Evolutionary transitions from camouflage to aposematism: Hidden signals play a pivotal role

Behavioural mimicry among poison frogs diverges during close-range encounters with predators

Aposematic species signal their unpalatability to potential predators with recognizable, and frequently conspicuous, colour patterns. These visual signals are often also associated with bold behaviour and a reduced propensity to escape from approaching predators. Bold behaviours may act as an aversive signal and allow defended prey to avoid the energetic/opportunity costs that arise from fleeing predators. For Batesian mimics, non-defended species which replicate the colours of defended models, behavioural mimicry may also improve mimic fidelity and reduce energetic/opportunity costs. However, as predators may test the honesty of aposematic signals through sampling behaviour, Batesian mimics can be at high risk during close-range interactions with predators. This raises the question of whether/when Batesian mimics should deviate from behavioural mimicry and initiate more extensive escape behaviour. Here, we exposed the chemically defended poison frog Ameerega bilinguis and its (non-toxic) Batesian mimic Allobates zaparo to a simulated predator encounter. We predicted Al. zaparo would escape to a greater distance and in a more erratic manner than Am. bilinguis. Yet, contrary to our predictions, Al. zaparo did not flee far from predators. It was, however, more likely to initiate escape prior to physical contact from the predator. We suggest that bold behaviour coupled with pre-emptive movement allows Al. zaparo to retain the benefits of behavioural mimicry while reducing the likelihood that predators will test signal honesty. Our data highlight that when examining the evolution of mimicry, we must consider both morphological and behavioural traits, as well as how risk to the prey may change how they behave throughout the predation sequence.

Turning lances into shields: flower mantids stretch their raptorial forelegs to avert and deflect predator attack

Evolutionary co-option, in which existing traits acquire novel adaptive functions, is a key strategy by which organisms adapt to new environmental challenges. Although such co-option has been widely documented at the genetic and morphological levels, its incidence at the behavioural level remains largely unknown. Mantids stretch their forelegs to capture prey; however, some flower mantids also perform foreleg stretches in the absence of prey. The current study tested whether this behaviour represents a novel function of the foreleg stretch, thus representing a case of behavioural co-option. Predator encounter behaviour assays revealed that foreleg stretching facilitates the escape of flower mantids from large predatory mantids by delaying predator approach or deflecting their attack towards less vulnerable body parts. Phylogenetic analysis suggested that the ancestral function of foreleg stretching involves prey capture, with the anti-predator function subsequently acquired in the flower mantid clade, coinciding with the diversification of large-sized mantids, the most likely invertebrate predators of flower mantids. This study provides a case of behavioural co-option, where a predator uses its predatory organ as a defensive implement to cope with its own predators. These findings further suggest that behavioural co-option may be common in nature, meriting more comprehensive studies.

Parasitism as a potential driver of aposematism

Aposematic animals couple unprofitability to predators, such as toxicity, with a warning signal, such as conspicuous coloration. Although toxicity and coloration can provide effective protection against predation, these traits also play a major role in the context of parasitism. Many of the color pigments used by aposematic animals are components of anti-infection immunity. Moreover, toxic compounds are used by conspicuous animals as defenses against parasites and pathogens. Parasites and pathogens not only pose selection on coloration through immunity and toxicity, they also play a major role in sexual selection, with mate choice often depending on conspicuous coloration and anti-infection toxicity. Consequently, parasitism is likely an important component in the evolution of traits that provide predator protection through aposematism.

Passive accumulation of alkaloids in inconspicuously colored frogs refines the evolutionary paradigm of acquired chemical defenses

Understanding the origins of novel, complex phenotypes is a major goal in evolutionary biology. Poison frogs of the family Dendrobatidae have evolved the novel ability to acquire alkaloids from their diet for chemical defense at least three times. However, taxon sampling for alkaloids has been biased towards colorful species, without similar attention paid to inconspicuous ones that are often assumed to be undefended. As a result, our understanding of how chemical defense evolved in this group is incomplete. Here, we provide new data showing that, in contrast to previous studies, species from each undefended poison frog clade have measurable yet low amounts of alkaloids. We confirm that undefended dendrobatids regularly consume mites and ants, which are known sources of alkaloids. Thus, our data suggest that diet is insufficient to explain the defended phenotype. Our data support the existence of a phenotypic intermediate between toxin consumption and sequestration - passive accumulation - that differs from sequestration in that it involves no derived forms of transport and storage mechanisms yet results in low levels of toxin accumulation. We discuss the concept of passive accumulation and its potential role in the origin of chemical defenses in poison frogs and other toxin-sequestering organisms. In light of ideas from pharmacokinetics, we incorporate new and old data from poison frogs into an evolutionary model that could help explain the origins of acquired chemical defenses in animals and provide insight into the molecular processes that govern the fate of ingested toxins.

