Visual illusions in predator–prey interactions: birds find moving patterned prey harder to catch

Antipredator defences in motion: animals reduce predation risks by concealing or misleading motion signals

Motion is a crucial part of the natural world, yet our understanding of how animals avoid predation whilst moving remains rather limited. Although several theories have been proposed for how antipredator defence may be facilitated during motion, there is often a lack of supporting empirical evidence, or conflicting findings. Furthermore, many studies have shown that motion often 'breaks' camouflage, as sudden movement can be detected even before an individual is recognised. Whilst some static camouflage strategies may conceal moving animals to a certain extent, more emphasis should be given to other modes of camouflage and related defences in the context of motion (e.g. flicker fusion camouflage, active motion camouflage, motion dazzle, and protean motion). Furthermore, when motion is involved, defence strategies are not necessarily limited to concealment. An animal can also rely on motion to mislead predators with regards to its trajectory, location, size, colour pattern, or even identity. In this review, we discuss the various underlying antipredator strategies and the mechanisms through which they may be linked to motion, conceptualising existing empirical and theoretical studies from two perspectives - concealing and misleading effects. We also highlight gaps in our understanding of these antipredator strategies, and suggest possible methodologies for experimental designs/test subjects (i.e. prey and/or predators) and future research directions.

Background matching can reduce responsiveness of jumping spiders to stimuli in motion

Motion and camouflage were previously considered to be mutually exclusive, as sudden movements can be easily detected. Background matching, for instance, is a well-known, effective camouflage strategy where the colour and pattern of a stationary animal match its surrounding background. However, background matching may lose its efficacy when the animal moves, as the boundaries of the animal become more defined against its background. Recent evidence shows otherwise, as camouflaged objects can be less detectable than uncamouflaged objects even while in motion. Here, we explored whether the detectability of computer-generated stimuli varies with the speed of motion, background (matching and unmatching) and size of stimuli in six species of jumping spiders (Araneae: Salticidae). Our results showed that, in general, the responsiveness of all six salticid species tested decreased with increasing stimulus speed regardless of whether the stimuli were conspicuous or camouflaged. Importantly, salticid responses to camouflaged stimuli were significantly lower compared with those to conspicuous stimuli. There were significant differences in motion detectability across species when the stimuli were conspicuous, suggesting differences in visual acuity in closely related species of jumping spiders. Furthermore, small stimuli elicited significantly lower responses than large stimuli across species and speeds. Our results thus suggest that background matching is effective even when stimuli are in motion, reducing the detectability of moving stimuli.

A framework for understanding post-detection deception in predator-prey interactions

Predators and prey exist in persistent conflict that often hinges on deception-the transmission of misleading or manipulative signals-as a means for survival. Deceptive traits are widespread across taxa and sensory systems, representing an evolutionarily successful and common strategy. Moreover, the highly conserved nature of the major sensory systems often extends these traits past single species predator-prey interactions toward a broader set of perceivers. As such, deceptive traits can provide a unique window into the capabilities, constraints and commonalities across divergent and phylogenetically-related perceivers. Researchers have studied deceptive traits for centuries, but a unified framework for categorizing different types of post-detection deception in predator-prey conflict still holds potential to inform future research. We suggest that deceptive traits can be distinguished by their effect on object formation processes. Perceptual objects are composed of physical attributes (what) and spatial (where) information. Deceptive traits that operate after object formation can therefore influence the perception and processing of either or both of these axes. We build upon previous work using a perceiver perspective approach to delineate deceptive traits by whether they closely match the sensory information of another object or create a discrepancy between perception and reality by exploiting the sensory shortcuts and perceptual biases of their perceiver. We then further divide this second category, sensory illusions, into traits that distort object characteristics along either the what or where axes, and those that create the perception of whole novel objects, integrating the what/where axes. Using predator-prey examples, we detail each step in this framework and propose future avenues for research. We suggest that this framework will help organize the many forms of deceptive traits and help generate predictions about selective forces that have driven animal form and behavior across evolutionary time.

