High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test

ebony underpins Batesian mimicry in melanic stoneflies

The evolution of Batesian mimicry - whereby harmless species avoid predation through their resemblance to harmful species - has long intrigued biologists. In rare cases, Batesian mimicry is linked to intraspecific colour variation, in which only some individuals within a population resemble a noxious 'model'. Here, we assess intraspecific colour variation within a widespread New Zealand stonefly, wherein highly melanized individuals of Zelandoperla closely resemble a chemically defended aposematic stonefly, Austroperla cyrene. We assess convergence in the colour pattern of these two species, compare their relative palatability to predators, and use genome-wide association mapping to assess the genetic basis of this resemblance. Our analysis reveals that melanized Zelandoperla overlap significantly with Austroperla in colour space but are significantly more palatable to predators, implying that they are indeed Batesian mimics. Analysis of 194,773 genome-wide SNPs reveals an outlier locus (ebony) strongly differentiating melanic versus non-melanic Zelandoperla. Genotyping of 338 specimens from a single Zelandoperla population indicates that ebony explains nearly 70% of the observed variance in melanism. As ebony has a well-documented role in insect melanin biosynthesis, our findings indicate this locus has a conserved function across deeply divergent hexapod lineages. Distributional records suggest a link between the occurrence of melanic Zelandoperla and the forested ecosystems where the model Austroperla is abundant, suggesting the potential for adaptive shifts in this system underpinned by environmental change.

Evolutionary origins of sexual dimorphism: Lessons from female-limited mimicry in butterflies

The striking female-limited mimicry observed in some butterfly species is a text-book example of sexually dimorphic trait submitted to intense natural selection. Two main evolutionary hypotheses, based on natural and sexual selection respectively, have been proposed. Predation pressure favoring mimicry toward defended species could be higher in females because of their slower flight, and thus overcome developmental constraints favoring the ancestral trait that limits the evolution of mimicry in males but not in females. Alternatively, the evolution of mimicry in males could be limited by female preference for non-mimetic males. However, the evolutionary origin of female preference for non-mimetic males remains unclear. Here, we hypothesize that costly sexual interactions between individuals from distinct sympatric species might intensify because of mimicry, therefore promoting female preference for non-mimetic trait. Using a mathematical model, we compare the evolution of female-limited mimicry when assuming either alternative selective hypotheses. We show that the patterns of divergence of male and female trait from the ancestral traits can differ between these selection regimes. We specifically highlight that divergence in female trait is not a signature of the effect of natural selection. Our results also evidence why female-limited mimicry is more frequently observed in Batesian mimics.

Concepts and contentions of coral snake resemblance: Batesian mimicry and its alternatives

Venomous coral snakes and non-venomous coral snake lookalikes are often regarded as a classic example of Batesian mimicry, whereby a harmless or palatable organism imitates a harmful or less palatable organism. However, the validity of this claim is questionable. The existing literature regarding coral snake mimicry presents a divisive stance on whether Batesian mimicry is occurring or whether the similarity between snakes is attributable to alternative factors. Here, we compile available literature on coral snake mimicry and assess the support for Batesian mimicry. We find that most of the recent relevant literature (after approximately 2000) supports the Batesian mimicry hypothesis. However, this is not strongly supported by empirical evidence. Potential considerations addressed here for both the Batesian and alternative hypotheses include the function of the colour pattern, predatory learning and the biogeographical distribution of similar snakes. The analyses performed previously by mimicry researchers show that the interpretation of the conditions for mimicry is not consistent throughout the scientific community when applied to coral snake systems. This review focuses on this division and stresses the need to reach an agreement about the adaptive significance of New World coral snakes and their lookalikes.

Mapping the evolution of accurate Batesian mimicry of social wasps in hoverflies

Hoverflies (Diptera: Syrphidae) provide an excellent opportunity to study the evolution of Batesian mimicry, where defenseless prey avoid predation by evolving to resemble defended "model" species. Although some hoverflies beautifully resemble their hymenopteran models, others seem to be poor mimics or are apparently nonmimetic. The reasons for this variation are still enigmatic despite decades of research. Here, we address this issue by mapping social-wasp mimicry across the phylogeny of Holarctic hoverflies. Using the "distance transform" technique, we calculate an objective measure of the abdominal pattern similarity between 167 hoverfly species and a widespread putative model, the social wasp, Vespula germanica. We find that good wasp mimicry has evolved several times, and may have also been lost, leading to the presence of nonmimics deep within clades of good mimics. Body size was positively correlated with similarity to the model, supporting previous findings that smaller species are often poorer mimics. Additionally, univoltine species were less accurate wasp mimics than multivoltine and bivoltine species. Hence, variation in the accuracy of Batesian mimics may reflect variation in the opportunity for selection caused by differences in prey value or signal perception (influenced by body size) and phenology or generation time (influenced by voltinism).

Is mimicry a diversification-driver in ants? Biogeography, ecology, ethology, genetics and morphology define a second West-Palaearctic Colobopsis species (Hymenoptera: Formicidae)

The West-Palaearctic Colobopsis ant populations have long been considered a single species (Colobopsis truncata). We studied the diversity of this species by employing a multidisciplinary approach and combining data from our surveys, museum and private collections, and citizen science platforms. As a result, we have revealed the existence of a second species, which we describe as Colobopsis imitans sp. nov., distributed allopatrically from Co. truncata and living in the Maghreb, Sicily and southern Iberia. While the pigmentation of Co. truncata is reminiscent of Dolichoderus quadripunctatus, that of Co. imitans is similar to Crematogaster scutellaris, with which Co. imitans lives in close spatial association, and whose foraging trails it habitually follows, similar to Camponotus lateralis and other ant-mimicking ants. The isolation between Co. imitans and Co. truncata seems to have occurred relatively recently because of significant, yet not extreme, morphometric differentiation, and to mtDNA polyphyly. Both Co. imitans and Co. truncata appear to employ mimicry of an unpalatable or aggressive ant species as an important defensive strategy; this ‘choice’ of a different model species is motivated by biogeographic reasons and appears to act as a critical evolutionary driver of their diversification.

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.

Sensory bias and signal detection trade-offs maintain intersexual floral mimicry

Mimicry is common in interspecies interactions, yet conditions maintaining Batesian mimicry have been primarily tested in predator-prey interactions. In pollination mutualisms, floral mimetic signals thought to dupe animals into pollinating unrewarding flowers are widespread (greater than 32 plant families). Yet whether animals learn to both correctly identify floral models and reject floral mimics and whether these responses are frequency-dependent is not well understood. We tested how learning affected the effectiveness and frequency-dependence of imperfect Batesian mimicry among flowers using the generalist bumblebee, Bombus impatiens, visiting Begonia odorata, a plant species exhibiting intersexual floral mimicry. Unrewarding female flowers are mimics of pollen-rewarding male flowers (models), though mimicry to the human eye is imperfect. Flower-naive bees exhibited a perceptual bias for mimics over models, but rapidly learned to avoid mimics. Surprisingly, altering the frequency of models and mimics only marginally shaped responses by naive bees and by bees experienced with the distribution and frequency of models and mimics. Our results provide evidence both of exploitation by the plant of signal detection trade-offs in bees and of resistance by the bees, via learning, to this exploitation. Critically, we provide experimental evidence that imperfect Batesian mimicry can be adaptive and, in contrast with expectations of signal detection theory, functions largely independently of the model and mimic frequency. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

The evolution of conspecific acceptance threshold models

How do organisms balance different types of recognition errors when cues associated with desirable and undesirable individuals or resources overlap? This is a fundamental question of signal detection theory (SDT). As applied in sociobiology, SDT is not limited to a single context or animal taxon, therefore its application can span what may be considered dissimilar systems. One of the applications of SDT is the suite of acceptance threshold models proposed by Reeve (1989), which analysed how individuals should balance acceptance and rejection errors in social discrimination decisions across a variety of recognition contexts, distinguished by how these costs and benefits relatively combine. We conducted a literature review to evaluate whether these models' specific predictions have been upheld. By examining over 350 research papers, we quantify how Reeve's models (Reeve 1989 Am. Nat. 133, 407-435 (doi:10.1086/284926)) have influenced the field of ecological and behavioural recognition systems research. We found overall empirical support for the predictions of the specific models proposed by Reeve, and argue for further expansion of their applications into more diverse taxonomic and additional recognition contexts. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Multiple models generate a geographical mosaic of resemblance in a Batesian mimicry complex