A Wide Color Gamut and Noniridescent Zinc-Anode Asymmetric Electrochromic Device for Self-Sustaining Color-Adaptive Bio-Camouflage System

Inspired by camouflage-colored organisms, the development of bio-camouflage systems using electrochromic (EC) technology has gained significant interest. However, existing EC systems struggle with achieving a wide color gamut, noniridescent colors, and self-sustainability. Herein, a self-sustainable color-adaptive bio-camouflage system integrating EC and nanogenerator (NG) technologies, enabling environmental color adaptation, and thermal regulation without an external power source is proposed. The system is based on a zinc-anode EC device (ZECD) with an asymmetric structure, incorporating flexible tungsten oxide and vanadium oxide electrodes. During the EC process, tungsten oxide shifts between blue and transparent, allowing near-infrared thermal modulation, while the vanadium oxide transitions from yellow to transparent. This design enables reversible near-infrared modulation and noniridescent color conversion among black, blue, green, yellow, and transparent. For the self-sustainability of the system, an electromagnetic and triboelectric hybrid NG that collects biomechanical energy is developed. In a typical driven cycle, the integrated system transitions colors and achieves significant near-infrared spectral modulation, demonstrating environmental adaptability and thermal regulation. Experiments on human skin and simulated chameleon color changes further confirm the system's effectiveness. This work highlights the potential of integrating EC and NG technologies to advance color-adaptive camouflage systems, opening new an avenue for bio-camouflage design.

Reduced palatability, fast flight, and tails: decoding the defence arsenal of Eudaminae skipper butterflies in a Neotropical locality

Prey often rely on multiple defences against predators, such as flight speed, attack deflection from vital body parts, or unpleasant taste, but our understanding on how often and why they are co-exhibited remains limited. Eudaminae skipper butterflies use fast flight and mechanical defences (hindwing tails), but whether they use other defences like unpalatability (consumption deterrence) and how these defences interact have not been assessed. We tested the palatability of 12 abundant Eudaminae species in Peru, using training and feeding experiments with domestic chicks. Further, we approximated the difficulty of capture based on flight speed and quantified it by wing loading. We performed phylogenetic regressions to find any association between multiple defences, body size, and habitat preference. We found a broad range of palatability in Eudaminae, within and among species. Contrary to current understanding, palatability was negatively correlated with wing loading, suggesting that faster butterflies tend to have lower palatability. The relative length of hindwing tails did not explain the level of butterfly palatability, showing that attack deflection and consumption deterrence are not mutually exclusive. Habitat preference (open or forested environments) did not explain the level of palatability either, although butterflies with high wing loading tended to occupy semi-closed or closed habitats. Finally, the level of unpalatability in Eudaminae is size dependent. Larger butterflies are less palatable, perhaps because of higher detectability/preference by predators. Altogether, our findings shed light on the contexts favouring the prevalence of single versus multiple defensive strategies in prey.

Polycomb group proteins confer robustness to aposematic coloration in the milkweed bug, Oncopeltus fasciatus

Aposematic coloration offers an opportunity to explore the molecular mechanisms underlying canalization. In this study, the role of epigenetic regulation underlying robustness was explored in the aposematic coloration of the milkweed bug, Oncopeltus fasciatus. Polycomb (Pc) and Enhancer of zeste (E(z)), which encode components of the Polycomb repressive complex 1 (PRC1) and PRC2, respectively, and jing, which encodes a component of the PRC2.2 subcomplex, were knocked down in the fourth instar of O. fasciatus. Knockdown of these genes led to alterations in scutellar morphology and melanization. In particular, when Pc was knocked down, the adults developed a highly melanized abdomen, head and forewings at all temperatures examined. In contrast, the E(z) and jing knockdown led to increased plasticity of the dorsal forewing melanization across different temperatures. Moreover, jing knockdown adults exhibited increased plasticity in the dorsal melanization of the head and the thorax. These observations demonstrate that histone modifiers may play a key role during the process of canalization to confer robustness in the aposematic coloration.

Light wavelength modulates search behavior performance in zebrafish

Visual systems have evolved to discriminate between different wavelengths of light. The ability to perceive color, or specific light wavelengths, is important as color conveys crucial information about both biotic and abiotic features in the environment. Indeed, different wavelengths of light can drive distinct patterns of activity in the vertebrate brain, yet what remains incompletely understood is whether distinct wavelengths can invoke etiologically relevant behavioral changes. To address how specific wavelengths in the visible spectrum modulate behavioral performance, we use larval zebrafish and a stereotypic light-search behavior. Prior work has shown that the cessation of light triggers a transitional light-search behavior, which we use to interrogate wavelength-dependent behavioral modulation. Using 8 narrow spectrum light sources in the visible range, we demonstrate that all wavelengths induce motor parameters consistent with search behavior, yet the magnitude of search behavior is spectrum sensitive and the underlying motor parameters are modulated in distinct patterns across short, medium, and long wavelengths. However, our data also establishes that not all motor features of search are impacted by wavelength. To define how wavelength modulates search performance, we performed additional assays with alternative wavelengths, dual wavelengths, and variable intensity. Last, we also tested blind larvae to resolve which components of wavelength dependent behavioral changes potentially include signaling from non-retinal photoreception. These findings have important implications as organisms can be exposed to varying wavelengths in laboratory and natural settings and therefore impose unique behavioral outputs.