Continuous Variation in an Aposematic Pattern Affects Background Contrast, but Is Not Associated With Differences in Microhabitat Use

Variation in aposematic signals was once predicted to be rare, yet in recent years it has become increasingly well documented. Despite increases in the frequency with which polytypism and polymorphism have been suggested to occur, population-wide variance is rarely quantified. We comprehensively sampled a subpopulation of the poison frog Oophaga sylvatica, a species which is polytypic across its distribution and also shows considerable within-population polymorphism. On one hand, color pattern polymorphism could be the result of multifarious selection acting to balance different signaling functions and leading to the evolution of discrete sub-morphs which occupy different fitness peaks. Alternatively, variance could simply be due to relaxed selection, where variation would be predicted to be continuous. We used visual modeling of conspecific and heterospecific observers to quantify the extent of within population phenotypic variation and assess whether this variation produced distinct signals. We found that, despite considerable color pattern variation, variance could not be partitioned into distinct groups, but rather all viewers would be likely to perceive variation as continuous. Similarly, we found no evidence that frog color pattern contrast was either enhanced or diminished in the frogs’ chosen microhabitats compared to alternative patches in which conspecifics were observed. Within population phenotypic variance therefore does not seem to be indicative of strong selection toward multiple signaling strategies, but rather pattern divergence has likely arisen due to weak purifying selection, or neutral processes, on a signal that is highly salient to both conspecifics and predators.

Motion: enhancing signals and concealing cues

Animal colour patterns remain a lively focus of evolutionary and behavioural ecology, despite the considerable conceptual and technical developments over the last four decades. Nevertheless, our current understanding of the function and efficacy of animal colour patterns remains largely shaped by a focus on stationary animals, typically in a static background. Yet, this rarely reflects the natural world: most animals are mobile in their search for food and mates, and their surrounding environment is usually dynamic. Thus, visual signalling involves not only animal colour patterns, but also the patterns of animal motion and behaviour, often in the context of a potentially dynamic background. While motion can reveal information about the signaller by attracting attention or revealing signaller attributes, motion can also be a means of concealing cues, by reducing the likelihood of detection (motion camouflage, motion masquerade and flicker-fusion effect) or the likelihood of capture following detection (motion dazzle and confusion effect). The interaction between the colour patterns of the animal and its local environment is further affected by the behaviour of the individual. Our review details how motion is intricately linked to signalling and suggests some avenues for future research.

This Review has an associated Future Leader to Watch interview with the first author.

Summary: While motion can reveal information about the signaller, motion can also be a means of concealing cues by reducing the likelihood of detection or the likelihood of capture following detection.

Do bovids evolve hindquarter markings for anti-predation?

Conspicuous coloration in animals serves many functions such as anti-predation. Anti-predation strategies include motion dazzle and flash behavior. Motion dazzle markings can reduce the probability of being preyed on because the predators misjudge their movement. In flash behavior, prey demonstrate conspicuous cue while fleeing; the predators follow them; however, the prey hide their markings and the predators assume that the prey has vanished. To investigate whether bovids use conspicuous hindquarter markings as an anti-predatory behavior, we undertook phylogenetically controlled analyses to explore under what physiological characteristics and environmental factors bovids might have this color pattern. The results suggested that rump patches and tail markings were more prevalent in bovids living in larger-sized groups, which supports the hypothesis of intraspecific communication. Moreover, we observed the occurrence of conspicuous white hindquarter markings in bovids having smaller body size and living in larger groups, suggesting a motion dazzle function. However, the feature of facultative exposing color patterns (flash markings) was not associated with body size, which was inconsistent with predictions and implied that bovids may not adopt this as an anti-predator strategy. It was concluded that species in bovids with conspicuous white hindquarter markings adopt motion dazzle as an anti-predation strategy while fleeing and escaping from being prey on.

In the corner of the eye: camouflaging motion in the peripheral visual field

Most animals need to move, and motion will generally break camouflage. In many instances, most of the visual field of a predator does not fall within a high-resolution area of the retina and so, when an undetected prey moves, that motion will often be in peripheral vision. We investigate how this can be exploited by prey, through different patterns of movement, to reduce the accuracy with which the predator can locate a cryptic prey item when it subsequently orients towards a target. The same logic applies for a prey species trying to localize a predatory threat. Using human participants as surrogate predators, tasked with localizing a target on peripherally viewed computer screens, we quantify the effects of movement (duration and speed) and target pattern. We show that, while motion is certainly detrimental to camouflage, should movement be necessary, some behaviours and surface patterns reduce that cost. Our data indicate that the phenotype that minimizes localization accuracy is unpatterned, having the mean luminance of the background, does not use a startle display prior to movement, and has short (below saccadic latency), fast movements.

From forest to city: urbanization modulates relative abundance of anti-predator coloration

Increased urbanization has resulted in community changes including alteration of predator communities. Little is known, however, about how such changes affect morphological anti-predator traits. Given the importance of coloration in predator avoidance, this trait in particular is expected to be susceptible to novel selective environments in urban areas. Here, we investigate the coloration pattern of a Neotropical anuran species, the túngara frog (Engystomops pustulosus), along an urbanization gradient. Túngara frogs have two distinct color patterns (unstriped and striped) which we found to occur at different frequencies along an urbanization gradient. Striped individuals increased in frequency with urbanization. To assess the strength of selection imposed by predators on the two color morphs, we deployed clay models of túngara frogs in forest and semi-urban populations. In addition, we examined microhabitat selection by individuals of the different morphs. We found higher predation rates associated with urbanization than forested areas. In particular, frogs from forested habitats had lower number of attacks by avian predators. Contrary to our predictions, however, predation rates were similar for both color morphs independent of urbanization. Also, coloration of the frogs did not affect their microhabitat preference. Overall, túngara frogs are more likely to have a striped coloration pattern in semi-urban areas where predation by birds is higher than in the forest. Our findings suggest that factors other than predation pressure shape the coloration pattern of urban frogs and emphasize the complex nature of effects that anthropogenic changes in habitat and predator communities may have on prey morphology.

Variation in activity rates may explain sex-specific dorsal color patterns in Habronattus jumping spiders

In many animals, color pattern and behavior interact to deceive predators. For mimics, such deception can range from precise (near-perfect mimicry) to only subtle resemblance (imperfect mimicry) and such strategies often differ by sex because of differing ecological selection pressures. In this field study, we examine variation in behavior and ecology that may be linked with sex differences in dorsal color pattern in three sympatric species of Habronattus jumping spiders (H. clypeatus, H. hallani, H. pyrrithrix). Males of these species have conspicuous dorsal patterning that is subtly reminiscent of the general color patterns of wasps and bees, while females are cryptic. We show that, compared with females, these conspicuous males exhibited increased leg-waving behavior outside of the context of courtship; such behavior is common in jumping spiders that mimic wasps and bees presumably because a mimic’s waving legs resemble antennae. Males of a fourth sympatric species (H. hirsutus) without conspicuous dorsal patterning did not exhibit increased leg-waving. These results are consistent with and offer preliminary support for the idea that male color and behavior may work together to deceive predators. We also examined whether higher movement rates of males (who must wander to find females) and/or different use of the microhabitat by the sexes could explain sexual dichromatism. We found that microhabitat use was similar for males and females, but males of all three conspicuously-colored species spent more time actively moving than females. To our knowledge, this is the first study to speculate that conspicuous male dorsal coloration in Habronattus may have a deceptive function, and to explore why dorsal coloration differs between the sexes.

What makes motion dazzle markings effective against predation?

