Acoustic mimicry in a predator-prey interaction

Buckling-induced sound production in the aeroelastic tymbals of Yponomeuta

The loss of elastic stability (buckling) can lead to catastrophic failure in the context of traditional engineering structures. Conversely, in nature, buckling often serves a desirable function, such as in the prey-trapping mechanism of the Venus fly trap (Dionaea muscipula). This paper investigates the buckling-enabled sound production in the wingbeat-powered (aeroelastic) tymbals of Yponomeuta moths. The hindwings of Yponomeuta possess a striated band of ridges that snap through sequentially during the up- and downstroke of the wingbeat cycle-a process reminiscent of cellular buckling in compressed slender shells. As a result, bursts of ultrasonic clicks are produced that deter predators (i.e. bats). Using various biological and mechanical characterization techniques, we show that wing camber changes during the wingbeat cycle act as the single actuation mechanism that causes buckling to propagate sequentially through each stria on the tymbal. The snap-through of each stria excites a bald patch of the wing's membrane, thereby amplifying sound pressure levels and radiating sound at the resonant frequencies of the patch. In addition, the interaction of phased tymbal clicks from the two wings enhances the directivity of the acoustic signal strength, suggesting an improvement in acoustic protection. These findings unveil the acousto-mechanics of Yponomeuta tymbals and uncover their buckling-driven evolutionary origin. We anticipate that through bioinspiration, aeroelastic tymbals will encourage novel developments in the context of multi-stable morphing structures, acoustic structural monitoring, and soft robotics.

Acoustic camouflage increases with body size and changes with bat echolocation frequency range in a community of nocturnally active Lepidoptera

Body size is an important trait in predator-prey dynamics as it is often linked to detection, as well as the success of capture or escape. Larger prey, for example, often runs higher risk of detection by their predators, which imposes stronger selection on their anti-predator traits compared to smaller prey. Nocturnal Lepidoptera (moths) vary strongly in body size, which has consequences for their predation risk, as bigger moths return stronger echoes for echolocating bats. To compensate for increased predation risk, larger moths are therefore expected to have improved anti-predator defences. Moths are covered by different types of scales, which for a few species are known to absorb ultrasound, thus providing acoustic camouflage. Here, we assessed whether moths differ in their acoustic camouflage in a size-dependent way by focusing on their body scales and the different frequency ranges used by bats. We used a sonar head to measure 3D echo scans of a total of 111 moth specimens across 58 species, from eight different families of Lepidoptera. We scanned all the specimens and related their echo-acoustic target strength to various body size measurements. Next, we removed the scales covering the thorax and abdomen and scanned a subset of specimens again to assess the sound absorptive properties of these scales. Comparing intact specimens with descaled specimens, we found almost all species to absorb ultrasound, reducing detection risk on average by 8%. Furthermore, the sound absorptive capacities of body scales increased with body size suggesting that larger species benefit more from acoustic camouflage. The size-dependent effect of camouflage was in particular pronounced for the higher frequencies (above 29 kHz), with moth species belonging to large-bodied families consequently demonstrating similar target strengths compared to species from small-bodied families. Finally, we found the families to differ in frequency range that provided the largest reduction in detection risk, which may be related to differences in predation pressure and predator communities of these families. In general, our findings have important implications for predator-prey interactions across eco-evolutionary timescales and may suggest that acoustic camouflage played a role in body size evolution of nocturnally active Lepidoptera.

Contrasting laboratory and field outcomes of bat-moth interactions

Predator-prey interactions are important but difficult to study in the field. Therefore, laboratory studies are often used to examine the outcomes of predator-prey interactions. Previous laboratory studies have shown that moth hearing and ultrasound production can help prey avoid being eaten by bats. We report here that laboratory behavioural outcomes may not accurately reflect the outcomes of field bat-moth interactions. We tested the success rates of two bat species capturing moths with distinct anti-bat tactics using behavioural experiments. We compared the results with the dietary composition of field bats using next-generation DNA sequencing. Rhinolophus episcopus and Rhinolophus osgoodi had a lower rate of capture success when hunting for moths that produce anti-bat clicks than for silent eared moths and earless moths. Unexpectedly, the success rates of the bats capturing silent eared moths and earless moths did not differ significantly from each other. However, the field bats had a higher proportion of silent eared moths than that of earless moths and that of clicking moths in their diets. The difference between the proportions of silent eared moths and earless moths in the bat diets can be explained by the difference between their abundance in bat foraging habitats. These findings suggest that moth defensive tactics, bat countertactics and moth availability collectively shape the diets of insectivorous bats. This study illustrates the importance of using a combination of behavioural experiments and molecular genetic techniques to reveal the complex interactions between predators and prey in nature.

A framework for understanding post-detection deception in predator-prey interactions

Predators and prey exist in persistent conflict that often hinges on deception-the transmission of misleading or manipulative signals-as a means for survival. Deceptive traits are widespread across taxa and sensory systems, representing an evolutionarily successful and common strategy. Moreover, the highly conserved nature of the major sensory systems often extends these traits past single species predator-prey interactions toward a broader set of perceivers. As such, deceptive traits can provide a unique window into the capabilities, constraints and commonalities across divergent and phylogenetically-related perceivers. Researchers have studied deceptive traits for centuries, but a unified framework for categorizing different types of post-detection deception in predator-prey conflict still holds potential to inform future research. We suggest that deceptive traits can be distinguished by their effect on object formation processes. Perceptual objects are composed of physical attributes (what) and spatial (where) information. Deceptive traits that operate after object formation can therefore influence the perception and processing of either or both of these axes. We build upon previous work using a perceiver perspective approach to delineate deceptive traits by whether they closely match the sensory information of another object or create a discrepancy between perception and reality by exploiting the sensory shortcuts and perceptual biases of their perceiver. We then further divide this second category, sensory illusions, into traits that distort object characteristics along either the what or where axes, and those that create the perception of whole novel objects, integrating the what/where axes. Using predator-prey examples, we detail each step in this framework and propose future avenues for research. We suggest that this framework will help organize the many forms of deceptive traits and help generate predictions about selective forces that have driven animal form and behavior across evolutionary time.