Antipredator defences in motion: animals reduce predation risks by concealing or misleading motion signals

Motion is a crucial part of the natural world, yet our understanding of how animals avoid predation whilst moving remains rather limited. Although several theories have been proposed for how antipredator defence may be facilitated during motion, there is often a lack of supporting empirical evidence, or conflicting findings. Furthermore, many studies have shown that motion often 'breaks' camouflage, as sudden movement can be detected even before an individual is recognised. Whilst some static camouflage strategies may conceal moving animals to a certain extent, more emphasis should be given to other modes of camouflage and related defences in the context of motion (e.g. flicker fusion camouflage, active motion camouflage, motion dazzle, and protean motion). Furthermore, when motion is involved, defence strategies are not necessarily limited to concealment. An animal can also rely on motion to mislead predators with regards to its trajectory, location, size, colour pattern, or even identity. In this review, we discuss the various underlying antipredator strategies and the mechanisms through which they may be linked to motion, conceptualising existing empirical and theoretical studies from two perspectives - concealing and misleading effects. We also highlight gaps in our understanding of these antipredator strategies, and suggest possible methodologies for experimental designs/test subjects (i.e. prey and/or predators) and future research directions.

The effect of biofluorescence on predation upon Cope's gray treefrog: A clay model experiment

Biofluorescence, the ability to absorb light and reemit it at a longer wavelength, is present in many taxa but has been examined only recently in amphibians. Over half of the studies documenting biofluorescence in the last century suggest this fluorescent signal may affect predation; however, to date, only one other experimental study has tested this hypothesis. To address this question, we experimentally tested the effect of biofluorescence on predation through the study of the Cope's Gray Treefrog, Hyla chrysoscelis. First, we quantified the spectral characteristics of a novel biofluorescence pattern in H. chrysoscelis. In both sexes of this species, the fluorescent signal is concentrated in an area that contains a proposed aposematic pattern to warn predators of the frog's toxic secretions. We hypothesized that the biofluorescent trait may increase the conspicuousness of this pattern and enable the frogs to deter predators more effectively. Second, we tested this prediction by conducting a clay model field experiment to assess differences in predation attempts on fluorescent versus non-fluorescent H. chrysoscelis models by various predator types. We found no effect of biofluorescence on the overall presence, type, or location of predation, suggesting that biofluorescence alone does not act as an antipredator signal of H. chrysoscelis. This study represents one of the first attempts to experimentally test the effect of biofluorescence on predation in any organism and the first to do so in amphibians. Further work is needed to explore the role of this trait in predation in other systems and to investigate alternative functions for the biofluorescent signal in H. chrysoscelis.

The global importance and interplay of colour-based protective and thermoregulatory functions in frogs

Small-scale studies have shown that colour lightness variation can have important physiological implications in ectotherms, with darker species having greater heating rates, as well as protection against pathogens and photooxidative damage. Using data for 41% (3059) of all known frog and toad species (Anura) from across the world, we reveal ubiquitous and strong clines of decreasing colour lightness towards colder regions and regions with higher pathogen pressure and UVB radiation. The relative importance of pathogen resistance is higher in the tropics and that of thermoregulation is higher in temperate regions. The results suggest that these functions influence colour lightness evolution in anurans and filtered for more similarly coloured species under climatic extremes, while their concurrent importance resulted in high within-assemblage variation in productive regions. Our findings indicate three important functions of colour lightness in anurans - thermoregulation, pathogen and UVB protection - and broaden support for colour lightness-environment relationships in ectotherms.

Selection on Visual Opsin Genes in Diurnal Neotropical Frogs and Loss of the SWS2 Opsin in Poison Frogs

Amphibians are ideal for studying visual system evolution because their biphasic (aquatic and terrestrial) life history and ecological diversity expose them to a broad range of visual conditions. Here, we evaluate signatures of selection on visual opsin genes across Neotropical anurans and focus on three diurnal clades that are well-known for the concurrence of conspicuous colors and chemical defense (i.e., aposematism): poison frogs (Dendrobatidae), Harlequin toads (Bufonidae: Atelopus), and pumpkin toadlets (Brachycephalidae: Brachycephalus). We found evidence of positive selection on 44 amino acid sites in LWS, SWS1, SWS2, and RH1 opsin genes, of which one in LWS and two in RH1 have been previously identified as spectral tuning sites in other vertebrates. Given that anurans have mostly nocturnal habits, the patterns of selection revealed new sites that might be important in spectral tuning for frogs, potentially for adaptation to diurnal habits and for color-based intraspecific communication. Furthermore, we provide evidence that SWS2, normally expressed in rod cells in frogs and some salamanders, has likely been lost in the ancestor of Dendrobatidae, suggesting that under low-light levels, dendrobatids have inferior wavelength discrimination compared to other frogs. This loss might follow the origin of diurnal activity in dendrobatids and could have implications for their behavior. Our analyses show that assessments of opsin diversification in across taxa could expand our understanding of the role of sensory system evolution in ecological adaptation.

The evolution and ecology of multiple antipredator defences

Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such "defence portfolios" that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies.