Theoretical investigations of automimicry: multiple trial learning and the palatability spectrum

Umbrella of protection: spatial and temporal dynamics in a temperate butterfly Batesian mimicry system

Batesian mimicry involves both spatial and temporal interactions between model, mimic and predator. Fundamental predictions in Batesian mimicry involve space, time and abundance; specifically, that the model and mimic are found in sympatry and that protection for the mimic is increased when predators interact with the model first and more frequently. Research has generally confirmed these predictions for Batesian mimicry at large spatial scales, with recent work on two nymphalid butterflies in western North America, the mimic Limenitis lorquini (Boisduval, 1852) and its model Adelpha californica (Butler, 1865) in western North America indicating that the mimic generally has lower abundance and emerges later in the season among widely separated populations in the California Coast Ranges and Sierra Nevada. However, no studies have investigated model–mimic dynamics at small scales in the temperate zone to test whether temporal habitat use and movements conform to predictions. If mimicry is as important a part of the biology of these temperate species as it is for their tropical counterparts, then in addition to emerging later and being less abundant overall, the mimic should be less widespread, should be less abundant in each habitat and should move less among available habitats. Our results using mark–release–recapture methods confirm these predictions and indicate that the mimic, L. lorquini, is enjoying an umbrella of protection against habitat specialist and generalist predators alike.

From cues to signals: evolution of interspecific communication via aposematism and mimicry in a predator-prey system

Current theory suggests that many signaling systems evolved from preexisting cues. In aposematic systems, prey warning signals benefit both predator and prey. When the signal is highly beneficial, a third species often evolves to mimic the toxic species, exploiting the signaling system for its own protection. We investigated the evolutionary dynamics of predator cue utilization and prey signaling in a digital predator-prey system in which prey could evolve to alter their appearance to mimic poison-free or poisonous prey. In predators, we observed rapid evolution of cue recognition (i.e. active behavioral responses) when presented with sufficiently poisonous prey. In addition, active signaling (i.e. mimicry) evolved in prey under all conditions that led to cue utilization. Thus we show that despite imperfect and dishonest signaling, given a high cost of consuming poisonous prey, complex systems of interspecific communication can evolve via predator cue recognition and prey signal manipulation. This provides evidence supporting hypotheses that cues may serve as stepping-stones in the evolution of more advanced communication and signaling systems that incorporate information about the environment.

Why are defensive toxins so variable? An evolutionary perspective

Defensive toxins are widely used by animals, plants and micro-organisms to deter natural enemies. An important characteristic of such defences is diversity both in the quantity of toxins and the profile of specific defensive chemicals present. Here we evaluate evolutionary and ecological explanations for the persistence of toxin diversity within prey populations, drawing together a range of explanations from the literature, and adding new hypotheses. We consider toxin diversity in three ways: (1) the absence of toxicity in a proportion of individuals in an otherwise toxic prey population (automimicry); (2) broad variation in quantities of toxin within individuals in the same population; (3) variation in the chemical constituents of chemical defence. For each of these phenomena we identify alternative evolutionary explanations for the persistence of variation. One important general explanation is diversifying (frequency- or density-dependent) selection in which either costs of toxicity increase or their benefits decrease with increases in the absolute or relative abundance of toxicity in a prey population. A second major class of explanation is that variation in toxicity profiles is itself nonadaptive. One application of this explanation requires that predator behaviour is not affected by variation in levels or profiles of chemical defence within a prey population, and that there are no cost differences between different quantities or forms of toxins found within a population. Finally, the ecology and life history of the animal may enable some general predictions about toxin variation. For example, in animals which only gain their toxins in their immature forms (e.g. caterpillars on host plants) we may expect a decline in toxicity during adult life (or at least no change). By contrast, when toxins are also acquired during the adult form, we may for example expect the converse, in which young adults have less time to acquire toxicity than older adults. One major conclusion that we draw is that there are good reasons to consider within-species variation in defensive toxins as more than mere ecological noise. Rather there are a number of compelling evolutionary hypotheses which can explain and predict variation in prey toxicity.

A MARKOV CHAIN MODEL OF PREDATOR-MODEL-MIMIC INTERACTIONS

The population dynamics for predator and prey environments have been studied extensively, and several major mathematical models have been introduced to quantify this. The situation becomes more complex, however, when the prey incorporates preservation strategies for survival. One of the most interesting approaches here is the use of mimicry of prey which are unacceptable to the predator, to avoid being consumed. Here we develop a Markov chain model of interactions between a predator and a prey population comprising unpalatable models, general mimics and specific mimics. This incorporates a simple stochastic procedure for the predator, enabling modifiable behavior to be modeled. We calculate equilibrium consumption probabilities and introduce a fitness measure for each type of prey. Finally, by taking into account the population size of each type of prey, we extend the previously reported notion of a predator benefit function for this more complex situation and provide various mathematical forms of optimal benefit for the predator under selected scenarios of biological importance.

The evolutionary stability of automimicry

Internal defences such as toxins cannot be detected from a distance by a predator, and are likely to be costly to produce and maintain. Populations of well-defended prey may therefore be vulnerable to invasion from rare 'cheater' mutants that do not produce the toxin themselves but obtain some protection from their resemblance to their better defended conspecifics (automimicry). Although it is well established that well-defended and weakly defended morphs may coexist stably in protected dimorphisms, recent theoretical work suggests that such dimorphisms would not be resistant to invasion by novel mutants with defence levels intermediate to those present. Given that most defences (including toxins) are likely to be continuous traits, this implies that automimicry may tend to be a transitory phenomenon, and thus less likely to explain variation in defence levels in nature. In contrast to this, we show that automimicry can also be evolutionarily stable for continuous traits, and that it may evolve under a wide range of conditions. A recently developed geometric method allows us to determine directly from a trade-off curve whether an evolutionarily stable defence dimorphism is at all possible, and to make some qualitative inferences about the ecological conditions that may favour it.

Spectrum of cyanide toxicity and allocation in Heliconius erato and Passiflora host plants

The larvae of three races of Heliconius erato were fed various species of Passiflora containing varying levels of cyanoglucosides. The mortality rate of larvae and pupae rose when larvae were fed species of Passiflora capable of releasing larger quantities of cyanide. When larvae were fed species of Passiflora with these properties, the resulting adult butterflies also released higher levels of cyanide. This may serve as a defense mechanism. The compounds responsible for the release of cyanide were not evenly distributed throughout the adult butterfly's body. The thorax contained the highest concentration of cyanogenic substances, followed by the head, wings, and abdomen. The younger tissues of Passiflora plants had higher levels of cyanide-releasing compounds than stems and mature leaves. Cyanogenic glycoside distribution within the plants is consistent with optimal allocation theory. The levels of cyanide-releasing substances in plants varied depending on the season.

How can automimicry persist when predators can preferentially consume undefended mimics?

It is common for species that possess toxins or other defences to advertise these defences to potential predators using aposematic ("warning") signals. There is increasing evidence that within such species, there are individuals that have reduced or non-existent levels of defence but still signal. This phenomenon (generally called automimicry) has been a challenge to evolutionary biologists because of the need to explain why undefended automimics do not gain such as a fitness advantage by saving the physiological costs of defence that they increase in prevalence within the population, hence making the aposematic signal unreliable. The leading theory is that aposematic signals do not stop all predatory attacks but rather encourage predators to attack cautiously until they have identified the defence level of a specific individual. They can then reject defended individuals and consume the undefended. This theory has recently received strong empirical support, demonstrating that high-accuracy discrimination appears possible. However, this raises a new evolutionary problem: if predators can perfectly discriminate the defended from the undefended and preferentially consume the latter, then how can automimicry persist? Here, we present four different mechanisms that can allow non-trivial levels of automimics to be retained within a population, even in the extreme case where predators can differentiate defended from undefended individuals with 100% accuracy. These involve opportunity costs to the predator of sampling carefully, temporal fluctuation in predation pressure, predation pressure being correlated with the prevalence of automimicry, or developmental or evolutionary constraints on the availability of defence. These mechanisms generate predictions as to the conditions where we would expect aposematically signalling populations to feature automimicry and those where we would not.

