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.     

Avian predators attack aposematic prey more forcefully when they are part of an aggregation

Defended insects often advertise their unprofitability to potential predators using conspicuous aposematic coloration. Many aposematic insects are also gregarious, and it has been suggested that the aggregation of defended prey may have facilitated the evolution of aposematic coloration. Empirical studies have demonstrated that birds are more wary of aggregated aposematic prey, and learn to avoid them more quickly than solitary prey. However, many aposematic insects survive being attacked by birds, and the effect of aggregation on post-attack survival has not previously been investigated. Using domestic chicks as predators and artificially manipulated mealworms as prey, we provide empirical evidence that predators attack aggregated aposematic prey more forcefully than solitary prey, reducing the likelihood of prey surviving an attack. Hence, we suggest that previous works concluding that aggregation was an important pre-requisite for the evolution of aposematism may have overestimated the fitness benefits of aggregation, since aggregated prey may be attacked less but are also less likely to survive an attack.     

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.     

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.     

Change in protective coloration in the striated shieldbug <Emphasis Type="Italic">Graphosoma lineatum</Emphasis> (Heteroptera: Pentatomidae): predator avoidance and generalization among different life stages

There are two major forms of protective coloration, camouflage and warning coloration, which often entail different colour pattern characteristics. Some species change strategy between or within life stages and one such example is the striated shieldbug, Graphosoma lineatum. The larvae and the pale brownish-and-black striated pre-diapause adults are more cryptic in the late summer environment than is the red-and black striation that the adults change to after diapause in spring. Here we investigate if the more cryptic pre-diapause adult and larval coloration may affect the aposematic function of the coloration as compared to the red adult form. In a series of trials we presented fifth instar larvae, pale or red adults to shieldbug-naïve domestic chicks, Gallus gallus domesticus, to investigate the birds’ initial wariness, avoidance learning, and generalization between the three prey types. The naïve chicks found the red adults most aversive followed by pale adults, and they found the larvae the least aversive. The birds did not find the larvae unpalatable and did not learn to avoid them, while they learned to avoid the two adult forms and then to a similar degree. Birds generalized asymmetrically between life stages, positively from larvae to adults and negatively from adults to larvae. We conclude that the lower conspicuousness in the pale forms of G. lineatum may entail a reduced aposematic function, namely a reduced initial wariness in inexperienced birds. The maintenance of the colour polymorphism is discussed.     

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”.     

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.     

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.     

Warning coloration in sawflyAthalia rosae larva and concealing coloration in butterflyPieris rapae larva feeding on similar plants evolved through individual selection

Green larvae of the butterfly Pieris rapae and black larvae of the sawfly Athalia rosae feed on green leaves of the same cruciferous plants. To demonstrate that P. rapae has concealing coloration and that A. rosae has warning coloration, the larvae of the two species were supplied to naive chicks Gallus gallus on white, green or black backgrounds. P. rapae larvae were palatable and their green body color acted as a concealing coloration. On the other hand, A. rosae larvae were unpalatable and their black body color acted as a warning coloration. There is a general consensus that warning coloration is an altruistic character which needs victims, and thus can evolve through kin selection or green beard selection. However, black A. rosae larvae were seldom injured by chicks' attack, in particular, on the green background. Therefore, the warning coloration of A. rosae larvae can be a selfish character and hence can evolve through individual selection as well as concealing coloration of P. rapae.     

Decision time and prey gregariousness influence attack probability in naïve and experienced predators

Aposematic coloration often has an element of conspicuousness. One suggested benefit of conspicuousness is that it enables the prey to be detected at a greater distance, allowing a predator more time to make a correct decision about attacking it and thus reducing possible recognition errors made by predators. I conducted an experiment, with chicks, Gallus gallus domesticus, as predators on live aposematic and nonaposematic prey, to investigate the effects of decision time and signal size on predator sampling behaviour. The chicks were subjected to different degrees of competition to influence how quickly decisions had to be made. Chicks in four treatment groups, either in the presence or absence of a competing chick, were presented with either solitary prey or prey in groups. In the presence of a competitor, chicks attacked the prey more often and more quickly and needed more attacks before they started to avoid the prey. With prey in groups, chicks took longer to attack, attacked less often, learnt to avoid prey more quickly and killed fewer aposematic prey. This experiment provides evidence for the importance of time and signal size for predators' attack decisions. More time to view prey prior to attack could produce a stronger image and thus encourage avoidance learning and produce a stronger neophobic avoidance effect. Copyright 2000 The Association for the Study of Animal Behaviour.     

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.     

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.     

