DIET QUALITY AFFECTS WARNING COLORATION INDIRECTLY: EXCRETION COSTS IN A GENERALIST HERBIVORE

Aposematic herbivores are under selection pressure from their host plants and predators. Although many aposematic herbivores exploit plant toxins in their own secondary defense, dealing with these harmful compounds might underlay costs. We studied whether the allocation of energy to detoxification and/or sequestration of host plant defense chemicals trades off with warning signal expression. We used a generalist aposematic herbivore Parasemia plantaginis (Arctiidae), whose adults and larvae show extensive phenotypic and genetic variation in coloration. We reared larvae from selection lines for small and large larval warning signals on Plantago lanceolata with either low or high concentration of iridoid glycosides (IGs). Larvae disposed of IGs effectively; their body IG content was low irrespective of their diet. Detoxification was costly as individuals reared on the high IG diet produced fewer offspring. The IG concentration of the diet did not affect larval coloration (no trade-off) but the wings of females were lighter orange (vs. dark red) when reared on the high IG diet. Thus, the difference in plant secondary chemicals did not induce variation in the chemical defense efficacy of aposematic individuals but caused variation in reproductive output and warning signals of females.     

Inversely related aposematic traits: reduced conspicuousness evolves with increased toxicity in a polymorphic poison-dart frog

Prevailing theory contends that aposematic coloration evolves in tandem with toxicity so that the evolution of increased toxicity will accompany the evolution of greater conspicuousness. Although variation in aposematic coloration within single species should be selectively constrained, because individuals varying from a predator-recognized warning signal will incur greater risk of predation, several species of poison-dart frogs display remarkable phenotypic variation. This variation may have evolved to match different levels of toxicity, and these species provide excellent opportunities to examine the evolution of aposematic coloration. Here, I test whether increased conspicuousness in the granular poison-dart frog evolved in tandem with increased toxicity. Contrary to classical predictions, toxicity assays, spectral reflectance measurements, and phylogenetic reconstruction reveal that the less conspicuous color morphs are actually significantly more toxic than the brightest, most conspicuous phenotypes and that the more toxic, less-conspicuous form evolved from a less toxic, more conspicuous ancestor. Through gas chromatography--mass spectrometry analysis of toxin profiles, I traced the increase in toxicity in the less-conspicuous populations to an acquisition of specific alkaloids, some of which are proven convulsants. These results challenge the tenet that increased conspicuousness always evolves with increased toxicity and support the idea that once aposematism has been established in a species, phenotypic variation may evolve from brightness and toxicity becoming decoupled.     

A parallel geographical mosaic of morphological and behavioural aposematic traits of the newt, Cynops pyrrhogaster (Urodela: Salamandridae)

Aposematic animals advertise their unprofitability to potential predators with conspicuous coloration, occasionally in combination with other life-history traits. Theory posits that selection on functionally interrelated aposematic characters promotes the unidirectional evolution of these characters, resulting in an increase or decrease in the effectiveness of the signal. To test whether this prediction applies on a microevolutionary scale, the intra- and interpopulational variations in aposematic coloration, behaviour (which enhances the effectiveness of the coloration) and body size of newts, Cynops pyrrhogaster (Urodela: Salamandridae), were investigated. A parallel geographical mosaic of variation in aposematic coloration and behaviour among populations, independent of body size, was found. Newts on islands displayed more conspicuous aposematic traits than those on the mainland, both morphologically and behaviourally. There was no significant relationship between variation in coloration and behaviour within populations. Male newts displayed more conspicuous coloration than females. Surveys of potential predators suggest that variable natural selection at a local scale, such as predation pressure, may primarily be responsible for the microevolution of variable aposematic traits in newts.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 613–622.     

