models. By contrast, starting from fledging time, cryptic and conspicuous models were attacked at similar rates, hinting at a lower selectivity by naïve juvenile birds compared with educated adult birds. Cryptic models exposed in a group together with conspicuous models were attacked by birds at a threefold lower rate than cryptic models exposed singly, thus supporting the aposematic commensalism hypothesis. However, this effect was not observed in mid- and late summer, presumably due to the lack of avoidance of conspicuous prey by the juvenile birds. We conclude that selection pressure on prey colouration weakens considerably when naïve birds dominate in the community, because the survival advantages of aposematic colouration are temporarily lost for both the conspicuous and their neighbouring cryptic prey.

Habitat disturbance alters color contrast and the detectability of cryptic and aposematic frogs

Animals use color both to conceal and signal their presence, with patterns that match the background, disrupt shape recognition, or highlight features important for communication. The forms that these color patterns take are responses to the visual systems that observe them and the environments within which they are viewed. Increasingly, however, these environments are being affected by human activity. We studied how pattern characteristics and habitat change may affect the detectability of three frog color patterns from the Bocas del Toro archipelago in Panama: Beige-Striped Brown Allobates talamancae and two spotted morphs of Oophaga pumilio, Black-Spotted Green and Black-Spotted Red. To assess detectability, we used visual modeling of conspecifics and potential predators, along with a computer-based detection experiment with human participants. Although we found no evidence for disruptive camouflage, we did find clear evidence that A. talamancae stripes are inherently more cryptic than O. pumilio spots regardless of color. We found no evidence that color pattern polytypism in O. pumilio is related to differences in the forest floor between natural sites. We did, however, find strong evidence that human disturbance affects the visual environment and modifies absolute and rank order frog detectability. Human-induced environmental change reduces the effectiveness of camouflage in A. talamancae, reduces detectability of Black-Spotted Green O. pumilio, and increases chromatic contrast, but not detectability, in Black-Spotted Red O. pumilio. Insofar as predators may learn about prey defenses and make foraging decisions based on relative prey availability and suitability, such changes may have wider implications for predator–prey dynamics.

How do predators generalize warning signals in simple and complex prey communities? Insights from a videogame

The persistence of distinct warning signals within and between sympatric mimetic communities is a puzzling evolutionary question because selection favours convergence of colour patterns among toxic species. Such convergence is partly shaped by predators' reaction to similar but not identical stimulus (i.e. generalization behaviour), and generalization by predators is likely to be shaped by the diversity of local prey. However, studying generalization behaviour is generally limited to simple variations of prey colour patterns. Here, we used a computer game played by humans as surrogate predators to investigate generalization behaviours in simple (4 morphs) and complex (10 morphs) communities of unprofitable (associated with a penalty) and profitable butterflies. Colour patterns used in the game are observed in the natural populations of unprofitable butterfly species such as Heliconius numata. Analyses of 449 game participants' behaviours show that players avoided unprofitable prey more readily in simple than in complex communities. However, generalization was observed only in players that faced complex communities, enhancing the protection of profitable prey that looked similar to at least one unprofitable morph. Additionally, similarity among unprofitable prey also reduced attack rates only in complex communities. These results are consistent with previous studies using avian predators but artificial colour patterns and suggest that mimicry is more likely to evolve in complex communities where increases in similarity are more likely to be advantageous.

Distance-dependent pattern blending can camouflage salient aposematic signals

The effect of viewing distance on the perception of visual texture is well known: spatial frequencies higher than the resolution limit of an observer's visual system will be summed and perceived as a single combined colour. In animal defensive colour patterns, distance-dependent pattern blending may allow aposematic patterns, salient at close range, to match the background to distant observers. Indeed, recent research has indicated that reducing the distance from which a salient signal can be detected can increase survival over camouflage or conspicuous aposematism alone. We investigated whether the spatial frequency of conspicuous and cryptically coloured stripes affects the rate of avian predation. Our results are consistent with pattern blending acting to camouflage salient aposematic signals effectively at a distance. Experiments into the relative rate of avian predation on edible model caterpillars found that increasing spatial frequency (thinner stripes) increased survival. Similarly, visual modelling of avian predators showed that pattern blending increased the similarity between caterpillar and background. These results show how a colour pattern can be tuned to reveal or conceal different information at different distances, and produce tangible survival benefits.

Warning coloration can be disruptive: aposematic marginal wing patterning in the wood tiger moth

Warning (aposematic) and cryptic colorations appear to be mutually incompatible because the primary function of the former is to increase detectability, whereas the function of the latter is to decrease it. Disruptive coloration is a type of crypsis in which the color pattern breaks up the outline of the prey, thus hindering its detection. This delusion can work even when the prey's pattern elements are highly contrasting; thus, it is possible for an animal's coloration to combine both warning and disruptive functions. The coloration of the wood tiger moth (Parasemia plantaginis) is such that the moth is conspicuous when it rests on vegetation, but when it feigns death and drops to the grass- and litter-covered ground, it is hard to detect. This death-feigning behavior therefore immediately switches the function of its coloration from signaling to camouflage. We experimentally tested whether the forewing patterning of wood tiger moths could function as disruptive coloration against certain backgrounds. Using actual forewing patterns of wood tiger moths, we crafted artificial paper moths and placed them on a background image resembling a natural litter and grass background. We manipulated the disruptiveness of the wing pattern so that all (marginal pattern) or none (nonmarginal pattern) of the markings extended to the edge of the wing. Paper moths, each with a hidden palatable food item, were offered to great tits (Parus major) in a large aviary where the birds could search for and attack the "moths" according to their detectability. The results showed that prey items with the disruptive marginal pattern were attacked less often than prey without it. However, the disruptive function was apparent only when the prey was brighter than the background. These results suggest that warning coloration and disruptive coloration can work in concert and that the moth, by feigning death, can switch the function of its coloration from warning to disruptive.

Reduced predation risk for melanistic pygmy grasshoppers in post-fire environments

The existence of melanistic (black) color forms in many species represents interesting model systems that have played important roles for our understanding of selective processes, evolution of adaptations, and the maintenance of variation. A recent study reported on rapid evolutionary shifts in frequencies of the melanistic forms in replicated populations of Tetrix subulata pygmy grasshoppers; the incidence of the melanistic form was higher in recently burned areas with backgrounds blackened by fire than in nonburned areas, and it declined over time in postfire environments. Here, we tested the hypothesis that the frequency shifts of the black color variant were driven, at least in part, by changes in the selective regime imposed by visual predators. To study detectability of the melanistic form, we presented human "predators" with images of black grasshoppers and samples of the natural habitat on computer screens. We demonstrate that the protective value of black coloration differs between burnt and nonburnt environments and gradually increases in habitats that have been more blackened by fire. These findings support the notion that a black color pattern provides improved protection from visually oriented predators against blackened backgrounds and implicate camouflage and predation as important drivers of fire melanism in pygmy grasshoppers.