Loss of conspicuous coloration has co-evolved with decreased body size in populations of poison dart frogs

Camouflaged plants are shorter than non-camouflaged plants in the alpine zone

Camouflage has been reported as a defensive strategy in plants, while our understanding of the evolution of such defensive coloration is still limited. In the present study, we tested the hypothesis that camouflaged plants are shorter than non-camouflaged ones in the same habitat. Based on a species list from the subnival zone from the Hengduan Mountains, SW China and the herbarium collection, we measured the plant heights of 2915 individuals from 621 species (either camouflaged or not), with elevation information as a reference. We show that camouflaged plants were significantly shorter than non-camouflaged ones, though the effects of phylogeny and elevation were considered. Interestingly, a negative correlation between plant height and elevation was found in non-camouflaged plants, but not in camouflaged ones. These results revealed the correlation between defensive coloration and plant height. Camouflage may have evolved from shorter ancestors because they may suffer stronger selection and provide a more efficient defence.

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.

Warning signals promote morphological diversification in fossorial uropeltid snakes (Squamata: Uropeltidae)

Many species possess warning colourations that signal unprofitability to predators. Warning colourations are also thought to provide prey with a ‘predator-free space’ and promote niche expansion. However, how such strategies release a species from environmental constraints and facilitate niche expansion is not clearly understood. Fossoriality in reptiles imposes several morphological limits on head and body size to facilitate burrowing underground, but many fossorial snakes live close to the surface and occasionally move above ground, exposing them to predators. In such cases, evolving antipredator defences that reduce predation on the surface could potentially relax the morphological constraints associated with fossoriality and promote morphological diversification. Fossorial uropeltid snakes possess varying degrees of conspicuous warning colourations that reduce avian predation when active above ground. We predicted that species with more conspicuous colourations will exhibit more robust body forms and show faster rates of morphological evolution because constraints imposed by fossoriality are relaxed. Using a comparative phylogenetic approach on the genus Uropeltis, we show that more conspicuous species tend to have more robust morphologies and have faster rates of head-shape evolution. Overall, we find that the evolution of warning colourations in Uropeltis can facilitate niche expansion by influencing rates of morphological diversification.

Preferences for and use of light microhabitats differ among and within populations of a polytypic poison frog

Anti-predator strategies can influence trade-offs governing other activities important to fitness. Crypsis, for example, might make conspicuous sexual display especially costly, whereas aposematism might reduce or remove such costs. We tested for correlates of anti-predator strategy in Oophaga pumilio, a polytypic poison frog with morphs spanning the crypsis–aposematism continuum. In the wild, males of visually conspicuous morphs display from conspicuous perches and behave as if they perceive predation risk to be low. We thus predicted that, given a choice of ambient light microhabitats, these males would use high ambient light conditions the most and be most likely to perch in high-light conditions. We found no evidence that differently colored male O. pumilio preferentially used bright microhabitats or that ambient light influenced perching in a morph-specific manner. Independent of light conditions, males from the most conspicuous population perched the least, but the most conspicuous individuals from a polymorphic population perched the most. These patterns suggest that preferences do not necessarily underlie among-morph differences observed in the wild. This could be explained, and remain consistent with theory, if risk aversion is shaped, in part, by experience.

Mate Choice versus Mate Preference: Inferences about Color-Assortative Mating Differ between Field and Lab Assays of Poison Frog Behavior

Codivergence of mating traits and mate preferences can lead to behavioral isolation among lineages in early stages of speciation. However, mate preferences limit gene flow only when expressed as mate choice, and numerous factors might be more important than preferences in nature. In the extremely color polytypic strawberry poison frog (Oophaga pumilio), female mate preferences have codiverged with color in most allopatric populations tested. Whether these lab-assayed preferences predict mating (gene flow) in the wild remains unclear. We observed courting pairs in a natural contact zone between red and blue lineages until oviposition or courtship termination. We found color-assortative mating in a disturbed habitat with high population density but not in a secondary forest with lower density. Our results suggest color-assortative O. pumilio mate choice in the wild but also mating patterns that do not match those predicted by lab-assayed preferences.

Phylogenetic comparative analysis supports aposematic colouration-body size association in millipede assassins (Hemiptera: Reduviidae: Ectrichodiinae)

The diversity of colour patterns and its importance in interactions with the environment make colouration in animals an intriguing research focus. Aposematic colouration is positively correlated with body size in certain groups of animals, suggesting that warning colours are more effective or that crypsis is harder to achieve in larger animals. Surprisingly, this relationship has not been recovered in studies investigating insects, which may have been confounded by a focus on aposematic taxa that are also gregarious. Millipede assassin bugs (Hemiptera: Reduviidae: Ectrichodiinae) comprise species with cryptic and aposematic colour patterns across a range of body sizes, are typically solitary as adults and are thus an excellent model for investigating a possible association between colouration and body size. Here, we use a comprehensive phylogeny for Ectrichodiinae, ancestral state reconstruction of colouration, and phylogenetic comparative methods to test for a colouration-body size association. The ancestor of Ectrichodiinae is reconstructed as cryptically coloured, with multiple subsequent transitions between aposematic and cryptic colouration. Aposematic colouration is positively associated with male body length and supports the hypothesis that selection on Ectrichodiinae body size may influence evolutionary transitions between aposematic and cryptic colouration or alternatively that selection for aposematic colouration influences body size evolution.

Does spatial variation in predation pressure modulate selection for aposematism?