Aposematism and gregariousness: the combined effect of group size and coloration on signal repellence

Many aposematic species have evolved an aggregated lifestyle, and one possible advantage of grouping in warningly coloured prey is that it makes the aposematic signal more effective by generating a greater aversion in predators. Here we investigate the effect of prey group size on predator behaviour, both when prey are aposematic and when they are not aposematic, to separate the effects of warning coloration and prey novelty. Naive domestic chicks (Gallus gallus domesticus) were presented with either solitary or groups of 3, 9 or 27 live larvae of the aposematic bug Tropidothorax leucopterus. Other naive chicks were presented with larvae of the non-aposematic bug Graptostethus servus either solitary or in groups of 27. Attack probability decreased with increasing group size of aposematic prey, both when birds were naive and when they had prior experience, whereas prey gregariousness did not affect the initial attack probability on the G. servus larvae. In a separate experiment, groups of mealworms were shown to be even more attractive than solitary mealworms to naive chicks. We conclude that the aversiveness of prey grouping in this study can be explained as increased signal repellence of specific prey coloration, in this case a classical warning coloration. These experiments thus support the idea of gregariousness increasing the signalling effect of warning coloration.     

Warning signal plasticity in hibiscus harlequin bugs

Color variation in aposematic (conspicuous and defended) prey should be suppressed by frequency-based selection by predators. However selection of color traits is confounded by the fact that coloration also plays an important role in many biological processes, and warning coloration may be constrained by biotic or abotic factors. Temperature, in particular the importance of thermoregulation, has been suggested as the source of much of the geographical variation in warning coloration we see in natural populations. Differential selection in different thermal environments may lead to developmentally canalized or ‘fixed’ differences between populations. Conversely, inter-population differences may be due to phenotypic plasticity, wherein trait expression is modified by environmental conditions. The hibiscus harlequin bug Tectocoris diophthalmus (Heteroptera: Scutelleridae), is a shieldback bug, with iridescent patches that show size variation between individuals, as well as inter-population variation with geographic patterning. This study aimed to identify environmental factors that drive the expression of this variable trait, using surveys, modeling, and experimental approaches. Surveys were taken at sites throughout Australia in three climate regions (tropical, subtropical, and temperate) at different time periods, and results were modeled with a multilevel ordinal regression. We tested for correlations between colouration and several biotic (density, host plant) and abiotic (temperature, rainfall) factors. We found strong phenotypic plasticity with respect to temperature and rainfall. Higher temperatures and increased rainfall were related to suppressed iridescence. A factorial experiment with tropical and temperate bugs in two climate-typical temperature regimes confirmed phenotypic plasticity in response to temperature, likely due to temperature sensitivity in melanin expression. Tropical and temperate populations showed striking differences between plasticity reaction norms, suggesting local evolution on the shape of phenotypic plasticity. We suggest that studying both biotic and abiotic selection pressures is important for understanding the causes of inter-population variation in aposematic signals.     

Aposematic coloration, luminance contrast, and the benefits of conspicuousness

Many organisms use warning, or aposematic, coloration to signaltheir unprofitability to potential predators. Aposematicallycolored prey are highly visually conspicuous. There is considerableempirical support that conspicuousness promotes the effectivenessof the aposematic signal. From these experiments, it is welldocumented that conspicuous, unprofitable prey are detectedsooner and aversion learned faster by the predator as comparedwith cryptic, unprofitable prey. Predators also retain memoryof the aversion longer when prey is conspicuous. The presentstudy focused on the elements of conspicuousness that conferthese benefits of aposematic coloration. Drawing on currentunderstanding of animal vision, we distinguish 2 features ofwarning coloration: high chromatic contrast and high brightness,or luminance, contrast. Previous investigations on aposematicsignal efficacy have focused mainly on the role of high chromaticcontrast between prey and background, whereas little researchhas investigated the role of high luminance contrast. Usingthe Chinese mantid as a model predator and gray-painted milkweedbugs as model prey, we found that increased prey luminance contrastincreased detection of prey, facilitated predator aversion learning,and increased predator memory retention of the aversive response.Our results suggest that the luminance contrast component ofaposematic coloration can be an effective warning signal betweenthe prey and predator. Thus, warning coloration can even evolveas an effective signal to color blind predators.     

