A simulation of evasive mimicry in the wild

Selection for evasive mimicry imposed by an arthropod predator

It has long been hypothesized that a species that is relatively easy to catch by predators may face selection to resemble a species that is harder to catch. Several experiments using avian predators have since supported this 'evasive mimicry' hypothesis. However, the sudden movement of artificial evasive prey in each of the above experiments may have startled the predators, generating an avoidance response unrelated to difficulty of capture. Additionally in the above experiments the catchability of prey was all or nothing, while in nature predators may occasionally catch evasive prey or fail to catch slower species, which might inhibit learning. Here, using mantids as predators, we conducted an experimental test of the evasive mimicry hypothesis that circumvents these limitations, using live painted calyptrate flies with modified evasive capabilities as prey. We found that mantids readily learned to avoid pursuing the more evasive prey types. Warning signals based on evasiveness and their associated mimicry may be widespread phenomena in nature. These findings not only further support its plausibility but demonstrate that even arthropod predators can select for it.

Shape of Evasive Prey Can Be an Important Cue That Triggers Learning in Avian Predators

Advertising escape ability could reduce predatory attacks. However, the effectiveness of certain phenotypic cues (e.g., color, shape, and size) in signaling evasiveness is still unknown. Understanding the role of such signals in driving predator learning is important to infer the evolutionary mechanisms leading to convergent evasiveness signals among prey species (i.e., evasive mimicry). We aim to understand the role of the color pattern (white patches on dark background) and morphology (extended butterfly hindwings) in driving learning and avoidance of escaping prey by surrogate avian predators, the European blue tit. These cues are common in butterflies and have been suspected to advertise escape ability in nature. We use dummy butterflies harboring shape and color patterns commonly found in skippers (family Hesperiidae). The prey models displayed the studied phenotypical cues (hindwing tails and white bands) in factorial combinations, and we tested whether those cues were learned as evasive signals and were generalised to different phenotypes. Our results suggest that hindwing tails and white bands can be associated with prey evasiveness. In addition, wild blue tits might learn and avoid attacking prey models bearing the studied phenotypic cues. Although blue tits seem to have an initial preference for the phenotype consisting of white patches and hindwing tails, the probability of attacking it was substantially reduced once the cues were associated with escaping ability. This suggests that the same morphological cues might be interchangeable as preference/avoidance signals. Further investigation of the salience of hindwing tails vs. white bands as cues for escaping ability, revealed that predators can associate both color pattern and shape to the difficulty of capture, and possibly generalize their aversion to other prey harboring those cues. More studies with larger sample sizes are needed to confirm this trend. Altogether, our results highlight the hitherto overlooked role of shape (butterfly hindwing tails) for signaling prey unprofitability.

Hard to catch: experimental evidence supports evasive mimicry

Most research on aposematism has focused on chemically defended prey, but the signalling difficulty of capture remains poorly explored. Similar to classical Batesian and Müllerian mimicry related to distastefulness, such 'evasive aposematism' may also lead to convergence in warning colours, known as evasive mimicry. A prime candidate group for evasive mimicry are Adelpha butterflies, which are agile insects and show remarkable colour pattern convergence. We tested the ability of naive blue tits to learn to avoid and generalize Adelpha wing patterns associated with the difficulty of capture and compared their response to that of birds that learned to associate the same wing patterns with distastefulness. Birds learned to avoid all wing patterns tested and generalized their aversion to other prey to some extent, but learning was faster with evasive prey compared to distasteful prey. Our results on generalization agree with longstanding observations of striking convergence in wing colour patterns among Adelpha species, since, in our experiments, perfect mimics of evasive and distasteful models were always protected during generalization and suffered the lowest attack rate. Moreover, generalization on evasive prey was broader compared to that on distasteful prey. Our results suggest that being hard to catch may deter predators at least as effectively as distastefulness. This study provides empirical evidence for evasive mimicry, a potentially widespread but poorly understood form of morphological convergence driven by predator selection.

Conspicuous colours reduce predation rates in fossorial uropeltid snakes

Uropeltid snakes (Family Uropeltidae) are non-venomous, fossorial snakes that are found above ground occasionally, during which time they are exposed to predation. Many species are brightly coloured, mostly on the ventral surface, but these colours are expected to have no function below the ground. Observations have shown that the cephalic resemblance (resemblance to heads) of uropeltid tails may direct attacks of predators towards the hardened tails, thereby potentially increasing handling times for predators. Experiments have also shown that predators learn to avoid prey that are non-toxic and palatable but are difficult to capture, hard to process or require long handling time when such prey advertise their unprofitability through conspicuous colours. We here postulate that uropeltid snakes use their bright colours to signal long handling times associated with attack deflection to the tails, thereby securing reduced predation from predators that can learn to associate colour with handling time. Captive chicken experiments with dough models mimicking uropeltids indicate that attacks were more common on the tail than on the head. Field experiments with uropeltid clay models show that the conspicuous colours of these snakes decrease predation rates compared to cryptic models, but a novel conspicuous colour did not confer such a benefit. Overall, our experiments provide support for our hypothesis that the conspicuous colours of these snakes reduce predation, possibly because these colours advertise unprofitability due to long handling times.

