Gabriella Gamberale-Stille

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 predators 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.

Domestic chicks primarily attend to colour, not pattern, when learning an aposematic coloration

Aposematic conspicuous coloration consists of one or a few bright colours, often in combination with a black defined internal pattern. The function of conspicuousness in aposematism has been ascrib ...

Fruit or aposematic insect? Context-dependent colour preferences in domestic chicks

Colours are common stimuli in signalling systems. Requirements to function well as a signal sometimes conflict between different signallers, and the same colour stimulus is used to convey completely different messages to the same receiver. Fruits and aposematic insects both use red coloration as a signal, in the former case to signal profitability and in the latter case as a warning signal. In two experiments, we investigated whether the domestic chick, an omnivorous predator, differed in its unconditioned preference or avoidance of red and green stimuli depending on whether or not the stimulus was an insect. The experiments were designed as preference tests between red and green painted prey. The prey were live insects and artificial fruits (experiment 1), and, to investigate the effect of movement, live and dead insects (experiment 2). The chicks did not show any difference in pecking preference between red and green when fruit–like stimuli were used, but when the prey were insects, green prey were strongly preferred to red prey, and prey movement did not affect this bias. Thus, young chicks may recognize prey as insects and then discriminate between different prey colorations, or one type of food may elicit an unlearned colour preference–avoidance response that is absent with another type of food.

Stimulus Salience as an Explanation for Imperfect Mimicry

UNLABELLED The theory of mimicry explains how a mimic species gains advantage by resembling a model species [1-3]. Selection for increased mimic-model similarity should then result in accurate mimicry, yet there are many surprising examples of poor mimicry in the natural world [4-8]. The existence of imperfect mimics remains a major unsolved conundrum. We propose and experimentally test a novel explanation of the phenomenon. We argue that predators perceive prey as having several traits, but that the traits differ in their importance for learning. When predators learn to discriminate prey, high-salience traits overshadow other traits, leaving them under little or no selection for similarity, and allow imperfect mimicry to succeed. We tested this idea experimentally, using blue tits as predators and artificial prey with three prominent traits: color, pattern, and shape. We found that otherwise imperfect color mimics were avoided about as much as perfect mimics, whereas pattern and shape mimics did not gain from their similarity to the model. All traits could separately be perceived and learned by the predators, but the color trait was learned at a higher rate, implying that it had higher salience. We conclude that difference in salience between components of prey appearance is of major importance in explaining imperfect mimicry. VIDEO ABSTRACT

Effects of food plant and group size on predator defence: differences between two co-occurring aposematic Lygaeinae bugs

1. The role of food plant and aggregation on the defensive properties of two aposematic sympatrically occurring seed bugs, Tropidothorax leucopterus and Lygaeus equestris (Lygaeinae, Heteroptera), was investigated. Larvae reared on seeds either of their natural host plant Vincetoxicum hirundinaria (Asclepiadaceae) or of sunflower Helianthus annuus were subjected to predation by chicks.

FEATURE SALTATION AND THE EVOLUTION OF MIMICRY

In Batesian mimicry, a harmless prey species imitates the warning coloration of an unpalatable model species. A traditional suggestion is that mimicry evolves in a two‐step process, in which a large mutation first achieves approximate similarity to the model, after which smaller changes improve the likeness. However, it is not known which aspects of predator psychology cause the initial mutant to be perceived by predators as being similar to the model, leaving open the question of how the crucial first step of mimicry evolution occurs. Using theoretical evolutionary simulations and reconstruction of examples of mimicry evolution, we show that the evolution of Batesian mimicry can be initiated by a mutation that causes prey to acquire a trait that is used by predators as a feature to categorize potential prey as unsuitable. The theory that species gain entry to mimicry through feature saltation allows us to formulate scenarios of the sequence of events during mimicry evolution and to reconstruct an initial mimetic appearance for important examples of Batesian mimicry. Because feature‐based categorization by predators entails a qualitative distinction between nonmimics and passable mimics, the theory can explain the occurrence of imperfect mimicry.

FEATURE THEORY AND THE TWO-STEP HYPOTHESIS OF MÜLLERIAN MIMICRY EVOLUTION

The two‐step hypothesis of Müllerian mimicry evolution states that mimicry starts with a major mutational leap between adaptive peaks, followed by gradual fine‐tuning. The hypothesis was suggested to solve the problem of apostatic selection producing a valley between adaptive peaks, and appears reasonable for a one‐dimensional phenotype. Extending the hypothesis to the realistic scenario of multidimensional phenotypes controlled by multiple genetic loci can be problematic, because it is unlikely that major mutational leaps occur simultaneously in several traits. Here we consider the implications of predator psychology on the evolutionary process. According to feature theory, single prey traits may be used by predators as features to classify prey into discrete categories. A mutational leap in such a trait could initiate mimicry evolution. We conducted individual‐based evolutionary simulations in which virtual predators both categorize prey according to features and generalize over total appearances. We found that an initial mutational leap toward feature similarity in one dimension facilitates mimicry evolution of multidimensional traits. We suggest that feature‐based predator categorization together with predator generalization over total appearances solves the problem of applying the two‐step hypothesis to complex phenotypes, and provides a basis for a theory of the evolution of mimicry rings.

Colour and pattern similarity in mimicry: evidence for a hierarchical discriminative learning of different components

Many aposematic species combine their bright colours with a black pattern that produces internal contrasts. Studies have shown that birds often pay attention to some parts of a signalling pattern a ...

Seasonal ontogenetic colour plasticity in the adult striated shieldbug Graphosoma lineatum (Heteroptera) and its effect on detectability

Camouflage and warning colouration are two important forms of protective colouration. We have studied the detectability of two seasonal colourations in the aposematic striated shieldbug, Graphosoma lineatum. The typical colouration of this insect is red and black, which is also the colouration of the reproductive post-hibernation bugs in our study area in south central Sweden. However, the majority of newly eclosed adults in late summer exhibit a ‘pale’ (light brownish, non-red) and black striation, and these bugs appear quite cryptic to the human eye when sitting on the dried stems and umbels of their host plants. In experiments using photographs of prey in the late-summer habitat shown on a computer screen, we compared the time to detection by human subjects of bugs, which had been manipulated to show either of the two typical seasonal colourations. Time to detection was significantly longer for the pale and black than for the red-and-black striation in images with the bug photographed at two different distances. This indicates that the pale pre-hibernation striation may have a cryptic function. In a separate experiment, we tested detectability of striated and non-striated manipulations of bug pre-hibernation colouration against the late-summer background, and found that time to detection was significantly longer for the striated bugs. We discuss potential functional explanations for the seasonal ontogenetic colour plasticity and suggest that the epidermal pale colour in the late summer provides a benefit of increased camouflage.

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.