Brighter-colored paper wasps (Polistes dominula) have larger poison glands

ABSTRACT: INTRODUCTION: Aposematism is a defense system against predators consisting of the toxicity warning using conspicuous coloration. If the toxin production and aposematic coloration is costly, only individuals in good physical condition can simultaneously produce abundant poison and striking coloration. In such cases, the aposematic coloration not only indicates that the animal is toxic, but also the toxicity level of individuals. The costs associated to the production of aposematic coloration would ensure that individuals indicate honestly their toxicity levels. In the present study, we examine the hypothesis that a positive correlation exists between the brightness of warning coloration and toxicity level using as a model the paper wasp (Polistes dominula). RESULTS: We collected wasps from 30 different nests and photographed them to measure the brightness of warning coloration in the abdomen. We also measured the volume of the poison gland, as well as the length, and the width of the abdomen. The results show a positive relationship between brightness and poison-gland size, which remained positive even after controlling the body size and abdomen width. CONCLUSION: The results suggest that the coloration pattern of these wasps is a true sign of toxicity level: wasps with brighter colors are more poisonous (they have larger poison glands).     

A role for phenotypic plasticity in the evolution of aposematism

The evolution of warning coloration (aposematism) has been difficult to explain because rare conspicuous mutants should suffer a higher cost of discovery by predators relative to the cryptic majority, while at frequencies too low to facilitate predator aversion learning. Traditional models for the evolution of aposematism have assumed conspicuous prey phenotypes to be genetically determined and constitutive. By contrast, we have recently come to understand that warning coloration can be environmentally determined and mediated by local prey density, thereby reducing the initial costs of conspicuousness. The expression of density-dependent colour polyphenism is widespread among the insects and may provide an alternative pathway for the evolution of constitutive aposematic phenotypes in unpalatable prey by providing a protected intermediate stage. If density-dependent aposematism can function as an adaptive intermediate stage for the evolution of constitutive aposematic phenotypes, differential reaction norm evolution is predicted among related palatable and unpalatable prey populations. Here, I present empirical evidence that indicates that (i) the expression of density-dependent colour polyphenism has differentially evolved between palatable and unpalatable populations of the grasshopper Schistocerca emarginata (= lineata) (Orthoptera: Acrididae), and (ii) variation in plasticity between these populations is commensurate with the expected costs of conspicuousness.     

No evidence for differential survival or predation between sympatric color morphs of an aposematic poison frog

Because variation in warning signals slows down the predator education process, aposematic theory predicts that animal warning signals should be monomorphic. Yet, warning color polytypisms are not uncommon in aposematic species. In cases where warning signal variants are separated geographically, adaptation to local predators could explain this variation. However, this cannot explain the persistence of sympatric polymorphisms in aposematic taxa. The strawberry poison frog (Oophaga pumilio) exhibits both allopatric and sympatric warning color variation in and around the Bocas del Toro archipelago of Panama. One explanation that has been proposed for the rapid diversification of O. pumilio coloration in this archipelago is low predation; if island populations have few predators, stabilizing selection would be relaxed opening the door for diversification via selection or genetic drift. Using a combination of mark-recapture and clay model studies, we tested for differences in survival and predation among sympatric red and yellow color morphs of O. pumilio from Bastimentos Island. We found no evidence for differential survival or predation in this population, despite the fact that one morph (red) is more common and widely distributed than the other (yellow). Even in an area of the island where the yellow morph is not found, predator attack rates were similar among morphs. Visual modeling suggests that yellow and red morphs are distinguishable and conspicuous against a variety of backgrounds and by viewers with different visual systems. Our results suggest that general avoidance by predators of red and yellow, both of which are typical warning colors used throughout the animal kingdom, may be contributing to the apparent stability of this polymorphism.     

Experienced chicks show biased avoidance of stronger signals: an experiment with natural colour variation in live aposematic prey

An important factor for understanding the evolution of warning coloration in unprofitable prey is the synergistic effect produced by predator generalisation behaviour. Warning coloration can arise and become stabilised in a population of solitary prey if more conspicuous prey benefit from a predator's previous interaction with less conspicuous prey. This study investigates whether domestic chicks (Gallus gallus domesticus) show a biased generalisation among live aposematic prey by using larvae of three species of seed bugs (Heteroptera: Lygaeidae) that are of similar shape but vary in the amount of red in the coloration. After positive experience of edible brownish prey, chicks in two reciprocal experiments received negative experience of either a slightly red or a more red distasteful larva. Attacking birds were then divided into two treatment groups, – one presented with the same prey again, and one presented with either a less red or a more red larva. Birds with only experience of edible prey showed no difference in attack probability of the two aposematic prey types. Birds with experience of the less red prey biased their avoidance so that prey with a more red coloration was avoided to a higher degree, whereas birds with experience of the more red prey avoided prey with the same, but not less red coloration. Thus, we conclude that bird predators may indeed show a biased generalisation behaviour that could select for and stabilise an aposematic strategy in solitary prey. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carotenoid-based colour polyphenism in a moth species: search for fitness correlates

