A direct comparison of the effectiveness of two anti-predator strategies under field conditions

Different behavioral profiles between invasive and native nudibranchs: means for invasion success?

Behavior is predicted to be a primary determinant of the success of the invasion process during the early phases of colonization. Comparing invaders with sympatric native species may provide a good approach to unravel behavioral traits involved in an invasion process. In this study, we carried out an experimental simulation of the introduction and the acclimatization phase into a new environment and assessed the expression of activity, alertness, and habituation in an invasive Mediterranean population of the South African nudibranch Godiva quadricolor comparing its profiles with those of the sympatric Mediterranean native nudibranchs Cratena peregrina and Caloria quatrefagesi. Individuals of these 3 species were subjected to 3 behavioral tests: spontaneous activity, carried out in the introduction phase (immediately after sampling) and after a week of acclimatization; alert test, in which a potential threat was simulated by means of a tactile stimulus, and habituation test, in which the same alert test stimulus was repeated 5 times at 30-min intervals. The invasive G. quadricolor showed higher levels of exploration activity, thigmotaxis, alertness, and sensitization than the native species. These behavioral traits may represent pivotal drivers of the ongoing invasion process.

Sphingid caterpillars conspicuous patches do not function as distractive marks or warning against predators

To avoid predation by visual predators, caterpillars can be cryptic to decrease detectability or aposematic to warn predators of potential unpalatability. However, for some species, it is not clear if conspicuous patches are selected to avoid predation. For example, Pandora sphinx (Eumorpha pandorus, Lepidoptera: Sphingidae) caterpillars are assumed to be palatable and have both cryptic (green, brown) and conspicuous (orange, red) color morphs. Five lateral, off-white to yellow patches on either side may serve as a warning for predators or to draw attention away from the caterpillar's form to function as distractive marks. We conducted a field study in three temperate fragmented forests in Massachusetts to investigate the potential utility of E. pandorus coloration and conspicuous patches. Using four plasticine caterpillar prey model treatments, green and red with and without lateral conspicuous patches, we tested the effects of color, patch patterning, and seasonality on attack rates by a variety of taxa. We found that 43% of the prey models (n = 964) had bite marks by an array of predators including arthropods (67.5%), birds (18.2%), rodents (11.5%), and large mammals (2.8%). Arthropods as dominant predators align with conclusions from previous studies of prey models placed near ground level. Attack rates peaked for arthropods in late August and early September but were more constant across trials for vertebrates. Arthropods, a heterogeneous group, as indicated by the variety of bite marks, showed significantly higher attack rates on green colored prey models and a tendency of higher attack on solid (non-patch patterned) prey models. Vertebrates, more visually oriented predators, had significantly higher attack rates on red colored prey models and patch patterned prey models. Thus, our results did not suggest that conspicuous patch patterning reduced predation and therefore, we did not find support for the distractive mark hypothesis or warning hypothesis. Further, our study shows clear contrasting interpretations by different predators regarding visual defensive strategies.

Testing bird-driven diurnal trade-offs of the moon moth's anti-bat tail

Traits are often caught in a dynamic tension of countervailing evolutionary pressures. Trade-offs can be imposed by predators evolutionarily curtailing the conspicuousness of a sexually selected trait, or acting in opposition to another natural selection pressure, for instance, a different predator with a divergent hunting strategy. Some moon moths (Saturniidae) have long hindwing tails that thwart echolocating bat attacks at night, allowing the moth to escape. These long tails may come at a cost, however, if they make the moth's roosting form more conspicuous to visually foraging predators during the day. To test this potential trade-off, we offered wild-caught Carolina wrens (Thryothorus ludovicianus) pastry dough models with real Actias luna wings that were either intact or had tails experimentally removed. We video recorded wrens foraging on models and found that moth models with tails did not experience increased detection and attack by birds. Thus, this elaborate trait, while obvious to human observers, does not seem to come at a cost of increased avian predator attention. The evolution of long hindwing tails, likely driven by echolocating predators at night, does not seem to be limited by opposing diurnal constraints. This study demonstrates the importance of testing presumed trade-offs and provides hypotheses for future testing.