Adults of Alderflies, Fishflies, and Dobsonflies (Megaloptera) Expel Meconial Fluid When Disturbed

Chemical secretions are an effective means by which insects may deter potential enemies, already being studied extensively with regard to their chemicals, synthesis, toxicity, and functions. However, these defensive secretions have been little studied in Megaloptera. Herein, the fluid expelling behavior of adult alderflies (Sialidae), fishflies (Corydalidae: Chauliodinae), and dobsonflies (Corydalidae: Corydalinae), all of the order Megaloptera, is described in detail regarding the timing and possible function of this behavior. When disturbed artificially, both males and females could expel fluid from the anus. However, the frequency of expelling was much lower in alderflies than in fishflies and dobsonflies. The amount of expelled fluid relative to body weight was also smaller in alderflies. In fishflies and dobsonflies, the amount of expelled fluid decreased with adult age, probably because the fluid is little replenished once expelled. The cream-colored fluid seems to be meconial fluid produced via the Malpighian tubules at the pupal stage, which is usually discharged at adult emergence in most other holometabolous insects. However, adult fishflies and dobsonflies often expel it vigorously by bending their abdomen when disturbed after emergence. Thus, the fluid expelling may be an anti-predatory behavior, particularly in younger adults that can expel a relatively large amount of fluid.

High duty cycle moth sounds jam bat echolocation: bats counter with compensatory changes in buzz duration

Tiger moth species vary greatly in the number of clicks they produce and the resultant duty cycle. Signals with higher duty cycles are expected to more effectively interfere with bat sonar. However, little is known about the minimum duty cycle of tiger moth signals for sonar jamming. Is there a threshold that allows us to classify moths as acoustically aposematic versus sonar jammers based on their duty cycles? We performed playback experiments with three wild-caught adult male bats, Eptesicus fuscus. Bat attacks on tethered moths were challenged using acoustic signals of Bertholdia trigona with modified duty cycles ranging from 0 to 46%. We did not find evidence for a duty cycle threshold; rather, the ability to jam the bat's sonar was a continuous function of duty cycle consistent with a steady increase in the number of clicks arriving during a critical signal processing time window just prior to the arrival of an echo. The proportion of successful captures significantly decreased as the moth duty cycle increased. Our findings suggest that moths cannot be unambiguously classified as acoustically aposematic or sonar jammers based solely on duty cycle. Bats appear to compensate for sonar jamming by lengthening the duration of their terminal buzz and they are more successful in capturing moths when they do so. In contrast to previous findings for bats performing difficult spatial tasks, the number of sonar sound groups decreased in response to high duty cycles and did not affect capture success.

Body size in Batesian mimicry

A variety of traits is available for predators to distinguish unpalatable prey from palatable Batesian mimics. Among them, body size has received little attention as a possible mimetic trait. Size should influence predator behaviour if it shows variation between models and mimics, is detectable by the predator in question, and is not overshadowed by other traits more salient to the predator. Simple predictions within mimetic populations are that perfect mimics receive the lowest predation rate. However, prey body size is typically tightly linked to the nutritional yield and handling time for a successful predator, as well as likely being correlated with a model’s levels of defence. In certain circumstances, these confounding factors might mean that (a) selection pressures on a mimic’s size either side of the model’s phenotype are not symmetrical, (b) the optimal body size for a mimic is not necessarily equal to that of the model, and/or (c) for predators, attacking better mimics of a model’s body size more readily is adaptive. I discuss promising avenues for improving our understanding of body size as a mimetic trait, including the importance of treatments that range in both directions from the model’s size. Further work is required to understand how body size ranks in saliency against other mimetic traits such as pattern. Comparative studies could investigate whether mimics are limited to resembling only models that are already similar in size.

Anti-bat ultrasound production in moths is globally and phylogenetically widespread

Warning signals are well known in the visual system, but rare in other modalities. Some moths produce ultrasonic sounds to warn bats of noxious taste or to mimic unpalatable models. Here, we report results from a long-term study across the globe, assaying moth response to playback of bat echolocation. We tested 252 genera, spanning most families of large-bodied moths, and document anti-bat ultrasound production in 52 genera, with eight subfamily origins described. Based on acoustic analysis of ultrasonic emissions and palatability experiments with bats, it seems that acoustic warning and mimicry are the raison d'être for sound production in most moths. However, some moths use high-duty-cycle ultrasound capable of jamming bat sonar. In fact, we find preliminary evidence of independent origins of sonar jamming in at least six subfamilies. Palatability data indicate that jamming and warning are not mutually exclusive strategies. To explore the possible organization of anti-bat warning sounds into acoustic mimicry rings, we intensively studied a community of moths in Ecuador and, using machine-learning approaches, found five distinct acoustic clusters. While these data represent an early understanding of acoustic aposematism and mimicry across this megadiverse insect order, it is likely that ultrasonically signaling moths comprise one of the largest mimicry complexes on earth.

