Aposematic (warning) coloration associated with thorns in higher plants

Aposematic coloration, a well-known phenomenon in animals, has been given little attention in plants. Here I discuss two types of conspicuousness of thorns which are typical of many plant species: (1) colorful thorns, and (2) white spots, or white and colorful stripes, associated with thorns in leaves and stems. Both types of aposematic coloration predominate the spine system of taxa rich with spiny species-Cacti, the genera Agave, Aloe and Euphorbia. The phenomena have been recorded here in over a thousand species originating in several continents of both the Old and New World. I propose that this is a case of vegetal aposematic coloration analogous to such coloration of poisonous animals, and which communicates between plants and herbivores.     

The Potential Anti-Herbivory Role of Microorganisms on Plant Thorns

Thorns, spines and prickles are some of the anti-herbivore defenses that plants have evolved. They were recently found to be commonly aposematic (warning coloration). However, the physical anti-herbivore defense executed by these sharp structures seems to be only the tip of the iceberg. We show that thorns of various plant species commonly harbor an array of aerobic and anaerobic pathogenic bacteria including Clostridium perfringens the causative agent of the life-threatening gas gangrene, Bacillus anthracis, and Pantoea agglomerans. Septic inflammation caused by plant thorn injury can result not only from bacteria. Medical literature indicates that thorns, spines or prickles also introduce pathogenic fungi into animals or humans. Dermatophytes that cause subcutaneous mycoses are unable to penetrate the skin and must be introduced into the subcutaneous tissue by a puncture wound. The common microorganism-thorn combinations seem to have been an important contributor to the fact that so many plant thorns are aposematically colored, as a case of convergent evolution of aposematism in these organisms.Key Words: aposematism, herbivory, pathogen, spine, thorn, bacillus anthracis, clostridium perfringens, sporotrichosis, Mycetoma, subcutaneous mycotic disease     

Effects of spines and thorns on Australian arid zone herbivores of different body masses

Summary We investigated the effects of thorns and spines on the feeding of 5 herbivore species in arid Australia. The herbivores were the rabbit (Oryctolagus cuniculus), euro kangaroo (Macropus robustus), red kangaroo (Macropus rufus), sheep (Ovis aries), and cattle (Bos taurus). Five woody plants without spines or thorns and 6 woody plants with thorns were included in the study. The spines and thorns were not found to affect the herbivores' rates of feeding (items ingested/min), but they did reduce the herbivores' rates of biomass ingestion (g-dry/item). The reduction in biomass ingested occurred in two ways: at a given diameter, twigs with spines and thorns had less mass than undefended plants, and the herbivores consumed twigs with smaller diameters on plants with spines and thorns. The relative importance of the two ways that twigs with spines and thorns provided less biomass varied with herbivore body mass. Reduced twig mass was more important for small herbivores, while large herbivores selected smaller diameters. The effectiveness of spines and thorns as anti-herbivore defenses did not vary with the evolutionary history of the herbivores (i.e. native vs. introduced). Spines and thorns mainly affected the herbivores' selection of maximum twig sizes (reducing diameter and mass), but the minimum twig sizes selected were also reduced.     

Potential aposematism in an insular tree species: are signals dishonest early in ontogeny?

Recent investigations have suggested that some plants are aposematic. Our understanding of how aposematism varies through plant ontogeny, however, is incomplete. Furthermore, the potential for lower leaf surfaces to signal to vertebrate herbivores that are viewing leaves from below has not been investigated. Here, we investigate ontogenetic changes in leaf colour in Pseudopanax crassifolius (Araliaceae), a tree species that is endemic to New Zealand. We demonstrate that P. crassifolius produces lateral leaf spines that peak in size during the sapling stage of development. Spots of brightly coloured tissues on the upper leaf surfaces may be warning signals. The intensity of these signals, however, peaked at the seedling stage, providing a dishonest signal of defence. Conversely, signals on lower leaf surfaces peaked in the sapling stage, providing an honest defensive signal later in ontogeny. Lateral leaf spines and all potential warning colours were absent in adults, after they grow above the reach of the largest known native megaherbivores (moa – Aves: Dinornithiformes). Overall, these results suggest that aposematism may vary predictably through plant ontogeny in response to the changing perspective of herbivores as plants grow vertically.     