Carotenoid‐based integumental coloration is often associated with individual performance in various animals. This is because the limited amount of the pigment has to be allocated to different vital functions. However, most of the evidence for the carotenoid‐based trade‐off comes from vertebrate studies, and it is unclear if this principle can be applied to insects. This possibility was investigated in Orgyia antiqua L. (Lepidoptera: Lymantriidae). The larvae of this species are polyphenic in their coloration, varying from a highly conspicuous combination of yellow hair tufts on black background to cryptic appearance with brown hair tufts. The conspicuous larvae are aposematic, advertising their aversive hairiness. The maintenance of different colour morphs in O. antiqua requires explanation, as an aposematic signal is expected to evolve towards monomorphism. Chromatographic analysis showed that the yellow coloration of the hair is based on the carotenoid pigment lutein (α‐carotene‐3,3’‐diol). The colour of hair tufts was dependent on their carotenoid content. This justifies an expectation of carotenoid‐based physiological trade‐offs between aposematic coloration and individual performance. To test this hypothesis, we monitored life histories of differently coloured larvae reared on various host plants, recording their body sizes, growth rates, and mortalities in each instar. There was a significant but relatively low heritability of tuft coloration, which allowed us to expect environmental effects. We found no phenotypic associations between hair tuft colour and performance indices in O. antiqua larvae, neither did the quality of host plant affect the frequency of colour morphs. However, the frequency of colour morphs differed between larval instars. Our results suggest that carotenoid‐mediated physiological trade‐offs are not involved in the maintenance of colour morphs in O. antiqua larvae, and factors other than individual condition should be responsible for the observed variability.     

Ontogenetic colour change and the evolution of aposematism: a case study in panic moth caterpillars

1. Aposematism is a widely used antipredator strategy in which an organism possesses both warning coloration and unprofitable characters. Theoretical evidence suggests that aposematic colour should develop when high opportunity costs imposed by crypsis force an organism to engage in conspicuous behaviours. Hence, it is expected that ontogenetic colour change (OCC) in larval insects should include aposematism when foraging needs compel behavioural modifications that preclude a continued state of crypsis. 2. To test this idea, I first investigated whether OCC in caterpillars of the panic moth Saucrobotys futilalis was indicative of a switch from cryptic to aposematic coloration. I then examined the context of panic moth OCC as it related to foraging patterns and behavioural conspicuousness. 3. Early Saucrobotys instars are a cryptic green, but later instars become progressively more orange and develop black spots. Early instar larvae forage cryptically on the inner parenchyma of silked-together host plant leaves to avoid predation, but are rapidly forced to engage in conspicuous foraging behaviours as they outgrow the resources afforded by their shelters. Both coloration and behaviour reach maximal conspicuousness in final instar larvae. 4. As predicted, OCC encompassed a change from crypsis to aposematism in Saucrobotys. Aposematic function was demonstrated by changes in both antipredator behaviour patterns and effectiveness of predator deterrence in early and late instars. Moreover, increased opportunity costs of crypsis and behavioural conspicuousness coincided with the onset of aposematic coloration. 5. This pattern of OCC suggests that aposematic coloration in Saucrobotys develops as a response to constraints imposed by crypsis. Moreover, my study illustrates the importance of the study of ontogenetic patterns in determining how behaviour, morphology, and predator responses interact to influence the initial evolution of phenomena such as aposematism.     