Automimicry destabilizes aposematism: predator sample-and-reject behaviour may provide a solution

Aposematism, the use of conspicuous colours to advertise unpalatability to predators, is perhaps the most studied signalling system in nature. However, its evolutionary stability remains paradoxical. The paradox is illustrated by the problem of automimicry. Automimics are palatable individuals within a population of unpalatable aposematics. Automimics benefit from predators avoiding warning coloration without carrying the models' cost of unpalatability, and should increase in the population, destabilizing the signalling system, unless selected against in some way. Cautious sampling, instead of avoidance, by predators may offer a solution to this problem. Here, we investigate the effect of automimic frequency on predator sampling behaviour, and whether predator sampling behaviour may provide a selection pressure against mimics. Domestic chicks (Gallus gallus domesticus) were subjected to the task of discriminating between green (signalling) and untreated brown chick crumbs. Some of the green crumbs were quinine treated and thus unpalatable. The frequency of palatable signalling prey items varied in four treatments; all unpalatable, low automimic frequency, high automimic frequency and all palatable. The results show that predator sampling behaviour is sensitive to automimic frequency and that predators may discriminate between models and mimics through sampling, and thereby benefit unprofitable prey. The results suggest somewhat surprisingly that aposematic signalling is stable only because of the actions of those predators not actually deterred by warning signals.

Correlations between adult mimicry and larval host plants in ithomiine butterflies

The apparent paradox of multiple coexisting wing pattern mimicry 'rings' in tropical butterflies has been explained as a result of microhabitat partitioning in adults. However, very few studies have tested this hypothesis. In neotropical forests, ithomiine butterflies dominate and display the richest diversity of mimicry rings. We show that co-mimetic species occupy the same larval host-plant species significantly more often than expected in two out of five communities that we surveyed; in one of these, the effect remains significant after phylogenetic correction. This relationship is most probably a result of a third correlated variable, such as microhabitat. Host-plant microhabitat may constrain adult movement, or host-plant choice may depend on butterfly microhabitat preferences and mimicry associations. This link between mimicry and host plant could help explain some host-plant and mimicry shifts, which have been important in the radiation of this speciose tropical group.

Batesian mimics influence mimicry ring evolution

Mathematical models of mimicry typically involve artificial prey species with fixed colorations or appearances; this enables a comparison of predation rates to demonstrate the level of protection a mimic might be afforded. Fruitful theoretical results have been produced using this method, but it is also useful to examine the possible evolutionary consequences of mimicry. To that end, we present individual-based evolutionary simulation models where prey colorations are free to evolve. We use the models to examine the effect of Batesian mimics on Müllerian mimics and mimicry rings. Results show that Batesian mimics can potentially incite Müllerian mimicry relationships and encourage mimicry ring convergence.

What kind of signals do mimetic tiger moths send? A phylogenetic test of wasp mimicry systems (Lepidoptera: Arctiidae: Euchromiini)

Mimicry has been examined in field and laboratory studies of butterflies and its evolutionary dynamics have been explored in computer simulations. Phylogenetic studies examining the evolution of mimicry, however, are rare. Here, the phylogeny of wasp-mimicking tiger moths, the Sphecosoma group, was used to test evolutionary predictions of computer simulations of conventional Müllerian mimicry and quasi-Batesian mimicry dynamics. We examined whether mimetic traits evolved individually, or as suites of characters, using concentrated change tests. The phylogeny of these moth mimics revealed that individual mimetic characters were conserved, as are the three mimetic wasp forms: yellow Polybia, black Polybia and Parachartergus mimetic types. This finding was consistent with a 'supergene' control of linked loci and the Nicholson two-step model of mimicry evolution. We also used a modified permutation-tail probability approach to examine the rate of mimetic-type evolution. The observed topology, hypothetical Müllerian and Batesian scenarios, and 1000 random trees were compared using Kishino-Hasegawa tests. The observed phylogeny was more consistent with the predicted Müllerian distribution of mimetic traits than with that of a quasi-Batesian scenario. We suggest that the range of discriminatory abilities of the predator community plays a key role in shaping mimicry dynamics.