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

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

植物警戒色的研究进展

植物为适应植食动物的取食压力而进化出物理、化学等多种防御机制,以把植食伤害降到最低程度,但动物不断的抽样尝试行为还是让有防御行为的植物受到伤害。因此,向潜在的植食动物传达自己的防御信号对植物是有益的。颜色作为一种稳定有效的视觉信号通常是花和果实的诱惑信号,某些情况下也是一种警戒防御信号,植食动物经过抽样学习后能识别这种防御信号并主动回避,从而形成了植物的警戒色。起源于猎物-捕食者关系的警戒色理论在动物界得到了充分研究,但植物警戒色却不为人所知,直到2001年Hamilton关于秋季树叶颜色的信号假说公开发表后,才引起人们对植物警戒色的初步研究。如今在早秋变色树种、幼叶、多剌植物、植物繁殖器官都发现了警戒色的一些例证,尽管有些还不太明确甚至存在争议,但至少为植物警戒色的进一步研究奠定了基础。植物营养体颜色在时空上的多态性变化值得人们更深入地研究,防御权衡假说也预示了防御有害植食动物的警戒作用存在于繁殖器官的可能性,研究它们生理和生态适应意义有利于人们更深程度地理解植物-动物之间的复杂关系。     

Weapon (thorn) automimicry and mimicry of aposematic colorful thorns in plants

In order to further characterize the function of coloration in plants as defense against herbivory, two types of thorn mimicry are described: (1) A unique type of weapon (thorn) automimicry (within the same individual) that was previously known only in animals, and (2) mimicry of aposematic colorful thorns, by colorful elongated and pointed plant organs (buds, leaves and fruit) that, despite their appearance, are not sharp. Some thorny plants including dozens of species of Agave, one species of Aloe and a palm species have thorn-like imprints or colorations on their leaves, constituting thorn automimicry by giving the impression of more extensive thorns. The mimicry of aposematic colorful thorns is a typical case of Batesian mimicry, but the thorn automimicry is a special intra-organismic Batesian mimicry. I propose that both types of mimicry serve as anti-herbivore mechanisms.     