Honest signaling and the uses of prey coloration

Abstract Although signal reliability is of fundamental importance to the understanding of animal communication, the extent of signal honesty in relation to antipredator warning signals has received relatively little attention. A recent theoretical model that assumed a physiological linkage between pigmentation and toxicity suggested that (aposematic) warning signals may often be reliable, in the sense that brightness and toxicity are positively correlated within prey populations. Two shortcomings of the model were (1) the requirement among predators for an innate aversion to brightly colored prey and (2) the assumption that prey can generate only bright coloration and not cryptic coloration. We evaluated the generality of predictions of reliable signaling when these shortcomings were removed. Without innate avoidance of bright prey, we found a positive brightness-toxin correlation when conspicuous prey coloration provided an additional fitness benefit unrelated to predation. Initially, this correlation could evolve for reasons unrelated to prey signaling; hence, aposematism might represent a striking example of exaptation. Given a choice between using pigmentation for bright or for cryptic coloration, crypsis was favored only in conditions of very low or very high resource levels. In the latter case, toxicity correlated positively with degree of cryptic coloration. Predictions of toxin-signal correlation appear robust, but we can identify interesting conditions in which signal reliability is not predicted.     

No evidence of quantitative signal honesty across species of aposematic burnet moths (Lepidoptera: Zygaenidae)

Many defended species use conspicuous visual warning signals to deter potential predators from attacking. Traditional theory holds that these signals should converge on similar forms, yet variation in visual traits and the levels of defensive chemicals is common, both within and between species. It is currently unclear how the strength of signals and potency of defences might be related: conflicting theories suggest that aposematic signals should be quantitatively honest, or, in contrast, that investment in one component should be prioritized over the other, while empirical tests have yielded contrasting results. Here, we advance this debate by examining the relationship between defensive chemicals and signal properties in a family of aposematic Lepidoptera, accounting for phylogenetic relationships and quantifying coloration from the perspective of relevant predators. We test for correlations between toxin levels and measures of wing colour across 14 species of day‐flying burnet and forester moths (Lepidoptera: Zygaenidae), protected by highly aversive cyanogenic glucosides, and find no clear evidence of quantitative signal honesty. Significant relationships between toxin levels and coloration vary between sexes and sampling years, and several trends run contrary to expectations for signal honesty. Although toxin concentration is positively correlated with increasing luminance contrast in forewing pattern in 1 year, higher toxin levels are also associated with paler and less chromatically salient markings, at least in females, in another year. Our study also serves to highlight important factors, including sex‐specific trends and seasonal variation, that should be accounted for in future work on signal honesty in aposematic species.     

Warning signal properties covary with toxicity but not testosterone or aggregate carotenoids in a poison frog

Aposematic (warning) coloration is a highly conspicuous trait that is found throughout the animal kingdom. In several aposematic species, warning signals have been co-opted for use in conspecific communication systems; for example, in the toxic and bright orange Solarte population of the strawberry poison frog (Oophaga [Dendrobates] pumilio), the brightness of male warning coloration serves as a sexual signal by both attracting females and repelling rivals. Here, we investigate correlations between bright male warning coloration and several physiological characteristics (e.g., circulating testosterone and carotenoids and noxious alkaloids in the skin), to gain insights into the mechanisms underlying the signal variation in this population and to inform hypotheses regarding the evolutionary stability of this trait. We find that although measures of male brightness (viewer-dependent or viewer-independent) do not correlate with two classic correlates of sexually selected traits (circulating testosterone and aggregate carotenoids in the skin), male reflectance does show a positive correlation with concentrations of two xanthophyll carotenoids. Total reflectance (a viewer-independent measure of male brightness) also shows a negative relationship with aggregate pumiliotoxin in the skin, which is considered one of the major classes of defensive alkaloids in this species. Because the alkaloids used in this species’ chemical defense are acquired from dietary sources, the magnitude of the reflectance intensity of a male’s warning signal can potentially provide viewers with reliable information regarding territory quality, health, and/or current condition.     

The effects of predator learning, forgetting, and recognition errors on the evolution of warning coloration