Southeast Asian clearwing moths buzz like their model bees

Background

The endless struggle to survive has driven harmless species to evolve elaborate strategies of deceiving predators. Batesian mimicry involves imitations of noxious species’ warning signals by palatable mimics. Clearwing moths (Lepidoptera: Sesiidae), incapable of inflicting painful bites or stings, resemble bees or wasps in their morphology and sometimes imitate their behaviours. An entirely unexplored type of deception in sesiids is acoustic mimicry. We recorded the buzzing sounds of two species of Southeast Asian clearwing moths, Heterosphecia pahangensis and H. hyaloptera and compared them to their visual model bee, Tetragonilla collina, and two control species of bees occurring in the same habitat. Recordings were performed on untethered, flying insects in nature.

Results

Based on eight acoustic parameters and wingbeat frequencies calculated from slow-motion videos, we found that the buzzes produced by both clearwing moths highly resemble those of T. collina but differ from the two control species of bees.

Conclusions

Acoustic similarities to bees, alongside morphological and behavioural imitations, indicate that clearwing moths display multimodal mimicry of their evolutionary models.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00419-8.

Convergent Evolution of Wingbeat-Powered Anti-Bat Ultrasound in the Microlepidoptera

Bats and moths provide a textbook example of predator-prey evolutionary arms races, demonstrating adaptations, and counter adaptations on both sides. The evolutionary responses of moths to the biosonar-led hunting strategies of insectivorous bats include convergently evolved hearing structures tuned to detect bat echolocation frequencies. These allow many moths to detect hunting bats and manoeuvre to safety, or in the case of some taxa, respond by emitting sounds which startle bats, jam their biosonar, and/or warn them of distastefulness. Until now, research has focused on the larger macrolepidoptera, but the recent discovery of wingbeat-powered anti-bat sounds in a genus of deaf microlepidoptera (Yponomeuta), suggests that the speciose but understudied microlepidoptera possess further and more widespread anti-bat defences. Here we demonstrate that wingbeat-powered ultrasound production, likely providing an anti-bat function, appears to indeed be spread widely in the microlepidoptera; showing that acoustically active structures (aeroelastic tymbals, ATs) have evolved in at least three, and likely four different regions of the wing. Two of these tymbals are found in multiple microlepidopteran superfamilies, and remarkably, three were found in a single subfamily. We document and characterise sound production from four microlepidopteran taxa previously considered silent. Our findings demonstrate that the microlepidoptera contribute their own unwritten chapters to the textbook bat-moth coevolutionary arms race.

Increased speed of movement reduced identification of Batesian ant-mimicking spiders by surrogate predators

In Batesian mimicry, the mimic gains protection from predators by imitating a noxious model. Some myrmecomorphic species use ants as models as ants have strong defensive capabilities. Ants are highly mobile models, and besides colour, shape, and size, mimics also imitate their movement. Yet, former studies focused mainly on static traits. Here, I tested the hypothesis that artificially increased speed of movement reduces the probability of the mimic being identified. First, images of 14 myrmecomorphic spider species and their models were used for humans to rank their mimetic accuracy. Humans were used as surrogate predators to obtain scores for each mimetic pair. In the second experiment, the effect of movement playback speed on the probability of identification was investigated, again using humans. Videos of mimics were played at different speeds, and the identification probability was recorded. While ants were correctly identified at any playback speed, the identification of myrmecomorphic spiders declined with increasing playback speed. In other words, the latency to correct identification increased with playback speed. Overall, mimics with higher accuracy scores were more difficult to identify while moving. The natural speed of movement of accurate mimics was similar to that of inaccurate ones. Movement is thus an important trait for myrmecomorphic species.

Survival Sounds in Insects: Diversity, Function, and Evolution

Insect defense sounds have been reported for centuries. Yet, aside from the well-studied anti-bat sounds of tiger moths, little is understood about the occurrence, function, and evolution of these sounds. We define a defense sound as an acoustic signal (air- or solid-borne vibration) produced in response to attack or threat of attack by a predator or parasitoid and that promotes survival. Defense sounds have been described in 12 insect orders, across different developmental stages, and between sexes. The mechanisms of defensive sound production include stridulation, percussion, tymbalation, tremulation, and forced air. Signal characteristics vary between species, and we discuss how morphology, the intended receiver, and specific functions of the sounds could explain this variation. Sounds can be directed at predators or non-predators, and proposed functions include startle, aposematism, jamming, and alarm, although experimental evidence for these hypotheses remains scant for many insects. The evolutionary origins of defense sounds in insects have not been rigorously investigated using phylogenetic methodology, but in most cases it is hypothesized that they evolved from incidental sounds associated with non-signaling behaviors such as flight or ventilatory movements. Compared to our understanding of visual defenses in insects, sonic defenses are poorly understood. We recommend that future investigations focus on testing hypotheses explaining the functions and evolution of these survival sounds using predator-prey experiments and comparative phylogenetics.

Abundance and richness of Arctiinae moths throughout the night in a Cerrado area

Abstract: The main goal of this work was to investigate how the abundance and richness of Arctiinae moths varies over time, during the night. Specifically, we analyzed the following questions: (1) Is there a relationship between Arctiinae abundance and richness with the temperature and relative humidity? (2) What are the hours of activity of each species of moth? (3) Does the species composition differ over night? (4) Is it necessary to sample this group of moths throughout the night to have a representative sample of the species? We sampled the moths in Emas National Park (17°49’-18°28’S and 52°39’-53°10’W), Brazil. We selected seven sampling points in an area of savanna. At each sampling point, we collected the Arctiinae moths with a light trap (with a 15-W black light fluorescent light bulb), reflected in a white cloth (2 x 3 m) extended vertically. We sampled the moths in seven consecutive nights (one night in each sampling point, from December 13 to December 19, 2012, from 7 p.m. until 7 a.m.). We divided the samplings in twelve periods over the night, with an hour each. At each period of time, we measured the temperature and the relative humidity with a digital termohygrometer. We sampled 149 individuals belonging to 17 species of Arctiinae moths. Most species (70.5%) were active only for one or two hours at night. The species differed in terms of time activity. The higher abundance occurred at 8 p. m. (44 individuals), followed by 38 individuals at 9 p. m. and 23 at 10 p. m. The species richness was also higher in the early hours of the night. The temperature was the only variable that showed a positive and significative relationship with the Arctiinae moth abundance. The species richness was not influenced neither by the temperature nor by the relative air humidity. The possible causes of the peak of abundance and species richness in specific hours of the night are discussed.

