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.     

Did aggregation favour the initial evolution of warning coloration? A novel world revisited

From experiments using novel prey signals to avoid innate reactions to traditional signals, Alatalo & Mappes (1996, Nature, 382, 708-710) concluded that gregariousness would have selected for warning coloration as it originated for the first time, whereas a solitary prey distribution would not. We have investigated this suggestion in experiments using the same novel prey and background symbols and wild-caught great tit, Parus major, predators. We compared the attack rate on cryptic unpalatable and aposematic unpalatable prey in either a solitary or an aggregated treatment. In the aggregated treatment we found no difference in attack rate on cryptic and aposematic prey. In the solitary treatment the attack rate on aposematic prey was significantly lower after one attack and at the end of the experiment. Thus, we conclude that, in so far as these experiments mimic an original predator-prey relationship, they do not give support to the idea that aggregation would have favoured the evolution of warning coloration in unpalatable prey. Copyright 2000 The Association for the Study of Animal Behaviour.     

The use of colour characters in phylogenetic reconstruction

The use of coloration as a source of characters in phylogenetic reconstruction is investigated using 54 published data sets. Studies were divided into two categories based on a priori postulated roles of the coloration: (1) aposematic and mimetic coloration and (2) nonaposematic, nonmimetic coloration plus dual signals. Colour characters superficially appear to provide similar phylogenetic signal to morphological ones in the case of aposematic and mimetic coloration but significantly less in other situations. However, the data indicated that the apparent signal in the aposematic/mimetic studies tends to be in greater conflict with the morphological signal. It is proposed that this reflects constraints in the evolution of colour characters that are part of aposematic/mimetic patterns and not that they are necessarily good indicators of phylogeny.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 193–202.     

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.     

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.     

Higher survival of an aposematic than of a cryptic form of a distasteful bug

Summary An experiment was performed to assess the relative survival of two forms of 5th instar larvae of Lygaeus equestris (Heteroptera, Lygaeidae) — the normal red form, called aposematic, and a mutant grey form, called cryptic — when given to hand-raised great tits (Parus major).Sixteen birds were presented with aposematic larvae and 16 were presented with cryptic larvae in 10 consecutive trials. One attack per trial was allowed. Both larval forms were presented against a background matching the grey larvae, but since both prey types were presented in a specific place known to the predator, detection rate for both was assumed to be unity.Birds learned to avoid both prey types. However, the survival of the aposematic larvae was higher than that of the cryptic ones due to three aspects of predator behaviour: i) a greater initial reluctance to attack, ii) a more rapid avoidance learning, and iii) a lower frequency of killing in an attack, when the prey was aposematic. Moreover, a greater number of birds learned to avoid prey without killing any individual, when the prey was aposematic. This result is considered to be due to prey coloration alone, since, in a separate test, no difference in prey distastefulness could be detected.This experiment shows that individual prey can benefit from being aposematic and indicates that individual selection can be a sufficient explanation for the evolution of aposematic coloration. It was concluded that, since the survivorship was 6.4 times higher for the aposematic prey, it could have a detection rate that is correspondingly higher than the cryptic in order for the two forms to have equal fitness.     

Evolution of larval gregariousness in relation to repellent defences and warning coloration in tree-feeding Macrolepidoptera: a phylogenetic analysis based on independent contrasts

Gregariousness in insects is often associated with aposematism, which has two distinct properties, repellent defence and warning coloration. Theoretically, both repellent defence and warning coloration are expected to facilitate the evolution of gregariousness. This paper investigates whether the likelihood for gregariousness to evolve is higher (1) in the presence of chemical/physical defence and (2) in the presence of warning coloration, in a sample of over 800 tree-living macrolepidopteran species. A new phylogenetic technique for investigating the correlation between two discrete characters, based on independent contrasts, is used. For each of nine contrasts, based on presence/absence of repellent defence that included transitions to gregariousness, the frequency of such transitions was highest in the lineage with repellent defence present. Similarly, out of 12 contrasts based on presence/absence of warning coloration 10 had the highest frequency of transitions to gregariousness in the lineage with warning coloration. Thus, gregariousness is more likely to evolve in lineages with repellent defence and in lineages with warning coloration, but it is concluded that, since these traits are strongly intercorrelated, it is very difficult to distinguish between their separate effects on the evolution of gregariousness. Our findings indicate, however, that potentially, the presence of repellent defence may be sufficient for the evolution of gregariousness.     