Batesian mimics-benign species that receive protection from predation by resembling a dangerous species-often occur with multiple model species. Here, we examine whether geographical variation in the number of local models generates geographical variation in mimic-model resemblance. In areas with multiple models, selection might be relaxed or even favour imprecise mimicry relative to areas with only one model. We test the prediction that model-mimic match should vary with the number of other model species in a broadly distributed snake mimicry complex where a mimic and a model co-occur both with and without other model species. We found that the mimic resembled its model more closely when they were exclusively sympatric than when they were sympatric with other model species. Moreover, in regions with multiple models, mimic-model resemblance was positively correlated with the resemblance between the model and other model species. However, contrary to predictions, free-ranging natural predators did not attack artificial replicas of imprecise mimics more often when only a single model was present. Taken together, our results suggest that multiple models might generate a geographical mosaic in the degree of phenotype matching between Batesian mimics and their models.

Frequency dependence shapes the adaptive landscape of imperfect Batesian mimicry

Despite more than a century of biological research on the evolution and maintenance of mimetic signals, the relative frequencies of models and mimics necessary to establish and maintain Batesian mimicry in natural populations remain understudied. Here we investigate the frequency-dependent dynamics of imperfect Batesian mimicry, using predation experiments involving artificial butterfly models. We use two geographically distinct populations of Adelpha butterflies that vary in their relative frequencies of a putatively defended model (Adelpha iphiclus) and Batesian mimic (Adelpha serpa). We found that in Costa Rica, where both species share similar abundances, Batesian mimicry breaks down, and predators more readily attack artificial butterfly models of the presumed mimic, A. serpa By contrast, in Ecuador, where A. iphiclus (model) is significantly more abundant than A. serpa (mimic), both species are equally protected from predation. Our results provide compelling experimental evidence that imperfect Batesian mimicry is frequency-dependent on the relative abundance of models and mimics in natural populations, and contribute to the growing body of evidence that complex dynamics, such as seasonality or the availability of alternative prey, influence the evolution of mimetic traits.

Evaluating the utility of camera traps in field studies of predation

Artificial prey techniques—wherein synthetic replicas of real organisms are placed in natural habitats—are widely used to study predation in the field. We investigated the extent to which videography could provide additional information to such studies. As a part of studies on aposematism and mimicry of coral snakes (Micrurus) and their mimics, observational data from 109 artificial snake prey were collected from video-recording camera traps in three locations in the Americas (terra firme forest, Tiputini Biodiversity Station, Ecuador; premontane wet forest, Nahá Reserve, Mexico; longleaf pine forest, Southeastern Coastal Plain, North Carolina, USA). During 1,536 camera days, a total of 268 observations of 20 putative snake predator species were recorded in the vicinity of artificial prey. Predators were observed to detect artificial prey 52 times, but only 21 attacks were recorded. Mammals were the most commonly recorded group of predators near replicas (243) and were responsible for most detections (48) and attacks (20). There was no difference between avian or mammalian predators in their probability of detecting replicas nor in their probability of attacking replicas after detecting them. Bite and beak marks left on clay replicas registered a higher ratio of avian:mammalian attacks than videos registered. Approximately 61.5% of artificial prey monitored with cameras remained undetected by predators throughout the duration of the experiments. Observational data collected from videos could provide more robust inferences on the relative fitness of different prey phenotypes, predator behavior, and the relative contribution of different predator species to selection on prey. However, we estimate that the level of predator activity necessary for the benefit of additional information that videos provide to be worth their financial costs is achieved in fewer than 20% of published artificial prey studies. Although we suggest future predation studies employing artificial prey to consider using videography as a tool to inspire new, more focused inquiry, the investment in camera traps is unlikely to be worth the expense for most artificial prey studies until the cost:benefit ratio decreases.

