Most animals including humans use vision to detect, identify, evaluate and respond to potential prey items in complex environments. Theories predict that predators' visual search performance is better when targets are similar than when targets are dissimilar and require divided attention, and this may contribute to colour pattern polymorphism in prey. Most prey also vary in size, but how size variation influences detectability and search performance of predators that utilize polymorphic prey has received little attention. To evaluate the effect of size variability on prey detection we asked human subjects to search for images of black, grey and striped pygmy grasshoppers presented on computer screens in size-variable (large, medium and small) or in size-invariable (all medium) sequences (populations) against photographs of natural grasshopper habitat. Results showed that size variability either increased or reduced detection of medium-sized targets depending on colour morph. To evaluate whether bias in perceived size varies depending on colour pattern, subjects were asked to discriminate between two grasshopper images of identical size that were presented in pairs against a monochromatic background. Subjects more often incorrectly classified one of the two identical-sized targets as being larger than the other in colour-dimorphic than in monomorphic presentations. The distinctly patterned (striped) morph elicited stronger size perception biases than the dorsally grey or black morphs, and striped grasshoppers were incorrectly classified more often as smaller than grey grasshoppers. The direction of the effect of size variability on detection changed across colour patterns as the bias in perceived size increased. Such joint effects of variation in size and colour pattern on detection and perception can impact the outcome of behavioural and evolutionary interactions between visually oriented predators and their camouflaged prey. This may have consequences for population dynamics, evolution of polymorphisms, community species composition and ecosystem functioning. (C) 2018 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Colour pattern polymorphism in animals can influence and be influenced by interactions between predators and prey. However, few studies have examined whether polymorphism is adaptive, and there is no evidence that the co-occurrence of two or more natural prey colour variants can increase survival of populations. Here we show that visual predators that exploit polymorphic prey suffer from reduced performance, and further provide rare evidence in support of the hypothesis that prey colour polymorphism may afford protection against predators for both individuals and populations. This protective effect provides a probable explanation for the longstanding, evolutionary puzzle of the existence of colour polymorphisms. We also propose that this protective effect can provide an adaptive explanation for search image formation in predators rather than search image formation explaining polymorphism.

Body size and coloration may contribute to variation in performance and fitness among individuals, for instance by influencing vulnerability to predators. Yet, the combined effect of size and colour pattern on susceptibility to visual predators has received little attention, particularly in camouflaged prey. In the colour polymorphic pygmy grasshopper Tetrix subulata (Linnaeus, 1758) females are larger than males although there is a size overlap between sexes. We investigated how body size and colour morph influenced detection of these grasshoppers, and whether differences in protective value among morphs change with size. We conducted a computer-based experiment and compared how human ‘predators’ detected images of large, intermediate or small grasshoppers belonging to black, grey or striped colour morphs when embedded in photographs of natural grasshopper habitats. We found that time to detection increased with decreasing size, that differences in time to detection of the black, grey and striped morphs depended differently on body size, and that no single morph provided superior or inferior protection in all three size classes. By comparing morph frequencies in samples of male and female grasshoppers from natural populations we also examined whether the joint effects of size and colour morph on detection could explain evolutionary dynamics in the wild. Morph frequency differences between sexes were largely in accordance with expectations from the results of the detection experiment. Our results demonstrate that body size and colour morph can interactively influence detection of camouflaged prey. This may contribute to the morph frequency differences between male and female pygmy grasshoppers in the wild. Such interactive effects may also influence the dynamics of colour polymorphisms, and contribute to the evolution of ontogenetic colour change and sexual dichromatism.

Polymorphism, the coexistence of two or more variants within a population, has served as a classic model system to address questions about the evolution and maintenance of intra-specific variation. It has been hypothesized that natural level of colour polymorphism may impair the search efficiency of visually oriented predators. To test this polymorphism protects hypothesis, we asked human participants to search for images of natural black, striped or grey Tetrix subulata grasshopper colour morphs presented against photographs of their natural habitat on computer screens. Fewer grasshoppers were detected when morphs were presented in mixed than in uniform sequences. All three morphs benefitted to comparable degrees, in terms of reduced detection, from being presented in polymorphic sequences. Our findings demonstrate that natural levels of polymorphic variation can impede the efficiency of visually oriented predators and increase survival of prey. This protective effect supports the limited attention hypothesis, explains why predators develop ‘search images’, may account for the spread and establishment of novel colour variants, and contribute to maintenance of polymorphisms.

In this thesis, I used the color polymorphic pygmy grasshopper, Tetrix subulata, as a model system in order to investigate the degree of niche differentiation between alternative color morphs that are present within a single population. First, I hypothesized that individuals belonging to different color morphs differ in their innate food preference due to somewhat dissimilar physiology, and in their actual food consumption due in part to differential habitat use. I found that alternative morphs differ in their preferred food (multiple-choice experiment in the lab), and in long-term consumption of food in the wild (stable isotopes analysis). Next, I investigated what kind of ecological constraints may result in differential habitat use? Firstly, by painting grasshoppers either black or white in order to influence their thermal properties, I demonstrated that thermoregulatory behavior results in differential microhabitat use in a way that may increase fitness. Secondly, in computer based experiments in which human ‘predators’ were asked to detect images of grasshoppers implemented in images of various environments, I further demonstrated that the degree to which a specific body coloration supply protection, changes across backgrounds, and that no single color pattern provides superior protection against all visual backgrounds. Thus, predator avoidance behavior may result in differential habitat use as well. Can these findings translate into an advantage of polymorphic relative to monomorphic populations?  I then evaluated one prediction from theory that posits that more variable founder groups will be more successful in establishing population in new environments. Indeed, I found higher number of individuals the following year in more variable founder groups in semi-natural conditions in field experiment. This thesis demonstrates, niche differentiation along several dimensions between alternative color morph in a generalist invertebrate. It provides experimental evidence supporting the notion that visual predation may be driving force behind evolutionary change in morph frequencies, and points to a possible advantage for color polymorphic relative to monomorphic groups in terms of a higher establishment success.

Background: Animal colour patterns offer good model systems for studies of biodiversity and evolution of local adaptations. An increasingly popular approach to study the role of selection for camouflage for evolutionary trajectories of animal colour patterns is to present images of prey on paper or computer screens to human 'predators'. Yet, few attempts have been made to confirm that rates of detection by humans can predict patterns of selection and evolutionary modifications of prey colour patterns in nature. In this study, we first analyzed encounters between human 'predators' and images of natural black, grey and striped colour morphs of the polymorphic Tetrix subulata pygmy grasshoppers presented on background images of unburnt, intermediate or completely burnt natural habitats. Next, we compared detection rates with estimates of capture probabilities and survival of free-ranging grasshoppers, and with estimates of relative morph frequencies in natural populations. Results: The proportion of grasshoppers that were detected and time to detection depended on both the colour pattern of the prey and on the type of visual background. Grasshoppers were detected more often and faster on unburnt backgrounds than on 50% and 100% burnt backgrounds. Striped prey were detected less often than grey or black prey on unburnt backgrounds; grey prey were detected more often than black or striped prey on 50% burnt backgrounds; and black prey were detected less often than grey prey on 100% burnt backgrounds. Rates of detection mirrored previously reported rates of capture by humans of free-ranging grasshoppers, as well as morph specific survival in the wild. Rates of detection were also correlated with frequencies of striped, black and grey morphs in samples of T. subulata from natural populations that occupied the three habitat types used for the detection experiment. Conclusions: Our findings demonstrate that crypsis is background-dependent, and implicate visual predation as an important driver of evolutionary modifications of colour polymorphism in pygmy grasshoppers. Our study provides the clearest evidence to date that using humans as 'predators' in detection experiments may provide reliable information on the protective values of prey colour patterns and of natural selection and microevolution of camouflage in the wild.