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.     

The model of early evolution of aposematic coloration

First stages of evolution of aposematic coloration include a region of negative selection. During these stages, individuals with aberrant coloration remain to be rare, while predators are still not able to associate coloration with unpalatability. The simulation model is proposed, in which this "problematic zone" is overcome by individual selection for the increasing of unpalatable prey conspicuity in a small unisexual population. It is shown that under this assumption aposematic coloration develops within a wide range of parameters such as the cost of unpalatability, the cost of coloring, the survival rate of unpalatable prey after being attacked by na?ve predator, the probability of discovering of differently colored preys by predator as well as the predator's learning rate and memory depth. Thus, the early evolution ofaposematic coloration does not require any unusual or unique set of circumstances; aposematic coloration along with concomitant Bates mimicry inevitably evolve within a wide range of initial conditions. The loss of cryptic coloration by the original form (e.g., due to a change of food preferences, and thereby the structure of a background coloring, changes in habitat structure, color mutations etc.) is one such condition.     

Cryptic female Strawberry poison frogs experience elevated predation risk when associating with an aposematic partner

Population divergence in sexual signals may lead to speciation through prezygotic isolation. Sexual signals can change solely due to variation in the level of natural selection acting against conspicuousness. However, directional mate choice (i.e., favoring conspicuousness) across different environments may lead to gene flow between populations, thereby delaying or even preventing the evolution of reproductive barriers and speciation. In this study, we test whether natural selection through predation upon mate‐choosing females can favor corresponding changes in mate preferences. Our study system, Oophaga pumilio, is an extremely color polymorphic neotropical frog with two distinctive antipredator strategies: aposematism and crypsis. The conspicuous coloration and calling behavior of aposematic males may attract both cryptic and aposematic females, but predation may select against cryptic females choosing aposematic males. We used an experimental approach where domestic fowl were encouraged to find digitized images of cryptic frogs at different distances from aposematic partners. We found that the estimated survival time of a cryptic frog was reduced when associating with an aposematic partner. Hence, predation may act as a direct selective force on female choice, favoring evolution of color assortative mating that, in turn, may strengthen the divergence in coloration that natural selection has generated.     

Rapid color evolution in an aposematic species: a phylogenetic analysis of color variation in the strikingly polymorphic strawberry poison-dart frog

Aposematism is one of the great mysteries of evolutionary biology. The evolution of aposematic coloration is poorly understood, but even less understood is the evolution of polymorphism in aposematic signals. Here, we use a phylogeographic approach to investigate the evolution of color polymorphism in Dendrobates pumilio, a well-known poison-dart frog (family Dendrobatidae), which displays perhaps the most striking color variation of any aposematic species. With over a dozen color morphs, ranging from bright red to dull green, D. pumilio provides an ideal opportunity to examine the evolution of color polymorphism and evolutionary shifts to cryptic coloration in an otherwise aposematic species. We constructed a phylogenetic tree for all D. pumilio color morphs from 3051bp of mtDNA sequence data, reconstructed ancestral states using parsimony and Bayesian methods, and tested the recovered tree against constraint trees using parametric bootstrapping to determine the number of changes to each color type. We find strong evidence for nearly maximal numbers of changes in all color traits, including five independent shifts to dull dorsal coloration. Our results indicate that shifts in coloration in aposematic species may occur more regularly than predicted and that convergence in coloration may indicate that similar forces are repeatedly driving these shifts.     

Teasing apart crypsis and aposematism – evidence that disruptive coloration reduces predation on a noxious toad

Both cryptic and aposematic colour patterns can reduce predation risk to prey. These distinct strategies may not be mutually exclusive, because the impact of prey coloration depends on a predator's sensory system and cognition and on the environmental background. Determining whether prey signals are cryptic or aposematic is a prerequisite for understanding the ecological and evolutionary implications of predator–prey interactions. This study investigates whether coloration and pattern in an exceptionally polymorphic toad, Rhinella alata, from Barro Colorado Island, Panama reduces predation via background matching, disruptive coloration, and/or aposematic signaling. When clay model replicas of R. alata were placed on leaf litter, the model's dorsal pattern – but not its colour – affected attack rates by birds. When models were placed on white paper, patterned and un‐patterned replicas had similar attack rates by birds. These results indicate that dorsal patterns in R. alata are functionally cryptic and emphasize the potential effectiveness of disruptive coloration in a vertebrate taxon.     