Characteristics of tiger moth (Erebidae: Arctiinae) anti-bat sounds can be predicted from tymbal morphology

Background

Acoustic signals are used by many animals to transmit information. Variation in the acoustic characteristics of these signals often covaries with morphology and can relay information about an individual’s fitness, sex, species, and/or other characteristics important for both mating and defense. Tiger moths (Lepidoptera: Erebidae: Arctiinae) use modified cuticular plates called “tymbal organs” to produce ultrasonic clicks which can aposematically signal their toxicity, mimic the signals of other species, or, in some cases, disrupt bat echolocation. The morphology of the tymbal organs and the sounds they produce vary greatly between species, but it is unclear how the variation in morphology gives rise to the variation in acoustic characteristics. This is the first study to determine how the morphological features of tymbals can predict the acoustic characteristics of the signals they produce.

Results

We show that the number of striations on the tymbal surface (historically known as “microtymbals”) and, to a lesser extent, the ratio of the projected surface area of the tymbal to that of the thorax have a strong, positive correlation with the number of clicks a moth produces per unit time. We also found that some clades have significantly different regression coefficients, and thus the relationship between microtymbals and click rate is also dependent on the shared ancestry of different species.

Conclusions

Our predictive model allows the click rates of moths to be estimated using preserved material (e.g., from museums) in cases where live specimens are unavailable. This has the potential to greatly accelerate our understanding of the distribution of sound production and acoustic anti-bat strategies employed by tiger moths. Such knowledge will generate new insights into the evolutionary history of tiger moth anti-predator defenses on a global scale.

Natural selection in mimicry

Biological mimicry has served as a salient example of natural selection for over a century, providing us with a dazzling array of very different examples across many unrelated taxa. We provide a conceptual framework that brings together apparently disparate examples of mimicry in a single model for the purpose of comparing how natural selection affects models, mimics and signal receivers across different interactions. We first analyse how model-mimic resemblance likely affects the fitness of models, mimics and receivers across diverse examples. These include classic Batesian and Müllerian butterfly systems, nectarless orchids that mimic Hymenoptera or nectar-producing plants, caterpillars that mimic inert objects unlikely to be perceived as food, plants that mimic abiotic objects like carrion or dung and aggressive mimicry where predators mimic food items of their own prey. From this, we construct a conceptual framework of the selective forces that form the basis of all mimetic interactions. These interactions between models, mimics and receivers may follow four possible evolutionary pathways in terms of the direction of selection resulting from model-mimic resemblance. Two of these pathways correspond to the selective pressures associated with what is widely regarded as Batesian and Müllerian mimicry. The other two pathways suggest mimetic interactions underpinned by distinct selective pressures that have largely remained unrecognized. Each pathway is characterized by theoretical differences in how model-mimic resemblance influences the direction of selection acting on mimics, models and signal receivers, and the potential for consequent (co)evolutionary relationships between these three protagonists. The final part of this review describes how selective forces generated through model-mimic resemblance can be opposed by the basic ecology of interacting organisms and how those forces may affect the symmetry, strength and likelihood of (co)evolution between the three protagonists within the confines of the four broad evolutionary possibilities. We provide a clear and pragmatic visualization of selection pressures that portrays how different mimicry types may evolve. This conceptual framework provides clarity on how different selective forces acting on mimics, models and receivers are likely to interact and ultimately shape the evolutionary pathways taken by mimetic interactions, as well as the constraints inherent within these interactions.

Phylogenomics resolves major relationships and reveals significant diversification rate shifts in the evolution of silk moths and relatives

BACKGROUND

Silkmoths and their relatives constitute the ecologically and taxonomically diverse superfamily Bombycoidea, which includes some of the most charismatic species of Lepidoptera. Despite displaying spectacular forms and diverse ecological traits, relatively little attention has been given to understanding their evolution and drivers of their diversity. To begin to address this problem, we created a new Bombycoidea-specific Anchored Hybrid Enrichment (AHE) probe set and sampled up to 571 loci for 117 taxa across all major lineages of the Bombycoidea, with a newly developed DNA extraction protocol that allows Lepidoptera specimens to be readily sequenced from pinned natural history collections.

RESULTS

The well-supported tree was overall consistent with prior morphological and molecular studies, although some taxa were misplaced. The bombycid Arotros Schaus was formally transferred to Apatelodidae. We identified important evolutionary patterns (e.g., morphology, biogeography, and differences in speciation and extinction), and our analysis of diversification rates highlights the stark increases that exist within the Sphingidae (hawkmoths) and Saturniidae (wild silkmoths).

CONCLUSIONS

Our study establishes a backbone for future evolutionary, comparative, and taxonomic studies of Bombycoidea. We postulate that the rate shifts identified are due to the well-documented bat-moth "arms race". Our research highlights the flexibility of AHE to generate genomic data from a wide range of museum specimens, both age and preservation method, and will allow researchers to tap into the wealth of biological data residing in natural history collections around the globe.