Selfish herd effects depend on prey crypsis

Determining why some animals form groups while others remain solitary is a longstanding goal in behavioural ecology. Group formation can help mitigate predation risk through various mechanisms, including risk dilution and group vigilance. The selfish herd hypothesis proposes that prey can reduce their risk by minimizing the area around which all points in that area are closer to them than to another conspecific (i.e. by minimizing their ‘domain of danger’ (DOD)). This hypothesis assumes that an individual's predation risk is proportional to the size of its DOD; however, the relationship between risk and proximity to conspecifics may depend on additional factors. Specifically, approaching conspecifics may be costly for prey that rely on crypsis because group formation increases detectability. Using plasticine model prey, we experimentally manipulated prey coloration as well as the DOD, and then tracked their ‘survival’ under natural field conditions. We found that an individual's predation risk increased with their DOD for conspicuous (red) prey, but decreased with the DOD in cryptic (green) prey. Our results are consistent with patterns in natural systems and indicate that the relationship between predation risk and DOD depends on additional factors like prey coloration.

Avian predators taste reject mimetic prey in relation to their signal reliability

Aposematic organisms defend themselves through various means to increase their unprofitability to predators which they advertise with conspicuous warning signals. Predators learn to avoid aposematic prey through associative learning that leads to lower predation. However, when these visual signals become unreliable (e.g., through automimicry or Batesian mimicry), predators may switch from using visual signals to taste sampling prey to choose among them. In this experiment, we tested this possibility in a field experiment where we released a total of 4800 mealworm prey in two clusters consisting of either: (i) undefended prey (injected with water) and (ii) model-mimics (injected with either quinine sulphate [models] or water [mimics]). Prey were deployed at 12 sites, with the mimic frequency of the model-mimics ranging between 0 and 1 (at 0.2 intervals). We found that taste rejection peaked at moderate mimic frequencies (0.4 and 0.6), supporting the idea that taste sampling and rejection of prey is related to signal reliability and predator uncertainty. This is the first time that taste-rejection has been shown to be related to the reliability of prey signals in a mimetic prey system.

Partial wing transparency works better when disrupting wing edges: Evidence from a field experiment

Lepidoptera-a group of insects in which wing transparency has arisen multiple times-exhibits much variation in the size and position of transparent wing zones. However, little is known as to how this variability affects detectability. Here, we test how the size and position of transparent elements affect the predation of artificial moths by wild birds in the field. Morphs with transparent elements touching wing borders showed a reduced predation risk, with the effect being the same regardless of the number of wing borders being touched. By contrast, transparent element size had little to no effect on predation risk. Overall, this experiment shows for the first time that transparency offers higher protection when it disrupts prey contour in terrestrial habitats.

The truth is in the detail: predators attack aposematic prey with less aggression than other prey types

Aposematic organisms are often unprofitable to predators (e.g. because of defensive chemicals) which they advertise with a conspicuous signal (e.g. bright and conspicuous colour signals). Aposematism is thought to reduce predation of prey because the colour signal increases the ability of predators to learn, recognize and remember the prey’s defensive properties. The efficacy of aposematism has been extensively documented in laboratory studies, although its benefits seem to be harder to demonstrate in the field. In this study, we compared the levels of partial and overall predation among four prey types (undefended and cryptic, undefended and warning coloured, defended and cryptic, and aposematic prey). Overall, predation of warning coloured and defended (aposematic) prey was lower than the predation for cryptic and undefended prey; however, it was the same as predation of cryptic and defended prey. Moreover, aposematic prey had higher levels of partial predation (where prey was not wholly consumed by the predator) and lower attack intensities. This suggests that prey were being taste sampled, but also might be better able to survive attacks. Therefore, the benefits of aposematism may lie not only in reducing outright predation, but also in altering a predator’s post-attack behaviour, thus leading to greater escape opportunities and post-attack survival of prey. These results reinforce the importance of examining predation in more detail rather than simply examining attack rates.