Müllerian mimicry in aposematic spiny plants

Müllerian mimicry is common in aposematic animals but till recently, like other aspects of plant aposematism was almost unknown. Many thorny, spiny and prickly plants are considered aposematic because their sharp defensive structures are colorful and conspicuous. Many of these spiny plant species (e.g., cacti and Agave in North American deserts; Aloe, Euphorbia and acacias with white thorns in Africa; spiny plants in Ohio; and spiny members of the Asteraceae in the Mediterranean basin) have overlapping territories, and also similar patterns of conspicuous coloration, and suffer from the evolutionary pressure of grazing by the same large herbivores. I propose that many of these species form Müllerian mimicry rings.Key words: aposematic coloration, defense, evolution, herbivory, müllerian mimicry, spines, thornsAposematic (warning) coloration is a biological phenomenon in which poisonous, dangerous or otherwise unpalatable organisms visually advertise these qualities to other animals. The evolution of aposematic coloration is based on the ability of target enemies to associate the visual signal with the risk, damage or non-profitable handling, and later to avoid such organisms as prey. Typical colors of aposematic animals are yellow, orange, red, purple, black, white or brown and combinations of these.^1–5 Many thorny, spiny and prickly plant species were proposed to be aposematic because their sharp defensive structures are usually colorful (yellow, orange, red, brown, black, white) and/or associated with similar conspicuous coloration.^5–22 Animal spines also have similar conspicuous coloration and were proposed to be aposematic.^1,5,17,23Several authors have proposed that mimicry of various types helps in plant defense, e.g.,^9,24–34 More specifically, Müllerian mimicry was already proposed to exist in several defensive plant signaling systems. The first was for several spiny species with white-variegated leaves.^8,10 The second was for some tree species with red or yellow poisonous autumn leaves.³⁵ The third cases are of a mixture of Müllerian and Batesian mimicry, of thorn auto-mimicry found in many Agave species.⁸Here I propose that many species of visually aposematic spiny plants of the following taxa: (1) Cactaceae, (2) the genus Agave, (3) the genus Aloe, (4) African thorny members of the genus Euphorbia, (5) African acacias with white thorns, (6) spiny vascular plants of southeastern Ohio, (7) spiny Near Eastern plants with white variegation on their leaves, (8) Near Eastern members of the Asteraceae with yellow spines, form Müllerian mimicry rings of spiny plants.To consider the existence of Müllerian mimicry rings in aposematic organisms, two factors are needed: (1) a similar signal, and (2) an overlapping distribution in respect to the territory of predators in animals, or herbivores in plants. I will show below that for the plant taxa proposed here to form Müllerian mimicry rings, both criteria operate.The accumulating data about the common association of plant defenses by spines with visual conspicuousness, along with the fact that many such species overlap in their habitat, raises the possibility of the broad phenomenon of existence of Müllerian mimicry rings in plants. Even from the limited number of publications proposing visual aposematism in spiny plants, the operation of vegetal Müllerian mimicry rings seems to be obvious. The phenomenon can now be traced to both the Old World (Asia, Africa and Europe) and the New World (North America). The best-studied cases include Cactaceae and the genera Agave, Aloe and Euphorbia,⁶ African acacias with white thorns,^12,15 Near Eastern spiny plants with white variegation on their leaves,^7,11 aposematic spiny vascular plants of southeastern Ohio,¹⁶ and many spiny Mediterranean species of the Asteraceae with yellow spines.²²In the four spiny taxa (Cactaceae and the genera Agave, Aloe and Euphorbia) that were the first to be proposed as visually aposematic⁶ there is a very strong morphological similarity. In cacti, there are two types of conspicuousness of spines that are typical of many plant species: (1) colorful spines, and (2) white spots, or white or colorful stripes, associated with spines on the stems. These two types of aposematic coloration also dominate the spine system of Agave, Aloe and Euphorbia. The fact that many species of three of these four spiny taxa (Agave, Aloe and Euphorbia) are also poisonous^36–38 further indicates their potential to form Müllerian mimicry rings.I propose that each of these groups for itself and some of these groups (e.g., Cactaceae and the genus Agave in North America; Aloe, Euphorbia and acacias in east and south Africa) that have overlapping distribution and share at least some of the herbivores, form Müllerian mimicry rings.The first Müllerian mimicry ring is of cacti and Agave that have an overlapping distribution over large areas in North America.^37,39 The large herbivores in North America disappeared not so long ago in evolutionary time scales and seem to have shaped the spiny defense of these plant taxa.⁴⁰The second Müllerian mimicry ring is of the spiny and thorny members of the African genera Aloe, Euphorbia and certain acacias with very conspicuous white thorns, which partly overlap in distribution and share various large mammalian herbivores.^12,15,36,41The third Müllerian mimicry ring is the outcome of the common presence of aposematic coloration in spiny vascular plants of southeastern Ohio,¹⁶ with color patterns in thorns and spines similar to those of Cactaceae and the genera Agave, Aloe and Euphorbia described in Lev-Yadun.⁶The next case of potential operation of Müllerian mimicry ring of spiny plants with overlapping territories that suffer from the same large herbivores, but on a much smaller geographical scale, has recently been proposed for several spiny species with white-variegated leaves,⁷ and later for more than 20 spiny species in the flora of Israel that have white markings associated with their spines.¹¹The last case of a probable Müllerian mimicry ring was described by Ronel et al.²² who while studying the spine system of Near Eastern spiny members of the Asteraceae, found 29 spiny species with yellow spines, and additional such species are expected to occur. Since some of these species and others with yellow spines also grow in southern Europe, it is clear that the same phenomenon is also common there.I conclude that Müllerian mimicry rings seem to be very common in plants, and that it is probable that many other spiny plants that form Müllerian mimicry rings are waiting to be studied. Such defensive rings are probably also formed by poisonous plants that share similar colors or odors.     