Local-Scale Bat Guild Activity Differs with Rice Growth Stage at Ground Level in the Philippines

High-flying insectivorous bats, as wide-ranging generalist insectivores, are valuable consumers of high-altitude migrating pests of rice in Southeast Asia. Here, we documented the behavior of relatively low-flying bats over irrigated rice to elucidate their potential role as predators of rice-associated pest insects in the Philippines. Specifically, we tested the local-scale effects of rice stage, particularly seedling and late vegetative stages, and time of night on acoustic activity of bats foraging near ground level within three functional guilds (based on foraging distance from background clutter). We also monitored bat activity from two 50 m-high towers to assess the vertical extent of relatively low-flying guilds, as well as document high-flying bat guild presence and temporal behavior. At ground level, the most active guild biased their activity and feeding over early growth stage fields, but also foraged at tower level. Activity of the bat guild adept at foraging closest to vegetation did not vary with time of night or rice stage and was absent from tower recordings. High-flying bats were predictably rare at rice level, but exhibited high foraging intensity at 50 m. Given the well-documented, sequential arrival of insect guilds with growth stage, these data suggest that at ground level edge-space bats may be important consumers of detritivores (e.g., mosquitoes). Moreover, our data suggest that just as habitat heterogeneity enhances the services of arthropod predators, these management practices also enhance bat activity and, presumably, their contribution to pest suppression.

Biomechanics of a moth scale at ultrasonic frequencies

The wings of moths and butterflies are densely covered in scales that exhibit intricate shapes and sculptured nanostructures. While certain butterfly scales create nanoscale photonic effects, moth scales show different nanostructures suggesting different functionality. Here we investigate moth-scale vibrodynamics to understand their role in creating acoustic camouflage against bat echolocation, where scales on wings provide ultrasound absorber functionality. For this, individual scales can be considered as building blocks with adapted biomechanical properties at ultrasonic frequencies. The 3D nanostructure of a full Bunaea alcinoe moth forewing scale was characterized using confocal microscopy. Structurally, this scale is double layered and endowed with different perforation rates on the upper and lower laminae, which are interconnected by trabeculae pillars. From these observations a parameterized model of the scale's nanostructure was formed and its effective elastic stiffness matrix extracted. Macroscale numerical modeling of scale vibrodynamics showed close qualitative and quantitative agreement with scanning laser Doppler vibrometry measurement of this scale's oscillations, suggesting that the governing biomechanics have been captured accurately. Importantly, this scale of B. alcinoe exhibits its first three resonances in the typical echolocation frequency range of bats, suggesting it has evolved as a resonant absorber. Damping coefficients of the moth-scale resonator and ultrasonic absorption of a scaled wing were estimated using numerical modeling. The calculated absorption coefficient of 0.50 agrees with the published maximum acoustic effect of wing scaling. Understanding scale vibroacoustic behavior helps create macroscopic structures with the capacity for broadband acoustic camouflage.

Fireflies thwart bat attack with multisensory warnings

Many defended animals prevent attacks by displaying warning signals that are highly conspicuous to their predators. We hypothesized that bioluminescing fireflies, widely known for their vibrant courtship signals, also advertise their noxiousness to echolocating bats. To test this postulate, we pit naïve big brown bats (Eptesicus fuscus) against chemically defended fireflies (Photinus pyralis) to examine whether and how these beetles transmit salient warnings to bats. We demonstrate that these nocturnal predators learn to avoid noxious fireflies using either vision or echolocation and that bats learn faster when integrating information from both sensory streams-providing fundamental evidence that multisensory integration increases the efficacy of warning signals in a natural predator-prey system. Our findings add support for a warning signal origin of firefly bioluminescence and suggest that bat predation may have driven evolution of firefly bioluminescence.

Adaptations for Substrate Gleaning in Bats: The Pallid Bat as a Case Study

Substrate gleaning is a foraging strategy in which bats use a mixture of echolocation, prey-generated sounds, and vision to localize and hunt surface-dwelling prey. Many substrate-gleaning species depend primarily on prey-generated noise to hunt. Use of echolocation is limited to general orientation and obstacle avoidance. This foraging strategy involves a different set of selective pressures on morphology, behavior, and auditory system organization of bats compared to the use of echolocation for both hunting and navigation. Gleaning likely evolved to hunt in cluttered environments and/or as a counterstrategy to reduce detection by eared prey. Gleaning bats simultaneously receive streams of echoes from obstacles and prey-generated noise, and have to segregate these acoustic streams to attend to one or both. Not only do these bats have to be exquisitely sensitive to the soft, low frequency sounds produced by walking/rustling prey, they also have to precisely localize these sounds. Gleaners typically use low intensity echolocation calls. Such stealth echolocation requires a nervous system that is attuned to low intensity sound processing. In addition, landing on the ground to hunt may bring gleaners in close proximity to venomous prey. In fact, at least 2 gleaning bat species are known to hunt highly venomous scorpions. While a number of studies have addressed adaptations for echolocation in bats that hunt in the air, very little is known about the morphological, behavioral, and neural specializations for gleaning in bats. This review highlights the novel insights gleaning bats provide into bat evolution, particularly auditory pathway organization and ion channel structure/function relationships. Gleaning bats are found in multiple families, suggesting convergent evolution of specializations for gleaning as a foraging strategy. However, most of this review is based on recent work on a single species – the pallid bat (Antrozous palli dus) – symptomatic of the fact that more comparative work is needed to identify the mechanisms that facilitate gleaning behavior.