Motion dazzle markings comprise patterns such as stripes and zig-zags that are postulated to protect moving prey by making predators misjudge the prey’s speed or trajectory. Recent experiments have provided conflicting results on their effect on speed perception and attack success. We focus on motion dazzle stripes and investigate the influence of four parameters—stripe orientation, stripe contrast, target size, and target speed—on perceived speed and attack success using a common experimental paradigm involving human “predators” attacking virtual moving targets on a computer touchscreen. We found that high-contrast stripes running parallel or perpendicular to the direction of motion reduce attack success compared to conspicuous uniform targets. Surprisingly, parallel stripes induced underestimation of speed, while perpendicular stripes induced overestimation of speed in relation to uniform black, suggesting that misjudgment of speed per se is sufficient to reduce attack accuracy. Across all the experiments, we found some support for parallel stripes inducing underestimation of target speed but these stripes reduced attack success only when targets were small, moved at an intermediate speed, and had high internal contrast. We suggest that prey features (e.g., size or speed) are an important determinant of capture success and that distortion of speed perception by a color pattern does not necessarily translate to reduced capture success of the prey. Overall, our results support the idea that striped patterns in prey animals can reduce capture in motion but are effective under a limited set of conditions.

The key role of behaviour in animal camouflage

Animal camouflage represents one of the most important ways of preventing (or facilitating) predation. It attracted the attention of the earliest evolutionary biologists, and today remains a focus of investigation in areas ranging from evolutionary ecology, animal decision-making, optimal strategies, visual psychology, computer science, to materials science. Most work focuses on the role of animal morphology per se, and its interactions with the background in affecting detection and recognition. However, the behaviour of organisms is likely to be crucial in affecting camouflage too, through background choice, body orientation and positioning; and strategies of camouflage that require movement. A wealth of potential mechanisms may affect such behaviours, from imprinting and self-assessment to genetics, and operate at several levels (species, morph, and individual). Over many years there have been numerous studies investigating the role of behaviour in camouflage, but to date, no effort to synthesise these studies and ideas into a coherent framework. Here, we review key work on behaviour and camouflage, highlight the mechanisms involved and implications of behaviour, discuss the importance of this in a changing world, and offer suggestions for addressing the many important gaps in our understanding of this subject.

Anuran predators overcome visual illusion: dazzle coloration does not protect moving prey

Predators everywhere impose strong selection pressures on the morphology and behavior of their prey, but the resulting antipredator adaptations vary greatly among species. Studies of adaptive coloration in prey species have generally focused on cryptic or aposematic prey, with little consideration of color patterns in palatable mobile prey. Complex color patterns have been proposed to decrease the ability of visual predators to capture moving prey (motion dazzle effect). Most support for this hypothesis, however, comes from experiments with human subjects and simulated prey. We tested the motion dazzle effect using, for the first time, natural predators (cane toads, Rhinella marina) and live prey (house crickets, Acheta domesticus) with altered color patterns. We found no support for the motion dazzle effect as striped crickets did not fare better than solid colored ones. Crickets that spent more time moving, however, were more likely to be eaten. Our results suggest that motion specialized visual predators such as toads overcome the motion dazzle effect and impose stronger selection pressure on prey behavior than on coloration. These findings emphasize the importance of sensory specializations of predators in mediating antipredator strategies.

Distance-dependent defensive coloration in the poison frog Dendrobates tinctorius, Dendrobatidae

Poison dart frogs provide classic examples of warning signals: potent toxins signaled by distinctive, conspicuous coloration. We show that, counterintuitively, the bright yellow and blue-black color of Dendrobates tinctorius (Dendrobatidae) also provides camouflage. Through computational modeling of predator vision, and a screen-based detection experiment presenting frogs at different spatial resolutions, we demonstrate that at close range the frog is highly detectable, but from a distance the colors blend together, forming effective camouflage. This result was corroborated with an in situ experiment, which found survival to be background-dependent, a feature more associated with camouflage than aposematism. Our results suggest that in D. tinctorius the distribution of pattern elements, and the particular colors expressed, act as a highly salient close range aposematic signal, while simultaneously minimizing detectability to distant observers.

