The effects of background coloration and dark spots on the risk of predation in poison frog models

Conspicuous stripes on prey capture attention and reduce attacks by foraging jumping spiders

Many animals avoid predation using aposematic displays that pair toxic/dangerous defences with conspicuous achromatic warning patterns, such as high-contrast stripes. To understand how these prey defences work, we need to understand the decision-making of visual predators. Here we gave two species of jumping spiders (Phidippus regius and Habronattus trimaculatus) choice tests using live termites that had their back patterns manipulated using paper capes (solid white, solid black, striped). For P. regius, black and striped termites were quicker to capture attention. Yet despite this increased attention, striped termites were attacked at lower rates than either white or black. This suggests that the termite's contrast with the background elicits attention, but the internal striped body patterning reduces attacks. Results from tests with H. trimaculatus were qualitatively similar but did not meet the threshold for statistical significance. Additional exploratory analyses suggest that attention to and aversion to stripes is at least partially innate and provide further insight into how decision-making played out during trials. Because of their rich diversity (over 6500 species) that includes variation in natural history, toxin susceptibility and degree of colour vision, jumping spiders are well suited to test broad generalizations about how and why aposematic displays work.

Pattern variation is linked to anti-predator coloration in butterfly larvae

Prey animals typically try to avoid being detected and/or advertise to would-be predators that they should be avoided. Both anti-predator strategies primarily rely on colour to succeed, but the specific patterning used is also important. While the role of patterning in camouflage is relatively clear, the design features of aposematic patterns are less well understood. Here, we use a comparative approach to investigate how pattern use varies across a phylogeny of 268 species of cryptic and aposematic butterfly larvae, which also vary in social behaviour. We find that longitudinal stripes are used more frequently by cryptic larvae, and that patterns putatively linked to crypsis are more likely to be used by solitary larvae. By contrast, aposematic larvae are more likely to use horizontal bands and spots, but we find no differences in the use of individual pattern elements between solitary and gregarious aposematic species. However, solitary aposematic larvae are more likely to display multiple pattern elements, whereas those with no pattern are more likely to be gregarious. Our study advances our understanding of how pattern variation, coloration and social behaviour covary across lepidopteran larvae, and highlights new questions about how patterning affects larval detectability and predator responses to aposematic prey.

Macroevolution of protective coloration across caterpillars reflects relationships with host plants

A critical function of animal coloration is avoiding attack, either by warning predators or reducing detectability. Evolution of these divergent strategies may depend on prey palatability and apparency to predators: conspicuous coloration may be favoured if species are distasteful, or habitats make hiding difficult; by contrast, camouflage may be effective if prey lack defences or environments are visually complex. For insect herbivores, host plants provide both chemical defence and the background against which they are detected or obscured; thus, plant traits may be key to coloration in these foundational terrestrial organisms. We use 1808 species of larval Lepidoptera to explore macroevolution of protective coloration strategy. We find that colour and pattern evolve jointly in caterpillars, similar to an array of species across the animal kingdom, while individual elements of coloration evolve closely with diet ecology. Consistent with key tenets of plant defence and plant-herbivore coevolutionary theory, conspicuous colours are associated with herbaceous host plants-thought to be defended by toxins-while camouflage colours and patterns are associated with woody plants and grasses. Contrary to theory, dietary specialization is not associated with conspicuous coloration. Our results add valuable insights into the evolutionary forces shaping colour and pattern in nature.

Colour, location and movement: what do models tell us about predation on colour morphs of a poison frog from eastern Amazonia?

Many dendrobatid frogs are known to be aposematic: brightly coloured and unpalatable to predators. To deceive predators, frog models used to test for predatory colour bias must be similar in size, colour, shape, and movement to frogs. We carried out an experiment with moving models of the species Adelphobates galactonotus, in two localities. A. galactonotus is a polytypic frog and each population of the species has a distinct colour. Birds and mammals were the vertebrates responsible for the marks on the models, but there was no difference in frequency of attacks on local-, non-local- and cryptic-colour models. Only invertebrates avoided cryptic models. Different populations of the species seem to be under different predation pressure, but colour differentiation in this species is probably related to other mechanisms, such as sexual selection.

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.

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.

Pattern contrast influences wariness in naïve predators towards aposematic patterns

An apparent and common feature of aposematic patterns is that they contain a high level of achromatic (luminance) contrast, for example, many warning signals combine black spots and stripes with a lighter colour such as yellow. However, the potential importance of achromatic contrast, as distinct from colour contrast, in reducing predation has been largely overlooked. Here, using domestic chicks as a model predator, we manipulated the degree of achromatic contrast in warning patterns to test if high luminance contrast in aposematic signals is important for deterring naïve predators. We found that the chicks were less likely to approach and eat prey with high contrast compared to low contrast patterns. These findings suggest that aposematic prey patterns with a high luminance contrast can benefit from increased survival through eliciting unlearned biases in naïve avian predators. Our work also highlights the importance of considering luminance contrast in future work investigating why aposematic patterns take the particular forms that they do.

Using three-dimensional printed models to test for aposematism in a carabid beetle

Many studies have demonstrated that bright colours sometimes evolve as warning coloration on the bodies of distasteful prey. However, few studies have demonstrated that the bright structural colours of beetles function as such aposematic signals for predators in the wild. To determine whether body colour might act as an aposematic signal in the carabid beetle Damaster blaptoides, we generated beetle models and conducted camera-trap and field experiments. Elaborate beetle models produced using a three-dimensional printer were used to determine which animals attack them in the wild. Red and black models were placed in forests to test which of the two types was attacked the least frequently. The camera-trap experiments indicated that mammals and birds were the potential predators of D. blaptoides. The field experiments revealed that predators attacked the red models significantly less frequently than the black models in each of three sites where red Damaster subspecies were distributed. In three sites where black Damaster subspecies were distributed, predators attacked both red and black models at similar rates. These results might imply that the predators learned more easily to avoid distasteful red beetles rather than black ones.

Avian learning favors colorful, not bright, signals

A few colors, such as red and yellow, are commonly found in aposematic (warning) signaling across taxa, independent of evolutionary relationships. These colors have unique traits (i.e., hue, brightness) that aid in their differentiation, and perhaps, their effectiveness in promoting avoidance learning. This repeated use calls into question the influence of selection on specific warning colors adopted by aposematic prey-predator systems. To disentangle the influence of color characteristics on this process, we trained week-old chickens (Gallus gallus domesticus) to learn to avoid distasteful food that was associated with one of three color signals (yellow, white, red) that varied in both hue and in brightness in order to assess which of these traits most influenced their ability to learn avoidance. Our results show that while chicks learned to avoid all three colors, avoidance was based on the hue, not brightness of the different signals. We found that yellow was the most effective for avoidance learning, followed by red, and finally white. Our results suggest that while these three colors are commonly used in aposematic signaling, predators' ability to learn avoidance differs among them. These results may explain why yellow is among the most common signals across aposematic taxa.