A phylogenomic analysis of lichen-feeding tiger moths uncovers evolutionary origins of host chemical sequestration

Host species utilize a variety of defenses to deter feeding, including secondary chemicals. Some phytophagous insects have evolved tolerance to these chemical defenses, and can sequester secondary defense compounds for use against their own predators and parasitoids. While numerous studies have examined plant-insect interactions, little is known about lichen-insect interactions. Our study focused on reconstructing the evolution of lichen phenolic sequestration in the tiger moth tribe Lithosiini (Lepidoptera: Erebidae: Arctiinae), the most diverse lineage of lichen-feeding moths, with 3000 described species. We built an RNA-Seq dataset and examined the adult metabolome for the presence of lichen-derived phenolics. Using the transcriptomic dataset, we recover a well-resolved phylogeny of the Lithosiini, and determine that the metabolomes within species are more similar than those among species. Results from an initial ancestral state reconstruction suggest that the ability to sequester phenolics produced by a single chemical pathway preceded generalist sequestration of phenolics produced by multiple chemical pathways. We conclude that phenolics are consistently and selectively sequestered within Lithosiini. Furthermore, sequestration of compounds from a single chemical pathway may represent a synapomorphy of the tribe, and the ability to sequester phenolics produced by multiple pathways arose later. These findings expand on our understanding of the interactions between Lepidoptera and their lichen hosts.

Sensing in a noisy world: lessons from auditory specialists, echolocating bats

All animals face the essential task of extracting biologically meaningful sensory information from the ‘noisy’ backdrop of their environments. Here, we examine mechanisms used by echolocating bats to localize objects, track small prey and communicate in complex and noisy acoustic environments. Bats actively control and coordinate both the emission and reception of sound stimuli through integrated sensory and motor mechanisms that have evolved together over tens of millions of years. We discuss how bats behave in different ecological scenarios, including detecting and discriminating target echoes from background objects, minimizing acoustic interference from competing conspecifics and overcoming insect noise. Bats tackle these problems by deploying a remarkable array of auditory behaviors, sometimes in combination with the use of other senses. Behavioral strategies such as ceasing sonar call production and active jamming of the signals of competitors provide further insight into the capabilities and limitations of echolocation. We relate these findings to the broader topic of how animals extract relevant sensory information in noisy environments. While bats have highly refined abilities for operating under noisy conditions, they face the same challenges encountered by many other species. We propose that the specialized sensory mechanisms identified in bats are likely to occur in analogous systems across the animal kingdom.

Evolutionary escalation: the bat-moth arms race

Echolocation in bats and high-frequency hearing in their insect prey make bats and insects an ideal system for studying the sensory ecology and neuroethology of predator-prey interactions. Here, we review the evolutionary history of bats and eared insects, focusing on the insect order Lepidoptera, and consider the evidence for antipredator adaptations and predator counter-adaptations. Ears evolved in a remarkable number of body locations across insects, with the original selection pressure for ears differing between groups. Although cause and effect are difficult to determine, correlations between hearing and life history strategies in moths provide evidence for how these two variables influence each other. We consider life history variables such as size, sex, circadian and seasonal activity patterns, geographic range and the composition of sympatric bat communities. We also review hypotheses on the neural basis for anti-predator behaviours (such as evasive flight and sound production) in moths. It is assumed that these prey adaptations would select for counter-adaptations in predatory bats. We suggest two levels of support for classifying bat traits as counter-adaptations: traits that allow bats to eat more eared prey than expected based on their availability in the environment provide a low level of support for counter-adaptations, whereas traits that have no other plausible explanation for their origination and maintenance than capturing defended prey constitute a high level of support. Specific predator counter-adaptations include calling at frequencies outside the sensitivity range of most eared prey, changing the pattern and frequency of echolocation calls during prey pursuit, and quiet, or 'stealth', echolocation.

Hearing diversity in moths confronting a neotropical bat assemblage

The tympanal ear is an evolutionary acquisition which helps moths survive predation from bats. The greater diversity of bats and echolocation strategies in the Neotropics compared with temperate zones would be expected to impose different sensory requirements on the neotropical moths. However, even given some variability among moth assemblages, the frequencies of best hearing of moths from different climate zones studied to date have been roughly the same: between 20 and 60 kHz. We have analyzed the auditory characteristics of tympanate moths from Cuba, a neotropical island with high levels of bat diversity and a high incidence of echolocation frequencies above those commonly at the upper limit of moths' hearing sensitivity. Moths of the superfamilies Noctuoidea, Geometroidea and Pyraloidea were examined. Audiograms were determined by non-invasively measuring distortion-product otoacoustic emissions. We also quantified the frequency spectrum of the echolocation sounds to which this moth community is exposed. The hearing ranges of moths in our study showed best frequencies between 36 and 94 kHz. High sensitivity to frequencies above 50 kHz suggests that the auditory sensitivity of moths is suited to the sounds used by sympatric echolocating bat fauna. Biodiversity characterizes predators and prey in the Neotropics, but the bat-moth acoustic interaction keeps spectrally matched.

A Comparative Analysis of Sonic Defences in Bombycoidea Caterpillars

Caterpillars have long been used as models for studying animal defence. Their impressive armour, including flamboyant warning colours, poisonous spines, irritating sprays, and mimicry of plant parts, snakes and bird droppings, has been extensively documented. But research has mainly focused on visual and chemical displays. Here we show that some caterpillars also exhibit sonic displays. During simulated attacks, 45% of 38 genera and 33% of 61 species of silk and hawkmoth caterpillars (Bombycoidea) produced sounds. Sonic caterpillars are found in many distantly-related groups of Bombycoidea, and have evolved four distinct sound types- clicks, chirps, whistles and vocalizations. We propose that different sounds convey different messages, with some designed to warn of a chemical defence and others, to startle predators. This research underscores the importance of exploring acoustic communication in juvenile insects, and provides a model system to explore how different signals have evolved to frighten, warn or even trick predators.