In some species of insects, individuals with fully developed wings and capable of flying coexist with flightless individuals that lack functional wings. Their diets may differ if long-winged individuals are more mobile and therefore likely to be better at finding and utilizing high quality food resources, or if they have different food preferences or physiological requirements. Despite its potential importance, differences in the diet of dispersal phenotypes have not been unequivocally demonstrated under natural conditions. To test for dietary divergence, we compared natural abundances of carbon and nitrogen stable isotope ratios (d¹³C and d¹⁵N) in long- and short-winged free ranging Tetrix subulata pygmy grasshoppers collected as adults from two natural populations. Overall, this comparison of stable isotopes indicated long-term differences in the diet of the two wing morphs in both populations, but not between males and females of the same morph. We conclude that it is likely that the dietary niches of the long winged and flightless individuals differ under natural conditions. This may reduce intra-specific competition, offset the expected trade-off between flight capacity and reproduction and promote ecological speciation.

Large founder groups and habitat match have been shown to increase the establishment success of reintroduced populations. Theory posits that the diversity of founder groups should also be important, but this has rarely been investigated. Here, experimental introductions of color-polymorphic Tetrix subulata pygmy grasshoppers into outdoor enclosures were used to test whether higher phenotypic diversity promotes establishment success. We show that the number of individuals present one year after introduction increases with color morph diversity in founder groups. Variance in establishment success did not decrease with increasing founder diversity, arguing against an important contribution of sampling effects or evolutionary rescue. Color morphs in T. subulata covary with a suite of other functionally important traits and utilize different resources. The higher establishment success in more diverse founder groups may therefore result, in part, from niche complementarity. Variation in establishment among groups was not associated with differences among source populations in reproductive capacities.

Gene flow is often regarded a random process that homogenizes differencesbetween populations and constrains local adaptation. However, the matching habitat choicehypothesis posits that individuals actively choose those microhabitats that best match theirspecific phenotype to maximize fitness. Dispersal (and possibly gene flow) may thus bedirected. Many studies report associations between habitats and phenotypes, but they mayreflect selection, plasticity or adaptation rather than matching choice. Here, we test twopredictions from the matching habitat choice hypothesis by manipulating the dorsal colourof Tetrix subulata, a pygmy grasshopper. (1) Is microhabitat choice flexible such thatdifferently manipulated phenotypes distribute themselves differently in a microclimaticand solar radiation mosaic? (2) If they do, are their fitness prospects higher in the morepreferred microhabitat? We find that individuals painted white or black do distributethemselves differently, with black individuals residing in habitats with less radiation, onaverage, than white individuals, demonstrating that microhabitat choices are plastic. Furthermore,white females had more hatchlings than black ones in the increased radiationtreatment, and this was mainly due to increased mortality of black females under increasedradiation. These findings provide rare experimental evidence in line with predictions fromthe matching habitat choice hypothesis.

The matching habitat choice hypothesis posits that individuals actively choose those microhabitats that best match their specificphenotype to maximize fitness. Despite the profound implications, matching habitat choice has not been unequivocally demonstrated. Weconducted two experiments to examine the impact of pigmentation pattern in the color polymorphic pygmy grasshopper Tetrix subulata onhabitat choice in a laboratory thermal mosaic arena. We found no behavioral differences in the thermal mosaic among pygmy grasshoppersbelonging to either pale, intermediate or dark natural color morphs. However, after manipulating the grasshoppers’ phenotype, the utilizationthrough time of warmer and colder parts of the arena was different for black-painted and white-painted individuals. White-paintedindividuals used warmer parts of the arena, at least during the initial stage of the experiment. We conclude that microhabitat choicerepresents a form of behavioural plasticity. Thus, even if the choice itself is flexible and not genetically determined, it can still lead to spatialgenetic structure in the population because the phenotypes themselves may be genetically mediated