Both Palatable and Unpalatable Butterflies Use Bright Colors to Signal Difficulty of Capture to Predators

Birds are able to recognize and learn to avoid attacking unpalatable, chemically defended butterflies after unpleasant experiences with them. It has also been suggested that birds learn to avoid prey that are efficient at escaping. This, however, remains poorly documented. Here, we argue that butterflies may utilize a variety of escape tactics against insectivorous birds and review evidence that birds avoid attacking butterflies that are hard to catch. We suggest that signaling difficulty of capture to predators is a widespread phenomenon in butterflies, and this ability may not be limited to palatable butterflies. The possibility that both palatable and unpalatable species signal difficulty of capture has not been fully explored, but helps explain the existence of aposematic coloration and escape mimicry in butterflies lacking defensive chemicals. This possibility may also change the role that putative Müllerian and Batesian mimics play in a variety of classical mimicry rings, thus opening new perspectives in the evolution of mimicry in butterflies.

Some Possible Cases of Escape Mimicry in Neotropical Butterflies

The possibility that escape or evasive mimicry evolved in butterflies and other prey insects in a similar fashion to classical Batesian and Müllerian mimicry has long been advanced in the literature. However, there is a general disagreement among lepidopterists and evolutionary biologists on whether or not escape mimicry exists, as well as in which mimicry rings this form of mimicry has evolved. Here, we review some purported cases of escape mimicry in Neotropical butterflies and suggest new mimicry rings involving several species of Archaeoprepona, Prepona, and Doxocopa (the "bright blue bands" ring) and species of Colobura and Hypna (the "creamy bands" ring) where the palatability of butterflies, their ability to escape predator attacks, geographic distribution, relative abundance, and co-occurrence in the same habitats strongly suggest that escape mimicry is involved. In addition, we also indicate other butterfly taxa whose similarities of coloration patterns could be due to escape mimicry and would constitute important case studies for future investigation.

Evasive mimicry: when (if ever) could mimicry based on difficulty of capture evolve?

We elucidate the conditions under which an easy-to-catch edible prey species may evolve to resemble another edible species that is much more difficult to capture ('evasive Batesian mimicry'), and the conditions under which two or more edible but hard-to-catch species evolve a common resemblance ('evasive Mullerian mimicry'). Using two complementary mathematical models, we argue that both phenomena are logically possible but that several factors will limit the prevalence of these forms of mimicry in nature. Evasive Batesian mimicry is most likely to arise when it is costly in time or energy for the predator species to pursue evasive prey, when mimics are encountered less frequently than evasive models and where there are abundant alternative prey. Evasive Mullerian mimicry, by contrast, is most likely to arise when evasive prey species differ in abundance, predators are slow to learn to avoid evasive prey and evading capture is costly to the prey. Unequivocal evidence for evasive Batesian or Mullerian mimicry has not yet been demonstrated in the field, and we argue that more empirical work is needed to test whether putative examples are indeed a result of selection to signal difficulty of capture.

Locomotor mimicry in butterflies? A critical review of the evidence

The hypothesis of locomotor mimicry in butterflies presented by Srygley ( Phil. Trans. R. Soc. Lond. B 343, 145—155 (1994)) is criticized as unparsimonious, from two perspectives. First, the existence of mimicry between palatable but unprofitable prey is disputed on theoretical and empirical grounds. The lack of a strong predator aversion stimulus seriously undermines the unprofitable prey scenario, and supposed cases of mimicry of unprofitable models are explicable by traditional mimetic modes. Second, correlations of phenotypic characters used to support alternative adaptive peaks for palatable and unpalatable butterflies are criticized for failing to account for phylogenetic relationships. Virtually all of the relevant variation in flight-related morphology is shown to be due to differences between clades, rather than mimicry groups. An alternative hypothesis emphasizing phylogenetic constraint in the evolution of morphological characters associated with predator avoidance is proposed. The ground rule — or perhaps doctrine would be a better term — is that adaptation is a special and onerous concept that should be used only where it is really necessary. When it must be recognized, it should be attributed to no higher a level of organization than is demanded by the evidence. In explaining adaptation, one should assume the adequacy of the simplest form of natural selection, that of alternate alleles in Mendelian populations, unless the evidence clearly shows that this theory does not suffice. G. C. Williams (1966, pp. 4—5).