Acoustic Aposematism and Evasive Action in Select Chemically Defended Arctiine (Lepidoptera: Erebidae) Species: Nonchalant or Not?

Tiger moths (Erebidae: Arctiinae) have experienced intense selective pressure from echolocating, insectivorous bats for over 65 million years. One outcome has been the evolution of acoustic signals that advertise the presence of toxins sequestered from the moths’ larval host plants, i.e. acoustic aposematism. Little is known about the effectiveness of tiger moth anti-bat sounds in their natural environments. We used multiple infrared cameras to reconstruct bat-moth interactions in three-dimensional (3-D) space to examine how functional sound-producing organs called tymbals affect predation of two chemically defended tiger moth species: Pygarctia roseicapitis (Arctiini) and Cisthene martini (Lithosiini). P. roseicapitis and C. martini with intact tymbals were 1.8 and 1.6 times less likely to be captured by bats relative to those rendered silent. 3-D flight path and acoustic analyses indicated that bats actively avoided capturing sound-producing moths. Clicking behavior differed between the two tiger moth species, with P. roseicapitis responding in an earlier phase of bat attack. Evasive flight behavior in response to bat attacks was markedly different between the two tiger moth species. P. roseicapitis frequently paired evasive dives with aposematic sound production. C. martini were considerably more nonchalant and employed evasion in fewer interactions. Our results show that acoustic aposematism is effective at deterring bat predation in a natural context and that this strategy is likely to be the ancestral function of tymbal organs within the Arctiinae.

Crying wolf to a predator: deceptive vocal mimicry by a bird protecting young

Animals often mimic dangerous or toxic species to deter predators; however, mimicry of such species may not always be possible and mimicry of benign species seems unlikely to confer anti-predator benefits. We reveal a system in which a bird mimics the alarm calls of harmless species to fool a predator 40 times its size and protect its offspring against attack. Our experiments revealed that brown thornbills (Acanthiza pusilla) mimic a chorus of other species' aerial alarm calls, a cue of an Accipiter hawk in flight, when predators attack their nest. The absence of any flying predators in this context implies that these alarms convey deceptive information about the type of danger present. Experiments on the primary nest predators of thornbills, pied currawongs (Strepera graculina), revealed that the predators treat these alarms as if they themselves are threatened by flying hawks, either by scanning the sky for danger or fleeing, confirming a deceptive function. In turn, these distractions delay attack and provide thornbill nestlings with an opportunity to escape. This sophisticated defence strategy exploits the complex web of interactions among multiple species across several trophic levels, and in particular exploits a predator's ability to eavesdrop on and respond appropriately to heterospecific alarm calls. Our findings demonstrate that prey can fool predators by deceptively mimicking alarm calls of harmless species, suggesting that defensive mimicry could be more widespread because of indirect effects on predators within a web of eavesdropping.

Identification of pyridine analogs as new predator-derived kairomones

In the wild, animals have developed survival strategies relying on their senses. The individual ability to identify threatening situations is crucial and leads to increase in the overall fitness of the species. Rodents, for example have developed in their nasal cavities specialized olfactory neurons implicated in the detection of volatile cues encoding for impending danger such as predator scents or alarm pheromones. In particular, the neurons of the Grueneberg ganglion (GG), an olfactory subsystem, are implicated in the detection of danger cues sharing a similar chemical signature, a heterocyclic sulfur- or nitrogen-containing motif. Here we used a “from the wild to the lab” approach to identify new molecules that are involuntarily emitted by predators and that initiate fear-related responses in the recipient animal, the putative prey. We collected urines from carnivores as sources of predator scents and first verified their impact on the blood pressure of the mice. With this approach, the urine of the mountain lion emerged as the most potent source of chemical stress. We then identified in this biological fluid, new volatile cues with characteristic GG-related fingerprints, in particular the methylated pyridine structures, 2,4-lutidine and its analogs. We finally verified their encoded danger quality and demonstrated their ability to mimic the effects of the predator urine on GG neurons, on mice blood pressure and in behavioral experiments. In summary, we were able to identify here, with the use of an integrative approach, new relevant molecules, the pyridine analogs, implicated in interspecies danger communication.

Tempo and mode of antibat ultrasound production and sonar jamming in the diverse hawkmoth radiation

The bat-moth arms race has existed for over 60 million y, with moths evolving ultrasonically sensitive ears and ultrasound-producing organs to combat bat predation. The evolution of these defenses has never been thoroughly examined because of limitations in simultaneously conducting behavioral and phylogenetic analyses across an entire group. Hawkmoths include >1,500 species worldwide, some of which produce ultrasound using genital stridulatory structures. However, the function and evolution of this behavior remain largely unknown. We built a comprehensive behavioral dataset of hawkmoth hearing and ultrasonic reply to sonar attack using high-throughput field assays. Nearly half of the species tested (57 of 124 species) produced ultrasound to tactile stimulation or playback of bat echolocation attack. To test the function of ultrasound, we pitted big brown bats (Eptesicus fuscus) against hawkmoths over multiple nights and show that hawkmoths jam bat sonar. Ultrasound production was immediately and consistently effective at thwarting attack and bats regularly performed catching behavior without capturing moths. We also constructed a fossil-calibrated, multigene phylogeny to study the evolutionary history and divergence times of these antibat strategies across the entire family. We show that ultrasound production arose in multiple groups, starting in the late Oligocene (∼ 26 Ma) after the emergence of insectivorous bats. Sonar jamming and bat-detecting ears arose twice, independently, in the Miocene (18-14 Ma) either from earless hawkmoths that produced ultrasound in response to physical contact only, or from species that did not respond to touch or bat echolocation attack.