Mimicry profiles are affected by human-induced habitat changes

Mimicry theory predicts that mimics in a Batesian mimicry complex evolve to resemble models closely, and that there is a limit on the numbers of mimics relative to models. For hoverflies (Diptera: Syrphidae), supposed mimics of social wasps (Hymenoptera: Vespidae, neither of these is true; many mimics are imperfect and in the UK and Europe they outnumber their models manifold. We hypothesized that the high abundance of mimics relative to models in the UK may be the result not just of mimic model dynamics, but of habitat changes caused by humans. Most of the larvae of poor mimics are aphidophagous, and changes from ancient forest to agricultural and/or urban habitats may have vastly augmented aphid numbers. Using new and literature data, we compared mimicry profiles of habitats differing in their degree of habitat disturbance. In both cases more highly disturbed habitats had proportionally more poor mimics and fewer high-fidelity mimics than less disturbed habitats. This supports the hypothesis that habitat change has an effect on model to mimic ratios.

Comparative unpalatability of mimetic viceroy butterflies (Limenitis archippus) from four south-eastern United States populations

Viceroy butterflies (Limenitis archippus), long considered palatable mimics of distasteful danaine butterflies, have been shown in studies involving laboratoryreared specimens to be moderately unpalatable to avian predators. This implies that some viceroys are Müllerian co-mimics, rather than defenseless Batesian mimics, of danaines. Here, I further test this hypothesis by assessing the palatability of wild-caught viceroys from four genetically and ecologically diverse populations in the southeastern United States. Bioassays revealed that viceroys sampled from three sites in Florida and one in South Carolina were all moderately unpalatable to captive redwinged blackbird predators, which ate fewer than half of the viceroy abdomens presented. Red-wings commonly exhibited long manipulation times and considerable distress behavior when attempting to eat a viceroy abdomen, and they taste-rejected over one-third of viceroys after a single peck. These findings, the first based on wild-caught butterflies, support the hypothesis that the viceroy-danaine relationship in some areas represents Müllerian mimicry, prompting a reassessment of selective forces shaping the interaction. Moreover, considerable variation in palatability of individual viceroys, and in behavior of individual birds, contributes to the complexity of chemical defense and mimicry in this system.

Mimetic gain in batesian and Müllerian mimicry

Starting from field investigations and experiments on mimetic butterfly populations a model for two mimetic species is developed. The model comprises various features such as the growth rates and carrying capacities of the two species, their unpalatability to predators, the recruitment and the training of the predators and, most important, the similarity of the two mimetic species. The model ranges from pure Batesian to pure Müllerian mimicry over a spectrum of possible cases. The mimetic gain is introduced as the relative increase in equilibrium density in a mimetic situation as compared to a situation where mimicry is not present. The dependence of this quantity on parameters as growth rate, carrying capacity, unpalatability, and similarity is investigated using numerical methods.

Coral Snake Mimicry: Does It Occur?

Field observations and experimental evidence refute previous objections to the coral snake mimicry hypothesis. Concordant color pattern variation spanning hundreds of miles and several presumed venomous models strongly suggests that several harmless or mildly venomous colubrid snakes are indeed mimics of highly venomous elapids.

Localization of Heart Poisons in the Monarch Butterfly

The cardiac glycosides that monarch butterflies sequester from milkweed plants during the larval stage differ remarkably in their emetic potency and are concentrated to different degrees in the various parts of the body as well as in the two sexes (Fig. 1). The very high concentrations of these compounds in the wings probably facilitate learned taste rejection in predators and account for the relatively high frequency of Danaid butterflies with beak-marked wings in natural populations. The cardiac glycosides in the abdomen have a much higher emetic potency than those in the rest of the body. Consequently, naive, extremely hungry, or forgetful birds which capture and peck off the wings but eat the abdomen discard the least emetic glycosides and ingest the most emetic, and thus again experience emesis. The nonrandom distribution of cardenolides in the wings, abdomen, and thorax, together with the fact that monarch males not only contain lower concentrations of cardiac glycosides than females but also contain cardenolides that are overall less emetic than those in females, is interpreted as evidence that these poisons are incorporated at a physiological cost. This cost, balanced against the benefits of protection from predation, provides a selective basis for the occurrence of both emetic and nonemetic individuals in natural populations. Since birds can discriminate emetic from nonemetic monarchs on the basis of taste, it is not necessary to invoke theories of kind of group selection to explain the evolution of this kind of unpalatability.