Mothers' Egg-pooling and Aposematic Offspring's Gregariousness: Cui bono?

Recently Sillén-Tullberg & Leimar (1988) modelled a general explanation for the evolution of gregariousness in prey organisms that live exposed, have no means of escape when discovered by a predator, and are small in relation to a potential predator (who thus can sample many prey individuals in one encounter). The model predicts that gregarious prey organisms of that type ought to be distasteful, and that the evolution of gregariousness will be favoured by aposematic coloration facilitating avoidance learning in a predator. Obviously, any protective power of grouping depends on group size. According to the Sillén-Tullberg & Leimar model, (1) “members of small groups may have a higher rate of death from predation than solitary individuals, but above a certain minimum group size, group members do better than solitary individuals; … as group size increases above the minimum value, group members suffer fewer and fewer deaths from predation”. They benefit from the “decreased risk of predator attack on any particular individual”, called dilution effect. (2) “The more prey specimens that the predator needs to sample during avoidance learning, the larger an aggregation needs to be in order for gregariousness to be advantageous”. It is further explained that (3) selection resulting from predation favours increase in group size until it “acts like a predator-satiation mechanism”.     

Adaptive change in protective coloration in adult striated shieldbugs Graphosoma lineatum (Heteroptera: Pentatomidae): test of detectability of two colour forms by avian predators

1. Protective coloration in insects may be aposematic or cryptic, and some species change defensive strategy between instars. In Sweden, the adult striated shieldbugs Graphosoma lineatum (Heteroptera: Pentatomidae) undergo a seasonal colour change from pale brown and black striation in the pre‐hibernating adults, to red and black striation in the same post‐hibernating individuals. To the human eye the pre‐hibernating adults appear cryptic against the withered late summer vegetation, whereas the red and black post‐hibernating adults appear aposematic. This suggests a possibility of a functional colour change. However, what is cryptic to the human eye is not necessarily cryptic to a potential predator. 2. Therefore we tested the effect of coloration in adult G. lineatum on their detectability for avian predators. Great tits (Parus major) were trained to eat sunflower seeds hidden inside the emptied exoskeletons of pale or red G. lineatum. Then the detection time for both colour forms was measured in a dry vegetation environment. 3. The birds required a longer time to find the pale form of G. lineatum than the red one. The pale form appears more cryptic on withered late summer vegetation than the red form, not only to the human eye but also to avian predators. The result supports the idea that the adult individuals of G. lineatum undergo a functional change from a cryptic protective coloration to an aposematic one.     

PHYLOGENETIC HYPOTHESES UNDER THE ASSUMPTION OF NEUTRAL QUANTITATIVE-GENETIC VARIATION

There are many situations in which the only available characters for reconstructing phylogenies are morphological. Those traits that are subject only to the forces of mutation and random genetic drift can be used to obtain unbiased estimates of phylogenetic relationships. However, the accurate recovery of a phylogeny from information on neutral characters requires the procurement of data for a large number of independent traits, individuals, and populations. Phylogenetic trees fit to data from more than five species will almost always contain topological errors, even with very large data sets. The population-genetic consequences of the neutral model are reviewed, and some statistical methods for testing whether the diversification of a phylogeny is compatible with such a model are outlined. The theory is then applied to a very large data set on cranial morphology in modern man. The results are consistent with the hypothesis that interracial differences in human skull dimensions are a simple consequence of random drift and mutation.     