Increased predation of nutrient-enriched aposematic prey

Avian predators readily learn to associate the warning coloration of aposematic prey with the toxic effects of ingesting them, but they do not necessarily exclude aposematic prey from their diets. By eating aposematic prey ‘educated’ predators are thought to be trading-off the benefits of gaining nutrients with the costs of eating toxins. However, while we know that the toxin content of aposematic prey affects the foraging decisions made by avian predators, the extent to which the nutritional content of toxic prey affects predators'' decisions to eat them remains to be tested. Here, we show that European starlings (Sturnus vulgaris) increase their intake of a toxic prey type when the nutritional content is artificially increased, and decrease their intake when nutritional enrichment is ceased. This clearly demonstrates that birds can detect the nutritional content of toxic prey by post-ingestive feedback, and use this information in their foraging decisions, raising new perspectives on the evolution of prey defences. Nutritional differences between individuals could result in equally toxic prey being unequally predated, and might explain why some species undergo ontogenetic shifts in defence strategies. Furthermore, the nutritional value of prey will likely have a significant impact on the evolutionary dynamics of mimicry systems.     

Avoidance of aposematic prey in European tits (Paridae): learned or innate?

Predators may either learn to avoid aposematic prey or may avoidit because of an innate bias. Learned as well as innate avoidancehas been observed in birds, but the existing evidence is basedon experiments with rather few unrelated model species. We comparedthe origin of avoidance in European species of tits (Paridae).First, we tested whether wild-caught birds (blue tits, greattits, crested tits, coal tits, willow tits, and marsh tits)avoid aposematic (red and black) adult firebugs Pyrrhocorisapterus (Heteroptera) more than nonaposematic (brown painted)ones. Larger proportion of birds avoided aposematic than brown-paintedfirebugs in majority of species (except coal tits). Second,we tested whether naive hand-reared birds of 4 species (bluetits, great tits, crested tits, and coal tits) attack or avoidaposematic and nonaposematic firebugs, both novel for them.Behavior of the naive blue tits and coal tits was similar tothat of the wild-caught birds; majority of them did not attackthe firebugs. Contrastingly, the naive great tits and crestedtits behaved differently than the wild-caught conspecific adults;majority of the wild-caught birds avoided the aposematic firebugs,whereas the naive birds usually did not show any initial avoidanceand had to learn to avoid the aposematic prey. Our results showthat the origin of avoidance may be different even in closelyrelated species. Because blue tits and coal tits avoided notonly aposematic firebugs but also their brown-painted form,we interpret their behavior as innate neophobia rather thaninnate bias against the warning coloration.     

Predators' toxin burdens influence their strategic decisions to eat toxic prey

Toxic prey advertise their unprofitability to predators via conspicuous aposematic coloration [1]. It is widely accepted that avoidance learning by naive predators is fundamental in generating selection for aposematism [2, 3] and mimicry [4, 5] (where species share the same aposematic coloration), and consequently this cognitive process underpins current evolutionary theory [5, 6]. However, this is an oversimplistic view of predator cognition and decision making. We show that predators that have learned to avoid chemically defended prey continue to attack defended individuals at levels determined by their current toxin burden. European starlings learned to discriminate between sequentially presented defended and undefended mealworms with different color signals. Once birds had learned to avoid the defended prey at a stable asymptotic level, we experimentally increased their toxin burdens, which reduced the number of defended prey that they ingested in the subsequent trial. This was due to the birds making strategic decisions to ingest defended prey on the basis of their visual signals. Birds are clearly able to learn about the nutritional benefits and defensive costs of eating defended prey, and they regulate their intake according to their current physiological state. This raises new perspectives on the evolution of aposematism, mimicry, and defense chemistry.     

Aversive Learning in the Praying Mantis (<Emphasis Type="Italic">Tenodera aridifolia</Emphasis>), a Sit and Wait Predator

Animals learn to associate sensory cues with the palatability of food in order to avoid bitterness in food (a common sign of toxicity). Associations are important for active foraging predators to avoid unpalatable prey and to invest energy in searching for palatable prey only. However, it has been suggested that sit-and-wait predators might rely on the opportunity that palatable prey approach them by chance: the most efficient strategy could be to catch every available prey and then decide whether to ingest them or not. In the present study, we investigated avoidance learning in a sit-and-wait predator, the praying mantis (Tenodera aridifolia). To examine the effects of conspicuousness and novelty of prey on avoidance learning, we used three different prey species: mealworms (novel prey), honeybees (novel prey with conspicuous signals) and crickets (familiar prey). We sequentially presented the prey species in pairs and made one of them artificially bitter. In the absence of bitterness, the mantises consumed bees and crickets more frequently than mealworms. When the prey were made bitter, the mantises still continued to attack bitter crickets as expected. However, they reduced their attacks on bitter mealworms more than on bitter bees. This contrasts with the fact that conspicuous signals (e.g. coloration in bees) facilitate avoidance learning in active foraging predators. Surprisingly, we found that the bitter bees were totally rejected after an attack whereas bitter mealworms were partially eaten (~35%). Our results highlight the fact that the mantises might maintain a selection pressure on bees, and perhaps on aposematic species in general.     