This paper demonstrates that the specifics of predator avoidance learning, information loss, and recognition errors may heavily influence the evolution of aposematism. I establish a mathematical model of the change in frequency over time of bright individuals of a distasteful prey species. Warning color spreads through green beard selection as reformulated by Guilford (1990); bright colored forms gain an advantage due to their phenotypic resemblance to other bright forms, which have been sampled by the predator. I use a general classical conditioning model to examine gradual predator learning and forgetting, and then consider the extreme of one-trial learning and no forgetting over time that may occur with very toxic prey. The advantage of conspicuous coloration under these latter conditions depends upon its role in lowering a constant probability of the prey being misidentified and thus mistakenly attacked by a predator, a rarely emphasized factor in the evolution of warning coloration. This constant probability of mistaken attacks can also be interpreted as a constant probability that forgetting has occurred (forgetting does not increase with time) or a periodic decision by the predator to resample avoided prey. I show that when predators learn and forget gradually, as under the general classical conditioning model, it is very difficult for aposematic coloration to become established unless bright individuals cross an often high threshold frequency through chance factors. In contrast, the conditions expected with highly toxic prey promote the evolution of warning coloration more easily, by means from the fixation of very bright mutations to the fixation of successive mutations each of which causes a small increase in a prey's conspicuousness. The results therefore predict that aposematic coloration may have evolved in a different manner in different predator and prey systems. They also suggest that it may be extremely difficult for warning coloration to evolve in more mildly toxic or distasteful prey outside of a mimicry system.     

Plant biological warfare: thorns inject pathogenic bacteria into herbivores

Thorns, spines and prickles are among the rich arsenal of antiherbivore defence mechanisms that plants have evolved. Many of these thorns are aposematic, that is, marked by various types of warning coloration. This coloration was recently proposed to deter large herbivores. Yet, the mechanical defence provided by thorns against large herbivores might be only the tip of the iceberg in a much more complicated story. Here we present evidence that thorns harbour an array of pathogenic bacteria that are much more dangerous to herbivores than the painful mechanical wounding by the thorns. Pathogenic bacteria like Clostridium perfringens, the causative agent of the life-threatening gas gangrene, and others, were isolated and identified from date palm (with green-yellow-black aposematic spines) and common hawthorn (with red aposematic thorns). These thorn-inhabiting bacteria have a considerable potential role in antiherbivory, and may have uniquely contributed to the common evolution of aposematism (warning coloration) in thorny plants.     

EVOLUTION OF GREGARIOUSNESS IN APOSEMATIC BUTTERFLY LARVAE: A PHYLOGENETIC ANALYSIS

Gregariousness ought to be disadvantageous for palatable organisms that live exposed and are relatively immobile and small in comparison to potential predators. Therefore, the idea that unpalatability generally evolves before egg clustering/larval gregariousness in butterflies was tested. Aposematic coloration in the larva was used as the criterion of unpalatability (it is argued that Batesian mimicry is rare in butterfly larvae), and the relative order of evolution of aposematism and gregariousness was inferred through phylogenetic analysis. Here, existing phylogenies were used, and the analysis was based on an assumption of a minimum number of evolutionary changes (parsimony). A total of 23 cases of independent evolution of gregariousness and 12 cases of independent evolution of aposematic coloration were found. In five cases, gregariousness evolved in cryptic species, the palatability of which is unknown. For lineages in which both unpalatability, as evidenced by aposematic coloration, and gregariousness were found and the two evolutionary events could be separated, unpalatability always preceded gregariousness: five cases of independent evolution of warning coloration were followed by a total of 15 cases of independent evolution of gregariousness. In no lineage did gregariousness evolve before warning coloration. It is thus concluded that unpalatability is an important predisposing factor for the evolution of egg clustering and larval gregariousness in butterflies. Insofar as kin selection is related to larval gregariousness, this study indicates that kin selection is of minor importance for the evolution of both unpalatability and warning coloration.     

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

Larger signal size is known to facilitate the learning process of predators to warning signals. Further, smaller objects are generally harder to detect than large, which suggests that smaller sized prey are less likely to benefit from an aposematic strategy compared to crypsis. However, whether body size changes in concert with shifts between crypsis and aposematism in natural populations, remains largely unexplored. I tested whether body size was larger in visually conspicuous population than in cryptic populations among recently diverged populations of the Strawberry Poison frog, Oophaga pumilio. By analysing spectral reflectance and body size data from individuals from 18 discrete populations I found a larger mean body size in conspicuous populations, which was confirmed by an analysis of a subset of 12 populations accounting for phylogenetic history. This shows that the loss of conspicuous colour likely co-evolved repeatedly with a decrease in body size. Thus, selection on body size may influence evolutionary shifts between aposematism and crypsis and vice versa.