Aposematism and mimicry in soft‐bodied beetles of the superfamily Cleroidea (Insecta)

The evolution of animal life strategies is among the main themes of current evolutionary biology. Checkered beetles, soft‐winged flower beetles and their allies (superfamily Cleroidea), exhibit well‐known aposematic colour patterns, particularly in the family Cleridae, which participate in mimicry complexes mostly with unpalatable beetles, ants and velvet ants representing a Müllerian–Batesian continuum. Many cleroids also exhibit attenuated hardening of cuticular layers resulting in a soft‐bodied appearance. Here, a molecular phylogenetic analysis of the entire Cleroidea was performed using sequences of two nuclear and two mitochondrial loci of ~4 kb total length. Inferred phylogenies were used to reconstruct ancestral colour patterns and involvement in mimicry complexes. The hypothesis of a soft‐bodied ancestor of Cleridae and allies was tested. The phylogenetic analyses corroborated the expanded Cleroidea concept including Byturidae and Biphyllidae formerly classified as Cucujoidea. Character state optimization showed cryptic coloration was the ancestral state in Cleroidea, from which aposematic coloration originated several times in distant cleroid lineages. Within Cleridae, mimicry also arose from an ancestor that was cryptic, and multiple lineages that mimicked unpalatable beetles (Chrysomelidae, Meloidae, Lycidae) and stinging Hymenoptera evolved. Aposematic coloration was acquired in all major clerid lineages including Thanerocleridae, which are either the sister group of Chaetosomatidae or Cleridae. These findings suggest that mimetic traits in the clerid clade evolved at various times, possibly soon after the origin of soft‐bodiedness. The adaptive value of aposematism in cleroids is likely to be enhanced in soft‐bodied species, as this trait provides limited means of protection against predators, and therefore may promote the acquisition of aposematic and mimetic coloration in various ecological situations.     

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.     

Understanding gregariousness in a larval Lepidopteran: the roles of host plant, predation, and microclimate

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development.
2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary.
3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies.
4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups.
5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species.
6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.     

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

Protective coloration is a well-known predator avoidance strategy in prey species. Aposematic species often display a contrasting color pattern consisting of dark spots of different shapes and sizes on a bright background coloration. Both elements, background color and spots are expected to serve different purposes. While the ecological function of the bright coloration has been addressed in many studies, the question of whether the interaction with differently sized spots influences predator behavior has received less attention by researchers. In a lowland rain forest in Costa Rica we used 2700 clay models that imitated the polytypic strawberry poison frog (Oophaga pumilio) as a proxy for an aposematic prey species. We manipulated the dorsal color pattern by using a local and a non-local aposematic and a non-local cryptic background color and combined them with black spots increasing in size (none, small, medium, large). The major objective was to test if spot size alters the survival rate of differently colored models. Background coloration and spot size were significant predictors of being attacked. However, the interaction between both effects was not. During five trials predators avoided the non-local aposematic color morph and did not discriminate between local aposematic and non-local cryptic models. Spot size and attack rate were negatively linear correlated which suggests that predator selection promotes the evolution of dark spots. We further conclude that spot size matters in a contrasting color pattern and plays an important role in predator avoidance.     

Reactions of hand-reared and wild-caught predators toward warningly colored, gregarious, and conspicuous prey

Recently there has been debate over the importance of innateavoidance of aposematic prey by predators, particularly birds.There is evidence that the predators have innate or unlearned,thus, inherited avoidance against certain colors, but whetherthere is any innate avoidance against gregariousness or conspicuousnessis unclear. Previously predator behavior toward these charactersof aposematic prey have been tested in separate experiments.We designed an experiment to separate inheritance toward color,gregariousness, and conspiucuosness. We simultaneously offeredthe predators warningly colored and nonwarningly colored preyitems, both aggregated and solitary, on white (conspicuous)or brown (cryptic) backgrounds. The predators we used were naive (handraised), wild-caught yearling and adult great tits (Parus major L.).The results confirm previous results regarding the innate avoidanceof color. Naive predators seemed to have a genetically or culturallytransmitted avoidance of yellow and black prey compared to brownprey. Surprisingly, yearling wild-caught great tits were moreselective than adults, which did not show as strong avoidanceof yellow and black prey. More importantly, birds did not findgregarious prey more aversive than single prey, which indicatesthat grouping alone does not serve as an innate avoidance signal.Conspicuousness itself was not aversive to the predators. Ourresults suggest that the avoidance against a particular colorpattern probably has an inherited basis, whereas gregariousand conspicuous characters of prey presumably aid the avoidancelearning.