Trade-offs direct the evolution of coloration in Galápagos land snails

Increasingly, multiple selective factors are recognized as jointly contributing to the evolution of morphology. What is not clear is how these forces vary across communities to promote morphological diversification among related species. In this study of Galápagos endemic snails (genus Naesiotus), we test several hypotheses of colour evolution. We observe mockingbirds (genus Mimus) predating live snails and find that avian predation selects against conspicuous shells. The evolutionary outcome of this selection is a diversity of shell colours across snails of the archipelago, each closely matching local backgrounds. We also find that snails more regularly exposed to the hot, equatorial sun reflect more light than shells of species from shadier habitats, suggesting a role for thermoregulatory constraints directing colour evolution. The signature of thermoregulatory selection is most clear in comparatively young communities (on the youngest islands), while the signature of selection from predators is most evident in older communities (on the older islands). Together, our findings point to a scenario of shifting selective forces along island ontogeny and community maturity that lead to the distribution of snail coloration we observe in Galápagos. Complex selective regimes such as these may have more responsibility for morphological diversity than is currently recognized.

A matter of proportion? Associational effects in larval anuran communities under fish predation

In Batesian mimicry, a species lacking defences against predators benefits from mimicking the aposematic signal of a defended species, while the model may incur the costs of reduced defensive efficacy. Similar reciprocal indirect effects may emerge even when the signal is not mimicked; termed associational effects, such interactions are well known in plants sharing herbivores but have received little attention in animal studies. We investigated associational interactions in a system where unequally defended prey (chemically defended Bufo bufo and undefended Rana temporaria tadpoles), sharing general morphology but not an aposematic signal, were exposed to predation by the carp Cyprinus carpio along a gradient of relative prey abundance. In the absence of fish, the assemblage composition had no effect on the survival of Rana, while that of Bufo decreased with increasing abundance of Rana. Fish reduced the survival of tadpoles from both species. However, increased relative abundance of Bufo in the community led to enhanced survival in both Bufo and Rana. Increasing relative proportions of heterospecifics reduced metamorph mass only in Bufo, indicating greater sensitivity to interspecific competition compared to Rana; the effect was reduced in the presence of fish. Our results show that undefended non-mimetic prey enjoy reduced predation with increasing relative abundance of chemically defended prey, which in turn suffer greater mortality with an increasing proportion of the undefended species. Associational resistance/susceptibility, driven by current assemblage composition, not by selection for resemblance, can shape the dynamics of mixed communities of defended and undefended prey in the absence of mimicry.

Geographic variation in mimetic precision among different species of coral snake mimics

Batesian mimicry is widespread, but whether and why different species of mimics vary geographically in resemblance to their model is unclear. We characterized geographic variation in mimetic precision among four Batesian mimics of coral snakes. Each mimic occurs where its model is abundant (i.e. in 'deep sympatry'), rare (i.e. at the sympatry/allopatry boundary or 'edge sympatry') and absent (i.e. in allopatry). Geographic variation in mimetic precision was qualitatively different among these mimics. In one mimic, the most precise individuals occurred in edge sympatry; in another, they occurred in deep sympatry; in the third, they occurred in allopatry; and in the fourth, precise mimics were not concentrated anywhere throughout their range. Mimicry was less precise in allopatry than in sympatry in only two mimics. We present several nonmutually exclusive hypotheses for these patterns. Generally, examining geographic variation in mimetic precision - within and among different mimics - offers novel insights into the causes and consequences of mimicry.

The genetics of adaptation to discrete heterogeneous environments: frequent mutation or large-effect alleles can allow range expansion

Range expansions are complex evolutionary and ecological processes. From an evolutionary standpoint, a populations' adaptive capacity can determine the success or failure of expansion. Using individual-based simulations, we model range expansion over a two-dimensional, approximately continuous landscape. We investigate the ability of populations to adapt across patchy environmental gradients and examine how the effect sizes of mutations influence the ability to adapt to novel environments during range expansion. We find that genetic architecture and landscape patchiness both have the ability to change the outcome of adaptation and expansion over the landscape. Adaptation to new environments succeeds via many mutations of small effect or few of large effect, but not via the intermediate between these cases. Higher genetic variance contributes to increased ability to adapt, but an alternative route of successful adaptation can proceed from low genetic variance scenarios with alleles of sufficiently large effect. Steeper environmental gradients can prevent adaptation and range expansion on both linear and patchy landscapes. When the landscape is partitioned into local patches with sharp changes in phenotypic optimum, the local magnitude of change between subsequent patches in the environment determines the success of adaptation to new patches during expansion.