Contrast to background influences predation on aposematic but not cryptic artificial caterpillars in a Brazilian coastal shrubland

Aposematism and crypticity are visual defensive strategies against predation; however, the relative effectiveness of these two strategies to reduce the risk of predation is not yet fully understood. We evaluated the risk of predation for caterpillars with cryptic and aposematic colouration as well as the probability of predation relative to the natural variation of contrast with the substrate. We expected that the two models would experience similar predation attempts and that the contrast with the substrate would be negatively related to the predation on aposematic mimic models and positively to the predation of cryptic models. Overall, 224 models were laid out along a transect and exposed to predation for five consecutive days during winter and autumn. Daily predation was 11.0% (winter) and 4.8% (autumn). Significant differences were not observed between predation rates on the two model types (50.6% aposematic). Most of the predated models had arthropod marks (86.4%) and only 13.6% had bird marks. The chance of predation was higher the greater the contrast between the aposematic mimic model and the substrate, although no relationship was observed for the cryptic model. Our results suggest that the two colour patterns do not differ in their defensive effectiveness and that micro-habitat selection might define the predation risk on aposematic mimic caterpillars in environments dominated by arthropod predators.

Why has transparency evolved in aposematic butterflies? Insights from the largest radiation of aposematic butterflies, the Ithomiini

Defended species are often conspicuous and this is thought to be an honest signal of defences, i.e. more toxic prey are more conspicuous. Neotropical butterflies of the large Ithomiini tribe numerically dominate communities of chemically defended butterflies and may thus drive the evolution of mimetic warning patterns. Although many species are brightly coloured, most are transparent to some degree. The evolution of transparency from a warning-coloured ancestor is puzzling as it is generally assumed to be involved in concealment. Here, we show that transparent Ithomiini species are indeed less detectable by avian predators (i.e. concealment). Surprisingly, transparent species are not any less unpalatable, and may in fact be more unpalatable than opaque species, the latter spanning a larger range of unpalatability. We put forth various hypotheses to explain the evolution of weak aposematic signals in these butterflies and other cryptic defended prey. Our study is an important step in determining the selective pressures and constraints that regulate the interaction between conspicuousness and unpalatability.

Transparency improves concealment in cryptically coloured moths

Predation is a ubiquitous and strong selective pressure on living organisms. Transparency is a predation defence widespread in water but rare on land. Some Lepidoptera display transparent patches combined with already cryptic opaque patches. A recent study showed that transparency reduced detectability of aposematic prey with conspicuous patches. However, whether transparency has any effect at reducing detectability of already cryptic prey is still unknown. We conducted field predation experiments with free avian predators where we monitored and compared survival of a fully opaque grey artificial form (cryptic), a form including transparent windows and a wingless artificial butterfly body. Survival of the transparent forms was similar to that of wingless bodies and higher than that of fully opaque forms, suggesting a reduction of detectability conferred by transparency. This is the first evidence that transparency decreases detectability in cryptic terrestrial prey. Future studies should explore the organization of transparent and opaque patches in animals and their interplay on survival, as well as the costs and other potential benefits associated with transparency on land.