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.     

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

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

Aposematism: what should our starting point be?

The evolution of aposematism is considered to be a major evolutionary problem because if new aposematic forms emerged in defended cryptic populations, they would face the dual problems of rarity and conspicuousness. We argue that this commonly assumed starting point might not have wide validity. We describe a novel evolutionary computer model in which prey evolve secondary defences and become conspicuous by moving widely over a visually heterogeneous habitat. Unless crypsis imposes high opportunity costs (for instance, preventing prey from efficient foraging, thermoregulation and communication), costly secondary defences are not predicted to evolve at all. However, when crypsis imposes opportunity costs, prey evolve secondary defences that facilitate raised behavioural conspicuousness as prey exploit opportunities within their environment. Optimal levels of secondary defence and of behavioural conspicuousness increase with population sizes and the costs imposed by crypsis. When prey are already conspicuous by virtue of their behaviours, the evolution of aposematic appearances (bright coloration, etc.) is much easier to explain because aposematic traits add little further costs of conspicuousness, but can bring large benefits.     

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.     

Avian predators taste-reject aposematic prey on the basis of their chemical defence

Avian predators learn to avoid defended insects on the basis of their conspicuous warning coloration. In many aposematic species, the level of chemical defence varies, with some individuals being more defended than others. Sequestration and production of defence chemicals is often costly and therefore less defended individuals enjoy the benefits of the warning signal without paying the full costs of chemical production. This is a fundamental theoretical problem for the evolutionary stability of aposematism, since less defended individuals appear to be at a selective advantage. However, if predators sample aposematic prey and selectively reject individuals on the basis of their chemical investment, aposematism could become evolutionarily stable. Previous research aimed at testing whether birds can use taste to discriminate between palatable and unpalatable prey has been confounded by other experimental factors. Here, we show that birds can taste and reject prey entirely on the basis of an individual's level of chemical defence and more importantly, they can make decisions on whether or not to consume a defended individual based upon their level of chemical investment. We discuss these results in relation to the evolution of aposematism, mimicry and defence chemistry.     

THERMOREGULATION CONSTRAINS EFFECTIVE WARNING SIGNAL EXPRESSION

Evolution of conspicuous signals may be constrained if animal coloration has nonsignaling as well as signaling functions. In aposematic wood tiger moth ( Parasemia plantaginis ) larvae, the size of a warning signal (orange patch on black body) varies phenotypically and genetically. Although a large warning signal is favored as an antipredator defense, we hypothesized that thermoregulation may constrain the signal size in colder habitats. To test this hypothesis, we conducted a factorial rearing experiment with two selection lines for larval coloration (small and large signal) and with two temperature manipulations (high and low temperature environment). Temperature constrained the size and brightness of the warning signal. Larvae with a small signal had an advantage in the colder environment, which was demonstrated by a faster development time and growth rate in the low temperature treatment, compared to larvae with a large signal. Interestingly, the larvae with a small signal were found more often on the plant than the ones with a large signal, suggesting higher basking activity of the melanic (small signal) individuals in the low temperature. We conclude that the expression of aposematic display is not only defined by its efficacy against predators; variation in temperature may constrain evolution of a conspicuous warning signal and maintain variation in it.     