Rapid evolution of mimicry following local model extinction

Batesian mimicry evolves when individuals of a palatable species gain the selective advantage of reduced predation because they resemble a toxic species that predators avoid. Here, we evaluated whether-and in which direction-Batesian mimicry has evolved in a natural population of mimics following extirpation of their model. We specifically asked whether the precision of coral snake mimicry has evolved among kingsnakes from a region where coral snakes recently (1960) went locally extinct. We found that these kingsnakes have evolved more precise mimicry; by contrast, no such change occurred in a sympatric non-mimetic species or in conspecifics from a region where coral snakes remain abundant. Presumably, more precise mimicry has continued to evolve after model extirpation, because relatively few predator generations have passed, and the fitness costs incurred by predators that mistook a deadly coral snake for a kingsnake were historically much greater than those incurred by predators that mistook a kingsnake for a coral snake. Indeed, these results are consistent with prior theoretical and empirical studies, which revealed that only the most precise mimics are favoured as their model becomes increasingly rare. Thus, highly noxious models can generate an 'evolutionary momentum' that drives the further evolution of more precise mimicry-even after models go extinct.

Imperfect mimicry and the limits of natural selection

Mimicry--when one organism (the mimic) evolves a phenotypic resemblance to another (the model) due to selective benefits--is widely used to illustrate natural selection's power to generate adaptations. However, many putative mimics resemble their models imprecisely, and such imperfect mimicry represents a specific challenge to mimicry theory and a general one to evolutionary theory. Here, we discuss 11 nonmutually exclusive hypotheses for imperfect mimicry. We group these hypotheses according to whether imperfect mimicry reflects: an artifact of human perception, which is not shared by any naturally occurring predators and therefore is not truly an instance of imperfect mimicry; genetic, developmental or time-lag constraints, which (temporarily) prevent a response to selection for perfect mimicry; relaxed selection, where imperfect mimicry is as adaptive as perfect mimicry; or tradeoffs, where imperfect mimicry is (locally) more adaptive than perfect mimicry. We find that the relaxed selection hypothesis has garnered the most support. However, because only a few study systems have thus far been comprehensively evaluated, the relative contributions of the various hypotheses toward explaining the evolution of imperfect mimicry remain unclear. Ultimately, clarifying why imperfect mimicry exists should provide critical insights into the limits of natural selection in producing complex adaptations.

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.

Predator cognition permits imperfect coral snake mimicry

Batesian mimicry is often imprecise. An underexplored explanation for imperfect mimicry is that predators might not be able to use all dimensions of prey phenotype to distinguish mimics from models and thus permit imperfect mimicry to persist. We conducted a field experiment to test whether or not predators can distinguish deadly coral snakes (Micrurus fulvius) from nonvenomous scarlet kingsnakes (Lampropeltis elapsoides). Although the two species closely resemble one another, the order of colored rings that encircle their bodies differs. Despite this imprecise mimicry, we found that L. elapsoides that match coral snakes in other respects are not under selection to match the ring order of their model. We suggest that L. elapsoides have evolved only those signals necessary to deceive predators. Generally, imperfect mimicry might suffice if it exploits limitations in predator cognitive abilities.

Mimics without models: causes and consequences of allopatry in Batesian mimicry complexes

Batesian mimicry evolves when a palatable species (the 'mimic') co-opts a warning signal from a dangerous species (the 'model') and thereby deceives its potential predators. Longstanding theory predicts that this protection from predation should break down where the model is absent. Thus, mimics are expected to only co-occur with their model. Yet, many mimics violate this prediction and occur in areas where their model is absent. Here, we discuss the causes and consequences of such allopatric mimics. We also describe how these 'rule-bending' mimics provide critical insights into diverse topics ranging from how Batesian mimicry evolves to its possible role in speciation.