First evidence for an aposematic function of a very common color pattern in small insects

Many small parasitoid wasps have a black head, an orange mesosoma and a black metasoma (BOB color pattern), which is usually present in both sexes. A likely function of this widespread pattern is aposematic (warning) coloration, but this has never been investigated. To test this hypothesis, we presented spider predators (Lyssomanes jemineus), both field-captured and bred in captivity from eggs, to four wasp genera (Baryconus, Chromoteleia, Macroteleia and Scelio), each genus being represented by a BOB morphospecies and black morphospecies. We also used false prey, consisting of lures made of painted rice grains. Behavioral responses were analyzed with respect to presence or absence of the BOB pattern. In order to better understand the results obtained, two additional studies were performed. First, the reflection spectrum of the cuticle of the wasp and a theoretical visual sensibility of the spider were used to calculate a parameter we called “absorption contrast” that allows comparing the perception contrast between black and orange in each wasp genus as viewed by the spider. Second, acute toxicity trials with the water flea, Daphnia magna, were performed to determine toxicity differences between BOB and non-BOB wasps. At least some of the results suggest that the BOB color pattern may possibly play an aposematic role.

The Natural World

This chapter examines how threats, counterthreats, warnings, feints, and deceptions are found throughout the natural world, in the daily lives of animals and even plants. Indeed, these can be seen in plants with thorns and poisons, as well as in animals growling, roaring, baring teeth, showing and exaggerating their weapons (or pretending to have weapons), misrepresenting their ferocity, puffing themselves up, and generally seeking to intimidate their rivals or potential predators. The chapter then considers the role of honesty versus deception: the evolution of warning coloration, whereby brightly colored poison arrow frogs, for example, inform would-be predators that eating them would be a bad idea; and mimicry, in which animals who are not themselves especially dangerous resemble others that are harmful to their predators and thus gain protection via the “empty threat” the former conveys. This, in turn, speaks to the intriguing question of whether a given threat is real or fake, honest or dishonest, and what difference—if any—this makes. The chapter also explains the hawk–dove model of the variations of animal threat, and looks at vocal threats and animal eavesdropping.

First evidence for an aposematic function of a very common color pattern in small insects

Many small parasitoid wasps have a black-orange-black (BOB) color pattern, which is usually present in both sexes. A likely function of this widespread pattern is aposematic (warning) coloration, but this has never been investigated. To test this hypothesis, we presented spider predators (Lyssomanes jemineus), both field-captured and lab-reared individuals, to a species with the BOB pattern and a congeneric all-black species in each of four scelionid genera (Baryconus, Chromoteleia, Macroteleia and Scelio). Each spider/wasp trial was filmed for 40 minutes under controlled conditions and three behavioral responses (detect, attack, avoid) were recorded in each of 136 trials, never using the same predator and prey more than once. In order to better understand the results obtained, two additional studies were performed. First, the reflection spectrum of the cuticle of the wasp and a theoretical visual sensibility model of the spider were used to calculate a parameter we called “absorption contrast” that allowed us to compare the perception contrast between black and orange in each wasp genus as viewed by the spider. Second, acute toxicity trials with the water flea, Daphnia magna, were performed to determine toxicity differences between BOB and non-BOB wasps. By combining the results from the three types of experiments, together with a statistical analysis, we confirmed that BOB color pattern plays an aposematic role.