Body size matters for aposematic prey during predator aversion learning

Aposematic prey advertise their toxicity to predators using conspicuous warning signals, which predators learn to use to reduce their intake of toxic prey. Like other types of prey, aposematic prey often differ in body size, both within and between species. Increasing body size can increase signal size, which make larger aposematic prey more detectable but also gives them a more effective and salient deterrent. However, increasing body size also increases the nutritional value of prey, and larger aposematic prey may make a more profitable meal to predators that are trading off the costs of eating toxins with the benefits of ingesting nutrients. We tested if body size, independent of signal size, affected predation of toxic prey as predators learn to reduce their attacks on them. European starlings (Sturnus vulgaris) learned to discriminate between defended (quinine-injected) and undefended (water-injected) mealworm prey (Tenebrio molitor) using visual signals. During this process, we found that birds attacked and ate more defended prey the larger they were. Body size does affect the probability that toxic prey are attacked and eaten, which has implications for the evolutionary dynamics of aposematism and mimicry (where species share the same warning pattern).

Avian vocal mimicry: a unified conceptual framework

Mimicry is a classical example of adaptive signal design. Here, we review the current state of research into vocal mimicry in birds. Avian vocal mimicry is a conspicuous and often spectacular form of animal communication, occurring in many distantly related species. However, the proximate and ultimate causes of vocal mimicry are poorly understood. In the first part of this review, we argue that progress has been impeded by conceptual confusion over what constitutes vocal mimicry. We propose a modified version of Vane-Wright's (1980) widely used definition of mimicry. According to our definition, a vocalisation is mimetic if the behaviour of the receiver changes after perceiving the acoustic resemblance between the mimic and the model, and the behavioural change confers a selective advantage on the mimic. Mimicry is therefore specifically a functional concept where the resemblance between heterospecific sounds is a target of selection. It is distinct from other forms of vocal resemblance including those that are the result of chance or common ancestry, and those that have emerged as a by-product of other processes such as ecological convergence and selection for large song-type repertoires. Thus, our definition provides a general and functionally coherent framework for determining what constitutes vocal mimicry, and takes account of the diversity of vocalisations that incorporate heterospecific sounds. In the second part we assess and revise hypotheses for the evolution of avian vocal mimicry in the light of our new definition. Most of the current evidence is anecdotal, but the diverse contexts and acoustic structures of putative vocal mimicry suggest that mimicry has multiple functions across and within species. There is strong experimental evidence that vocal mimicry can be deceptive, and can facilitate parasitic interactions. There is also increasing support for the use of vocal mimicry in predator defence, although the mechanisms are unclear. Less progress has been made in explaining why many birds incorporate heterospecific sounds into their sexual displays, and in determining whether these vocalisations are functionally mimetic or by-products of sexual selection for other traits such as repertoire size. Overall, this discussion reveals a more central role for vocal mimicry in the behavioural ecology of birds than has previously been appreciated. The final part of this review identifies important areas for future research. Detailed empirical data are needed on individual species, including on the structure of mimetic signals, the contexts in which mimicry is produced, how mimicry is acquired, and the ecological relationships between mimic, model and receiver. At present, there is little information and no consensus about the various costs of vocal mimicry for the protagonists in the mimicry complex. The diversity and complexity of vocal mimicry in birds raises important questions for the study of animal communication and challenges our view of the nature of mimicry itself. Therefore, a better understanding of avian vocal mimicry is essential if we are to account fully for the diversity of animal signals.

Niche-specific cognitive strategies: object memory interferes with spatial memory in the predatory bat Myotis nattereri

Related species with different diets are predicted to rely on different cognitive strategies: those best suited for locating available and appropriate foods. Here we tested two predictions of the niche-specific cognitive strategies hypothesis in bats, which suggests that predatory species should rely more on object memory than on spatial memory for finding food and that the opposite is true of frugivorous and nectivorous species. Specifically, we predicted that: (1) predatory bats would readily learn to associate shapes with palatable prey and (2) once bats had made such associations, these would interfere with their subsequent learning of a spatial memory task. We trained free-flying Myotis nattereri to approach palatable and unpalatable insect prey suspended below polystyrene objects. Experimentally naïve bats learned to associate different objects with palatable and unpalatable prey but performed no better than chance in a subsequent spatial memory experiment. Because experimental sequence was predicted to be of consequence, we introduced a second group of bats first to the spatial memory experiment. These bats learned to associate prey position with palatability. Control trials indicated that bats made their decisions based on information acquired through echolocation. Previous studies have shown that bat species that eat mainly nectar and fruit rely heavily on spatial memory, reflecting the relative consistency of distribution of fruit and nectar compared with insects. Our results support the niche-specific cognitive strategies hypothesis and suggest that for gleaning and clutter-resistant aerial hawking bats, learning to associate shape with food interferes with subsequent spatial memory learning.

Evolution of deceptive and true courtship songs in moths

Ultrasonic mating signals in moths are argued to have evolved via exploitation of the receivers' sensory bias towards bat echolocation calls. We have demonstrated that female moths of the Asian corn borer are unable to distinguish between the male courtship song and bat calls. Females react to both the male song and bat calls by “freezing”, which males take advantage of in mating (deceptive courtship song). In contrast, females of the Japanese lichen moth are able to distinguish between the male song and bat calls by the structure of the sounds; females emit warning clicks against bats, but accept males (true courtship song). Here, we propose a hypothesis that deceptive and true signals evolved independently from slightly different precursory sounds; deceptive/true courtship songs in moths evolved from the sounds males incidentally emitted in a sexual context, which females could not/could distinguish, respectively, from bat calls.