Beyond pollinator attraction: extra-floral displays deter herbivores in a Mediterranean annual plant

The evolution of several floral traits is thought to be driven by multiple selective agents, including pollinators and herbivores. Similar combinations of selection pressures may have shaped extra-floral traits. The conspicuous purple tufts of leaves (“flags”), which often terminate vertical inflorescences in the Mediterranean annual Salvia viridis, were shown to attract insect pollinators to the flowering patch. Here we test whether they also function as anti-herbivore signals. We determined the aposematic potential of S. viridis flags on three levels: concentrations of anthocyanins, suggested to function as aposematic visual signals, in leaves and flags; spectrometry to estimate whether the color-vision system of two common Mediterranean generalist herbivores (locusts and goats) can discriminate flags from leaves; and choice experiments to determine food preferences of the same herbivores. Anthocyanin concentrations in flags were >10-fold higher than in leaves. Flags exhibited peak reflectance at 450 and 700 nm wavelengths, while leaves reflected maximally at 550 nm. According to the Vorobyev-Osorio color vision model, these differences in color reflection are likely to allow visual discrimination by herbivores. Goats preferred feeding on clipped inflorescences over control inflorescences. Locusts preferred leaves over flags. To test whether this was due to deterrence from the flags’ coloration, we also offered them choice between leaves and a rare, white morph, of the flags. The locusts chose both equally immediately after presentation, but leaves attracted more individuals after 5 min of feeding. The locusts also preferred green cabbage over anthocyanin-rich red cabbage. These results support the function of colorful extra-floral displays as warning signals.     

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.     

Frequency-dependent taste-rejection by avian predation may select for defence chemical polymorphisms in aposematic prey

Chemically defended insects advertise their unpalatability to avian predators using conspicuous aposematic coloration that predators learn to avoid. Insects utilize a wide variety of different compounds in their defences, and intraspecific variation in defence chemistry is common. We propose that polymorphisms in insect defence chemicals may be beneficial to insects by increasing survival from avian predators. Birds learn to avoid a colour signal faster when individual prey possesses one of two unpalatable chemicals rather than all prey having the same defence chemical. However, for chemical polymorphisms to evolve within a species, there must be benefits that allow rare chemical morphs to increase in frequency. Using domestic chicks as predators and coloured crumbs for prey, we provide evidence that birds taste and reject proportionally more of the individuals with rare defence chemicals than those with common defence chemicals. This indicates that the way in which birds attack and reject prey could enhance the survival of rare chemical morphs and select for chemical polymorphism in aposematic species. This is the first experiment to demonstrate that predators can directly influence the form taken by prey's chemical defences.     

Effects of plant spinescence on large mammalian herbivores

Summary Plant thorns and spines had these effects on the feeding behaviour of the three species of browsing ungulate that we studied, kudu, impala and domestic goats: (i) bite sizes were restricted, in most cases to single leaves or leaf clusters; (ii) hooked thorns retarded biting rates; (iii) the acceptability of those plant species offering small leaf size in conjunction with prickles was lower, at least for the kudus, than those of other palatable plant species; (iv) the inhibitory effect of prickles on feeding was much less for the smaller impalas and goats than for the larger kudus; (v) from certain hook-thorned species the kudus bit off shoot ends despite their prickles; (vi) for certain straight-thorned species the kudus compensated partially for the slow eating rates obtained by extending their feeding durations per encounter. Most spinescent species were similar in their acceptability to the ungulates to unarmed palatable species, despite higher crude protein contents in their foliage than the latter. Such structural features furthermore reduce the tissue losses incurred by plants per encounter by a large ungulate herbivore, by restricting the eating rates that the animals obtain. In this way prickles function to restrict foliage losses to large herbivores below the levels that might otherwise occur.     

Aposematic coloration does not deter corallivory by fish on the coral <Emphasis Type="Italic">Montastraea cavernosa</Emphasis>

Predation on corals by visual predators is a significant source of partial or total mortality on coral reefs, and corals have evolved strategies, including chemical defenses, to deter predation. One mechanism that organisms use to communicate the presence of chemical defenses is aposematic coloration, or the display of bright coloration as a warning to visual predators such as fish. Corals exhibit multiple colors, and it has been hypothesized that one role for this variability in coloration is as an aposematic warning of adverse palatability. Here, we test green and orange color morphs of the Caribbean coral Montastraea cavernosa for the presence of chemical defenses and whether their differences in coloration elicited different feeding responses. While M. cavernosa is chemically defended, there is no difference in feeding deterrence between color morphs; thus, the different color morphs of this coral species do not appear to represent an example of aposematic 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.