It is widely believed that aposematic signals should be conspicuous, but in nature, they vary from highly conspicuous to near cryptic. Current theory, including the honest signal or trade‐off hypotheses of the toxicity–conspicuousness relationship, cannot explain why adequately toxic species vary substantially in their conspicuousness. Through a study of similarly toxic Danainae (Nymphalidae) butterflies and their mimics that vary remarkably in their conspicuousness, we show that the benefits of conspicuousness vary along a gradient of predation pressure. Highly conspicuous butterflies experienced lower avian attack rates when background predation pressure was low, but attack rates increased rapidly as background predation pressure increased. Conversely, the least conspicuous butterflies experienced higher attack rates at low predation pressures, but at high predation pressures, they appeared to benefit from crypsis. Attack rates of intermediately conspicuous butterflies remained moderate and constant along the predation pressure gradient. Mimics had a similar pattern but higher attack rates than their models and mimics tended to imitate the signal of less attacked model species along the predation pressure gradient. Predation pressure modulated signal fitness provides a possible mechanism for the maintenance of variation in conspicuousness of aposematic signals, as well as the initial survival of conspicuous signals in cryptic populations in the process of aposematic signal evolution, and an alternative explanation for the evolutionary gain and loss of mimicry.

Body size affects the evolution of eyespots in caterpillars

Many caterpillars have conspicuous eye-like markings, called eyespots. Despite recent work demonstrating the efficacy of eyespots in deterring predator attack, a fundamental question remains: Given their protective benefits, why have eyespots not evolved in more caterpillars? Using a phylogenetically controlled analysis of hawkmoth caterpillars, we show that eyespots are associated with large body size. This relationship could arise because (i) large prey are innately conspicuous; (ii) large prey are more profitable, and thus face stronger selection to evolve such defenses; and/or (iii) eyespots are more effective on large-bodied prey. To evaluate these hypotheses, we exposed small and large caterpillar models with and without eyespots in a 2 × 2 factorial design to avian predators in the field. Overall, eyespots increased prey mortality, but the effect was particularly marked in small prey, and eyespots decreased mortality of large prey in some microhabitats. We then exposed artificial prey to naïve domestic chicks in a laboratory setting following a 2 × 3 design (small or large size × no, small, or large eyespots). Predators attacked small prey with eyespots more quickly, but were more wary of large caterpillars with large eyespots than those without eyespots or with small eyespots. Taken together, these data suggest that eyespots are effective deterrents only when both prey and eyespots are large, and that innate aversion toward eyespots is conditional. We conclude that the distribution of eyespots in nature likely results from selection against eyespots in small caterpillars and selection for eyespots in large caterpillars (at least in some microhabitats).

No predatory bias with respect to colour familiarity for the aposematic Adelphobates galactonotus (Anura: Dendrobatidae)

Aposematic colouration deters visually oriented predators because conspicuous signals are easier to detect and associate with unpalatability. Consequently, brightly coloured prey that are novel are predicted to be preyed on more than those with bright but typical colours. Here we evaluated whether predatory bias is associated with the colour differences observed at two different localities for a large, conspicuously coloured and poisonous Amazonian frog, Adelphobates galactonotus. At each locality predation experiments were carried out using frog models of two naturally occurring colours of the study species (blue and orange) and a control (brown). We found no evidence that novel colours were more vulnerable to predation than local colours. These results do not therefore support our hypothesis that predatory bias explains the geographic variation of colour in A. galactonotus.

Coarse dark patterning functionally constrains adaptive shifts from aposematism to crypsis in strawberry poison frogs

Ecological specialization often requires tight coevolution of several traits, which may constrain future evolutionary pathways and make species more prone to extinction. Aposematism and crypsis represent two specialized adaptations to avoid predation. We tested whether the combined effects of color and pattern on prey conspicuousness functionally constrain or facilitate shifts between these two adaptations. We combined data from 17 natural populations of strawberry poison frogs, Oophaga pumilio with an experimental approach using digitalized images of frogs and chickens as predators. We show that bright coloration often co-occurs with coarse patterning among the natural populations. Dull green frogs with coarse patterning are rare in nature but in the experiment they were as easily detected as bright red frogs suggesting that this trait combination represents a transient evolutionary state toward aposematism. Hence, a gain of either bright color or coarse patterning leads to conspicuousness, but a transition back to crypsis would be functionally constrained in populations with both bright color and coarse patterning by requiring simultaneous changes in two traits. Thus, populations (or species) signaling aposematism by conspicuous color should be less likely to face an evolutionary dead end and more likely to radiate than populations with both conspicuous color and coarse patterning.

Does relaxed predation drive phenotypic divergence among insular populations?

The evolution of striking phenotypes on islands is a well-known phenomenon, and there has been a long-standing debate on the patterns of body size evolution on islands. The ecological causes driving divergence in insular populations are, however, poorly understood. Reduced predator fauna is expected to lower escape propensity, increase body size and relax selection for crypsis in small-bodied, insular prey species. Here, we investigated whether escape behaviour, body size and dorsal coloration have diverged as predicted under predation release in spatially replicated islet and mainland populations of the lizard species Podarcis gaigeae. We show that islet lizards escape approaching observers at shorter distances and are larger than mainland lizards. Additionally, we found evidence for larger between-population variation in body size among the islet populations than mainland populations. Moreover, islet populations are significantly more divergent in dorsal coloration and match their respective habitats poorer than mainland lizards. These results strongly suggest that predation release on islets has driven population divergence in phenotypic and behavioural traits and that selective release has affected both trait means and variances. Relaxed predation pressure is therefore likely to be one of the major ecological factors driving body size divergence on these islands.