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

Eyespot peek-a-boo: Leaf rolls enhance the antipredator effect of insect eyespots

Animal colour patterns are often accompanied by specific, synergistic behaviours to most effectively defend prey against visual predators. Given the inherent context-dependence of colour perception, understanding how these colour-behaviour synergies function in a species' natural environment is crucial. For example, refuge-building species create a unique visual environment where most (or all) of the body is obscured unless closely inspected. How these built environments affect the perception of defensive colour patterns by predators is not well understood. Using artificial caterpillars that resemble a refuge-building species with conspicuous markings (Papilio troilus; Lepidoptera: Papilionidae), I tested the hypothesis that leaf rolls amplify the antipredator effect of this species' eyespots. I compared wild avian predation rates on 659 artificial swallowtail-like caterpillars from four treatment groups: eyespotted and non-eyespotted, and presented in leaf rolls or on open leaves of live host plants. In support of my hypothesis, eyespots only reduced predation for larvae in leaf rolls. On open leaves, eyespots had no antipredator effect. I also found that leaf rolls reduced predation in general for both eyespotted and non-eyespotted prey. These results highlight the importance of considering relevant behaviours in studies of animal coloration whenever possible, including behaviours that influence colour perception indirectly (e.g. through habitat use or modification).

Diel activity correlates with colour pattern morphology of heterobranch sea slugs

Visually hunting predators drive the evolution and maintenance of bold visual defences, including aposematic signalling. Such visual signals must be encountered by predators in lighting conditions where the perception of colour patterns is possible. Therefore, aposematic colouration is predicted to evolve in species encountered by visually hunting predators during daytime, with diurnal species most at risk of visual predation. However, colour patterns in diurnal and nocturnal species are highly diverse, and little is known about the relationships between prey colouration and daytime activity. To investigate correlations between daytime activity and colour pattern phenotypes across species, we quantified colour patterns in 45 species (n = 346 individuals) of eastern Australian sea slugs (Gastropoda: Heterobranchia). We used 158 descriptors of colour pattern morphology using the Quantitative Colour Pattern Analysis (QCPA) framework, which models pattern appearance using the visual system of a potential predator, a triggerfish (Rhinecanthus aculeatus). We then used phylogenetic factor analysis to identify a single factor that was strongly correlated with daytime activity, comprising 55 individual colour pattern descriptors. This dimension of pattern variation identified daytime activity in 87% of species in our dataset. We found that daytime activity in heterobranch sea slugs was associated with a distinct, highly descriptive set of spatiochromatic image statistics. Specifically, diurnal species were more boldly patterned than nocturnal species, exhibiting increased colour, luminance and pattern contrast, indicating the presence of visual signalling and, thus, visual predation as a key driver of heterobranch sea slug phenotype. Our study sheds light on the ecology and evolution of defensive colouration in sea slugs and provides a novel and robust workflow for comparative phylogenetic studies considering colour pattern spaces described by high-dimensional datasets, which can be applied to other species and ecosystems.

The Dangers of Growing Old: Adult Moths Face Higher Predation Pressures than Caterpillars in Hyles lineata

Holometabolous insects display drastically different morphologies across life stages (i.e., larvae vs. adults). Morphological differences across life stages, such as different sizes and coloration, likely result in differential survival, as predators may find individuals of one life stage more conspicuous and/or more energetically profitable than another. Furthermore, prey conspicuousness may vary temporally because both the sensory environment and predator sensory abilities differ between day and night. Here, we investigated how the interaction between life stage (caterpillar vs. moth) and time of day (day vs. night) influences predation of the white-lined sphinx (Lepidoptera: Hyles lineata). We predicted that caterpillars would be less susceptible to predation than adult moths, as adults are larger and have a more conspicuous shape. After quantifying predation for 72 h during dawn and dusk using 199 plasticine replicas each of adults and caterpillars, predation on adult replicas was twice that of predation on caterpillar replicas. Furthermore, replicas were six times more likely to be predated on during the day than during the night. Lastly, attacks were made mainly by birds, which carried out 86% of the attacks on adult models and 85% of those on caterpillar models. These data support the hypothesis that predation rates differ across life stages in holometabolous insects. This research lays a foundation for further investigation into how specific differences in morphology across life stages affect predation and survival in holometabolous insects.

Evolutionary Drivers of Conspicuous Spots in Velvet Ants (Hymenoptera: Dasymutilla)

Predation plays an important role in animal evolution by selecting for antipredator adaptations. Antipredator color adaptations include conspicuous spots, which are believed to provide protection by deflecting attacks to harmful or peripheral body parts, deimatic signaling, or as conspicuous warning coloration. The utility of antipredator signals is context-dependent and may be influenced by the environment. In this study, we investigated the selective forces acting on the evolution of conspicuous spots on velvet ants (Mutillidae: Dasymutilla). We tested whether conspicuous spots in 80 species of velvet ants evolved in (i) forest-dwelling species, (ii) habitat-generalist species, or (iii) species predated by diverse birds and frogs. Results show that conspicuous spots are more likely to evolve in forest-dwelling species and in areas with more canopy cover, whereas species inhabiting open areas and deserts tend to lose them. Moreover, taxa with conspicuous spots transition between open and forested habitats less often. Spot presence was not associated with predator diversity. We suggest that spots in velvet ants require complex visual environments to be effective, which may limit their habitat occurrence. In simpler environments, carrying conspicuous spots could be costly due to increased exposure to visual predators.