Environment-dependent attack rates of cryptic and aposematic butterflies

Many organisms have evolved adaptive coloration that reduces their risk of predation. Cryptic coloration reduces the likelihood of detection/recognition by potential predators, while warning or aposematic coloration advertises unprofitability and thereby reduces the likelihood of attack. Although some studies show that aposematic coloration functions better at decreasing attack rate than crypsis, recent work has suggested and demonstrated that crypsis and aposematism are both successful strategies for avoiding predation. Furthermore, the visual environment (e.g., ambient lighting, background) affects the ability for predators to detect prey. We investigated these 2 related hypotheses using 2 well-known visually aposematic species of Heliconius butterflies, which occupy different habitats (open-canopy vs. closed-canopy), and one palatable, cryptic, generalist species Junonia coenia. We tested if the differently colored butterflies differ in attack rates by placing plasticine models of each of the 3 species in 2 different tropical habitats where the butterflies naturally occur: disturbed, open-canopy habitat and forested, closed-canopy habitat. The cryptic model had fewer attacks than one of the aposematic models. Predation rates differed between the 2 habitats, with the open habitat having much higher predation. However, we did not find an interaction between species and habitat type, which is perplexing due to the different aposematic phenotypes naturally occurring in different habitats. Our findings suggest that during the Panamanian dry season avian predation on perched butterflies is not a leading cause in habitat segregation between the 2 aposematic species and demonstrate that cryptically colored animals at rest may be better than aposematic prey at avoiding avian attacks in certain environments.

Avoidance of an aposematically coloured butterfly by wild birds in a tropical forest

1. Birds are considered to be the primary selective agents for warning colouration in butterflies, and select for aposematic mimicry by learning to avoid brightly coloured prey after unpleasant experiences. It has long been thought that bright colouration plays an important role in promoting the avoidance of distasteful prey by birds. 2. The hypothesis that warning colouration facilitates memorability and promotes predator avoidance was tested by means of a field experiment using distasteful model butterflies. Artificial butterflies with a Heliconius colour pattern unknown to local birds were generated using bird vision models, either coloured or achromatic, and hung in tree branches in a tropical forest. Two sequential trials were conducted at each site to test avoidance by naïve and experienced predators. 3. There was a significant reduction in predation in the second trial. Also, coloured models were attacked less than achromatic models. Specifically, coloured butterflies were attacked significantly less in the second trial, but there was no significant decrease in predation on achromatic models. 4. The present results imply an important role for colour in enhancing aversion of aposematic butterflies. It has also been demonstrated that previous experience of distasteful prey can lead to enhanced avoidance in subsequent trials, supporting mimicry theory.

A field demonstration of the costs and benefits of group living to edible and defended prey

Both theoretical and laboratory research suggests that many prey animals should live in a solitary, dispersed distribution unless they lack repellent defences such as toxins, venoms and stings. Chemically defended prey may, by contrast, benefit substantially from aggregation because spatial localization may cause rapid predator satiation on prey toxins, protecting many individuals from attack. If repellent defences promote aggregation of prey, they also provide opportunities for new social interactions; hence the consequences of defence may be far reaching for the behavioural biology of the animal species. There is an absence of field data to support predictions about the relative costs and benefits of aggregation. We show here for the first time using wild predators that edible, undefended artificial prey do indeed suffer heightened death rates if they are aggregated; whereas chemically defended prey may benefit substantially by grouping. We argue that since many chemical defences are costly to prey, aggregation may be favoured because it makes expensive defences much more effective, and perhaps allows grouped individuals to invest less in chemical defences.

Seasonal changes in predator community switch the direction of selection for prey defences

Insect communities consist of aposematic species with efficient warning colours against predation, as well as abundant examples of crypsis. To understand such coexistence, we here report results from a field experiment where relative survival of artificial larvae, varying in conspicuousness, was estimated in natural bird communities over an entire season. This takes advantage of natural variation in the proportion of naive predators: naivety peaks when young birds have just fledged. We show that the relative benefit of warning signals and crypsis changes accordingly. When naive birds are rare (early and late in the season), conspicuous warning signals improve survival, but conspicuousness becomes a disadvantage near the fledging time of birds. Such temporal structuring of predator-prey relationships facilitates the coexistence of diverse antipredatory strategies and helps explain two patterns we found in a 688-species community of Lepidoterans: larval warning signals remain rare and occur disproportionately often in seasons when predators are educated.