The current and future state of animal coloration research

Animal colour patterns are a model system for understanding evolution because they are unusually accessible for study and experimental manipulation. This is possible because their functions are readily identifiable. In this final paper of the symposium we provide a diagram of the processes affecting colour patterns and use this to summarize their functions and put the other papers in a broad context. This allows us to identify significant 'holes' in the field that only become obvious when we see the processes affecting colour patterns, and their interactions, as a whole. We make suggestions about new directions of research that will enhance our understanding of both the evolution of colour patterns and visual signalling but also illuminate how the evolution of multiple interacting traits works.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Diverted by dazzle: perceived movement direction is biased by target pattern orientation

'Motion dazzle' is the hypothesis that predators may misjudge the speed or direction of moving prey which have high-contrast patterning, such as stripes. However, there is currently little experimental evidence that such patterns cause visual illusions. Here, observers binocularly tracked a Gabor target, moving with a linear trajectory randomly chosen within 18° of the horizontal. This target then became occluded, and observers were asked to judge where they thought it would later cross a vertical line to the side. We found that internal motion of the stripes within the Gabor biased judgements as expected: Gabors with upwards internal stripe motion relative to the overall direction of motion were perceived to be crossing above Gabors with downwards internal stripe movement. However, surprisingly, we found a much stronger effect of the rigid pattern orientation. Patches with oblique stripes pointing upwards relative to the direction of motion were perceived to cross above patches with downward-pointing stripes. This effect occurred only at high speeds, suggesting that it may reflect an orientation-dependent effect in which spatial signals are used in direction judgements. These findings have implications for our understanding of motion dazzle mechanisms and how human motion and form processing interact.

Deceived by stripes: conspicuous patterning on vital anterior body parts can redirect predatory strikes to expendable posterior organs

Conspicuous coloration, which presumably makes prey more visible to predators, has intrigued researchers for long. Contrastingly coloured, conspicuous striped patterns are common among lizards and other animals, but their function is not well known. We propose and test a novel hypothesis, the 'redirection hypothesis', wherein longitudinal striped patterns, such as those found on the anterior body parts of most lacertilians, redirect attacks away from themselves during motion towards less vulnerable posterior parts, for example, the autotomous tail. In experiments employing human 'predators' attacking virtual prey on a touchscreen, we show that longitudinal striped patterns on the anterior half of prey decreased attacks to the anterior and increased attacks to the posterior. The position of stripes mattered-they worked best when they were at the anterior. By employing an adaptive psychophysical procedure, we show that prey with striped patterning are perceived to move slower, offering a mechanistic explanation for the redirective effect. In summary, our results suggest that the presence of stripes on the body (i.e. head and trunk) of lizards in combination with caudal autotomy can work as an effective anti-predator strategy during motion.

Dazzle camouflage, target tracking, and the confusion effect

The influence of coloration on the ecology and evolution of moving animals in groups is poorly understood. Animals in groups benefit from the “confusion effect,” where predator attack success is reduced with increasing group size or density. This is thought to be due to a sensory bottleneck: an increase in the difficulty of tracking one object among many. Motion dazzle camouflage has been hypothesized to disrupt accurate perception of the trajectory or speed of an object or animal. The current study investigates the suggestion that dazzle camouflage may enhance the confusion effect. Utilizing a computer game style experiment with human predators, we found that when moving in groups, targets with stripes parallel to the targets’ direction of motion interact with the confusion effect to a greater degree, and are harder to track, than those with more conventional background matching patterns. The findings represent empirical evidence that some high-contrast patterns may benefit animals in groups. The results also highlight the possibility that orientation and turning may be more relevant in the mechanisms of dazzle camouflage than previously recognized.