Out in the open: behavior’s effect on predation risk and thermoregulation by aposematic caterpillars

Warning coloration should be under strong stabilizing selection but often displays considerable intraspecific variation. Opposing selection on color by predators and temperature is one potential explanation for this seeming paradox. Despite the importance of behavior for both predator avoidance and thermoregulation, its role in mediating selection by predators and temperature on warning coloration has received little attention. Wood tiger moth caterpillars, Arctia plantaginis, have aposematic coloration, an orange patch on the black body. The size of the orange patch varies considerably: individuals with larger patches are safer from predators, but having a small patch is beneficial in cool environments. We investigated microhabitat preference by these caterpillars and how it interacted with their coloration. We expected caterpillar behavior to reflect a balance between spending time exposed to maximize basking and spending time concealed to avoid detection by predators. Instead, we found that caterpillars preferred exposed locations regardless of their coloration. Whether caterpillars were exposed or concealed had a strong effect on both temperature and predation risk, but caterpillars in exposed locations were both much warmer and less likely to be attacked by a bird predator (great tits, Parus major). This shared optimum may explain why we observed so little variation in caterpillar behavior and demonstrates the important effects of behavior on multiple functions of coloration.

Geographic mosaic of selection by avian predators on hindwing warning colour in a polymorphic aposematic moth

Warning signals are predicted to develop signal monomorphism via positive frequency-dependent selection (+FDS) albeit many aposematic systems exhibit signal polymorphism. To understand this mismatch, we conducted a large-scale predation experiment in four locations, among which the frequencies of hindwing warning coloration of aposematic Arctia plantaginis differ. Here we show that selection by avian predators on warning colour is predicted by local morph frequency and predator community composition. We found +FDS to be strongest in monomorphic Scotland, and in contrast, lowest in polymorphic Finland, where different predators favour different male morphs. +FDS was also found in Georgia, where the predator community was the least diverse, whereas in the most diverse avian community in Estonia, hardly any models were attacked. Our results support the idea that spatial variation in predator and prey communities alters the strength or direction of selection on warning signals, thus facilitating a geographic mosaic of selection.

Using three-dimensional printed models to test for aposematism in a carabid beetle

Many studies have demonstrated that bright colours sometimes evolve as warning coloration on the bodies of distasteful prey. However, few studies have demonstrated that the bright structural colours of beetles function as such aposematic signals for predators in the wild. To determine whether body colour might act as an aposematic signal in the carabid beetle Damaster blaptoides, we generated beetle models and conducted camera-trap and field experiments. Elaborate beetle models produced using a three-dimensional printer were used to determine which animals attack them in the wild. Red and black models were placed in forests to test which of the two types was attacked the least frequently. The camera-trap experiments indicated that mammals and birds were the potential predators of D. blaptoides. The field experiments revealed that predators attacked the red models significantly less frequently than the black models in each of three sites where red Damaster subspecies were distributed. In three sites where black Damaster subspecies were distributed, predators attacked both red and black models at similar rates. These results might imply that the predators learned more easily to avoid distasteful red beetles rather than black ones.

Uncovering the effects of Müllerian mimicry on the evolution of conspicuousness in colour patterns

The conspicuousness of colour pattern in defended species associates with a high detectability by predators, making its evolution puzzling. Müllerian mimicry, the convergence of warning coloration among defended prey species, is pervasive in communities of conspicuous prey, and mimicry switches, with mutant individuals having the same colour pattern as other co-mimetic species, may often associate with changes in conspicuousness. Yet, the implication of mimicry for the evolution of conspicuousness has not been considered. Here, we build a model describing the population dynamics of conspicuous defended prey to explore the invasion conditions of mutants that differ from other individuals by their conspicuousness. We assume that predation risk depends not only on the number of individuals sharing a given colour pattern within the population but also on the presence of co-mimetic species. We compare the evolutionary fates of mutant colour patterns (1) that are similar to the ancestral colour pattern and thus belong to the same mimicry ring (assemblage of co-mimetic species), or (2) that are different from the ancestral colour pattern and thus potentially belong to a distinct mimicry ring. Our analytical derivations show that (1) less conspicuous colour patterns are more likely to be selected within mimicry ring, and that (2) a mimicry switch lowering predation risk can promote the invasion of a more conspicuous colour pattern. We thus highlight that the variation in conspicuousness observed in the wild results not only from the characteristics of the colour pattern (detectability, salience) but also from the local composition of mimetic communities.