Benefit by contrast: an experiment with live aposematic prey

Aposematic prey often have a coloration that contrasts withthe background. One beneficial effect of such conspicuous colorationis that it produces faster and more durable avoidance by predators.Another suggested benefit is that prey that contrast with thebackground are more quickly discerned and recognized as unpalatableby experienced predators. To further investigate the effectsof prey contrast on predator behavior, I conducted an experimentwith young chicks (Gallus gallus domesticus) as predators onlive aposematic and nonaposematic prey. Birds with prior experienceof both prey types were allowed into an arena with both palatableprey and aposematic prey on backgrounds that either closelymatched or contrasted with the coloration of the aposematicprey. Also, the time a bird had available to decide to attacka prey was manipulated by including a competing chick or not.The experienced birds showed greater attack latencies for aposematicprey on more contrasting backgrounds, and aposematic prey werealso attacked to a greater extent when on a matching background.The presence of a competitor generated similar effects, wherebirds in high competition attacked more and faster comparedto birds subjected to lower degree of competition, but therewas no interaction between competition and contrast. Thus,the experiment provides evidence that prey contrast againstthe background may produce better recognition and avoidance,independently of predator viewing time.     

Bioluminescence in firefly larvae: A test of the aposematic display hypothesis (Coleoptera: Lampyridae)

We investigated the hypothesis that bioluminescence in firefly larvae (Coleoptera: Lampyridae) functions as an aposematic display. In two experiments, we confirmed the distastefulness of firefly larvae, and tested the hypothesis that a naive, nocturnal predator can learn to use light signals as aposematic cues for avoiding distasteful prey. Larvae were rejected as acceptable prey by 100% of the house mice (Mus musculus) tested. Mice learned to avoid bitter food associated with light cues significantly faster (P=0.003) than mice presented with food lacking light cues. We conclude that luminescent glowing in firefly larvae meets the requirements of an aposematic signal.     

THE EFFECT OF BIASED INCLUSION OF TAXA ON THE CORRELATION BETWEEN DISCRETE CHARACTERS IN PHYLOGENETIC TREES

In a published paper, a method for testing the correlation between two discrete characters was presented and applied to test whether in butterfly larvae origins of gregariousness are concentrated to lineages with aposematic coloration. The relationship was found to be nonsignificant. However, the butterfly data on which the test was applied had been compiled in another study to investigate evolutionary sequences and was biased, because there was an overrepresentation of aposematic, as compared to cryptic, branches in the sample. In the paper presented here, aposematic and cryptic clades of the original phylogeny were resolved to the same degree, and the resulting set of branches may be regarded as unbiased with respect to the hypothesis being tested. A method for testing the contingency of states in two characters was then applied to the new data set, resulting in a highly significant relationship between origins of gregariousness and aposematic coloration. I argue that when using statistical methods on phylogenetic data, it is crucial to resolve various parts of the phylogeny to the same comparable systematic unit in order not to get a distorted sample of taxa/branches.     

Contrast versus colour in aposematic signals

Conspicuousness is an important feature of warning coloration. One hypothesis for its function is that it increases signal efficacy by facilitating avoidance learning. An alternative, based on the handicap hypothesis, suggests that the degree of conspicuousness holds information directly about the quality of the prey, and that predators associate and learn about the conspicuousness of the coloration, and not the actual colour pattern. We studied the relative importance of signal contrast and the colours of signals for predator attention during discrimination. We used young chicks, Gallus gallus domesticus, as predators and small blue or red paper cones on either matching or contrasting paper backgrounds as stimuli associated with palatable or unpalatable chick crumbs. In four treatment groups, birds could use either cone and/or background colour, cone colour only, background colour only or cone-to-background contrast as cues for discrimination. Only birds in the contrast treatment failed to learn their discrimination task. Birds that had a choice between cone and background colour as cues used the cone colour and they learned the task faster than did birds that had to use background colour as a cue. The results suggest that birds primarily attend to the colours of signals and disregard contrast in discrimination tasks; they thus fail to support a handicap function of conspicuous aposematic coloration. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.