Backgrounds and the evolution of visual signals

Color signals which mediate behavioral interactions across taxa and contexts are often thought of as color 'patches' - parts of an animal that appear colorful compared to other parts of that animal. Color patches, however, cannot be considered in isolation because how a color is perceived depends on its visual background. This is of special relevance to the function and evolution of signals because backgrounds give rise to a fundamental tradeoff between color signal detectability and discriminability: as its contrast with the background increases, a color patch becomes more detectable, but discriminating variation in that color becomes more difficult. Thus, the signal function of color patches can only be fully understood by considering patch and background together as an integrated whole.

Predator selection on multicomponent warning signals in an aposematic moth

Aposematic prey advertise their unprofitability with conspicuous warning signals that are often composed of multiple color patterns. Many species show intraspecific variation in these patterns even though selection is expected to favor invariable warning signals that enhance predator learning. However, if predators acquire avoidance to specific signal components, this might relax selection on other aposematic traits and explain variability. Here, we investigated this idea in the aposematic moth Amata nigriceps that has conspicuous black and orange coloration. The size of the orange spots in the wings is highly variable between individuals, whereas the number and width of orange abdominal stripes remains consistent. We produced artificial moths that varied in the proportion of orange in the wings or the presence of abdominal stripes. We presented these to a natural avian predator, the noisy miner (Manorina melanocephala), and recorded how different warning signal components influenced their attack decisions. When moth models had orange stripes on the abdomen, birds did not discriminate between different wing signals. However, when the stripes on the abdomen were removed, birds chose the model with smaller wing spots. In addition, we found that birds were more likely to attack moths with a smaller number of abdominal stripes. Together, our results suggest that bird predators primarily pay attention to the abdominal stripes of A. nigriceps, and this could relax selection on wing coloration. Our study highlights the importance of considering individual warning signal components if we are to understand how predation shapes selection on prey warning coloration.

Heterozygote advantage and pleiotropy contribute to intraspecific color trait variability

The persistence of intrapopulation phenotypic variation typically requires some form of balancing selection because drift and directional selection eventually erode genetic variation. Heterozygote advantage remains a classic explanation for the maintenance of genetic variation in the face of selection. However, examples of heterozygote advantage, other than those associated with disease resistance, are rather uncommon. Across most of its distribution, males of the aposematic moth Arctia plantaginis have two hindwing phenotypes determined by a heritable one locus-two allele polymorphism (genotypes: WW/Wy = white morph, yy = yellow morph). Using genotyped moths, we show that the presence of one or two copies of the yellow allele affects several life-history traits. Reproductive output of both males and females and female mating success are negatively affected by two copies of the yellow allele. Females carrying one yellow allele (i.e., Wy) have higher fertility, hatching success, and offspring survival than either homozygote, thus leading to strong heterozygote advantage. Our results indicate strong female contribution especially at the postcopulatory stage in maintaining the color polymorphism. The interplay between heterozygote advantage, yellow allele pleiotropic effect, and morph-specific predation pressure may exert balancing selection on the color locus, suggesting that color polymorphism may be maintained through complex interactions between natural and sexual selection.

Genetic colour variation visible for predators and conspecifics is concealed from humans in a polymorphic moth

The definition of colour polymorphism is intuitive: genetic variants express discretely coloured phenotypes. This classification is, however, elusive as humans form subjective categories or ignore differences that cannot be seen by human eyes. We demonstrate an example of a ‘cryptic morph’ in a polymorphic wood tiger moth (Arctia plantaginis), a phenomenon that may be common among well‐studied species. We used pedigree data from nearly 20,000 individuals to infer the inheritance of hindwing colouration. The evidence supports a single Mendelian locus with two alleles in males: WW and Wy produce the white and yy the yellow hindwing colour. The inheritance could not be resolved in females as their hindwing colour varies continuously with no clear link with male genotypes. Next, we investigated if the male genotype can be predicted from their phenotype by machine learning algorithms and by human observers. Linear discriminant analysis grouped male genotypes with 97% accuracy, whereas humans could only group the yy genotype. Using vision modelling, we also tested whether the genotypes have differential discriminability to humans, moth conspecifics and their bird predators. The human perception was poor separating the genotypes, but avian and moth vision models with ultraviolet sensitivity could separate white WW and Wy males. We emphasize the importance of objective methodology when studying colour polymorphism. Our findings indicate that by‐eye categorization methods may be problematic, because humans fail to see differences that can be visible for relevant receivers. Ultimately, receivers equipped with different perception than ours may impose selection to morphs hidden from human sight.

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.