The development of behavioral defenses: a mechanistic analysis of vulnerability in red-eyed tree frog hatchlings

I examined the development and effectiveness of behavioral defensesof red-eyed tree frog hatchlings, Agalychnis callidryas, against predatoryshrimp Macrobrachium americanum. Arboreal eggs of A. callidryashatch early if attacked by egg predators and later if undisturbed,producing tadpoles that enter the water at different developmentalstages. Older hatchlings survive better than young hatchlings withaquatic predators, including shrimp. Hatchlings respond to shrimpby both increasing activity and avoiding the bottom microhabitat.Older hatchlings are more active and, in the presence of shrimp,avoid the bottom more than young hatchlings. These ontogeneticchanges in behavior improve survival. Specifically, the likelihoodof fleeing from shrimp increases with hatchling age, and fleeingis an effective defense. In contrast to results from experimentswith odonates, immobility does not reduce risk of shrimp attack, thusthere is no trade-off between fleeing and motionless crypsis.Shrimp spend most of their time on the bottom, where attacksare more often successful. Avoidance of the bottom microhabitatby tadpoles therefore improves survival. Evasive maneuvers alsofunction in defense, but evasiveness does not change with age.Morphology may limit microhabitat use by younger hatchlings.Failure to flee may reflect unresponsiveness to disturbance,which would reduce unnecessarily early hatching and limit exposureof young A. callidryas to aquatic predators.     

Prey subsidy or predator cue? Direct and indirect effects of caged predators on aquatic consumers and resources

The non-consumptive effects of predators on prey can affect prey phenotypes, potentially having important consequences for communities due to trait-mediated indirect interactions. Predicting non-consumptive effects and their impacts on communities can be difficult because predators can affect resources directly through nutrient cycling and indirectly by altering prey resource use, which can lead to complex interactions among resources and consumers. In this study we examined the effects of caged dragonfly predators on aquatic resources in the presence and absence of two focal herbivores, the tadpoles of Neotropical tree frogs Agalychnis callidryas and Dendropsophus ebraccatus. We crossed the presence/absence of caged dragonflies with four tadpole treatments: no tadpoles, each tadpole species alone, and both species together to examine interactions among tadpole composition, predator presence, and time on tadpole growth, resources, and zooplankton abundances. Predator effects on growth changed through ontogeny and was species-dependent. Predators initially reduced then dramatically increased A. callidryas growth, but had no effect on D. ebraccatus. Predators also increased the abundances of both periphyton and phytoplankton. However, there was no evidence of a trait-mediated trophic cascade (i.e., tadpole by predator interaction). Instead, nutrients from prey carcass subsidies likely played an increasingly important role in facilitating resources, and shaping tadpole growth, competitive interactions, and zooplankton abundances through time. In nutrient-poor aquatic systems the release of nutrients via the consumption of terrestrially derived prey items by aquatic predators may have important impacts on food webs by facilitating resources independent of the role of trait-mediated trophic cascades.     

To hatch and hatch not: similar selective trade-offs but different responses to egg predators in two closely related, syntopic treefrogs

Risk-sensitive hatching is adaptive for species facing a trade-off between egg-stage and post-hatching risks, and environmental variation in one or both stages. Such plasticity has been found in amphibians, fishes, reptiles and spiders, with red-eyed treefrogs (Agalychnis callidryas) being the best-studied case. We assessed hatching plasticity and egg- and larval-stage risks in a closely related, syntopic species, the gliding leaf-frog (Agalychnis spurrelli). We found a lower hatching response to egg-eating snakes in A. spurrelli (9–28% of embryos escaped) than in A. callidryas (59–80% escaped). Levels of snake predation were similarly high for clutches of both species monitored at a pond in Costa Rica, and in fish predation experiments early-hatched A. spurrelli tadpoles were more vulnerable than later hatchlings, as has been shown for A. callidryas. A. spurrelli thus face a risk trade-off similar to A. callidryas, and likely would benefit from predator-induced hatching; their lower responsiveness to snakes appears nonadaptive. A. spurrelli embryos showed a stronger hatching response (57% hatched in 1 h) to submergence underwater than to snake attacks even though submergence is a less frequent risk. This suggests they have a greater capacity for early hatching than is expressed in the context of snake attacks, but have much lower sensitivity to snake cues than to flooding cues. Development in A. spurrelli is accelerated compared to syntopic A. callidryas, and spontaneous hatching is earlier and more synchronous. This is congruent with predictions based on selection by egg predators in the absence of a strong escape hatching response.     

Are Aristolochic Acids Responsible for the Chemical Defence of Aposematic Larvae of Battus polydamas (L.) (Lepidoptera: Papilionidae)?

Aristolochic acids (AAs) are thought to be responsible for the chemical protection of the aposematic larvae Battus polydamas (L.) (Papilionidae: Troidini) against predators. These compounds are sequestered by larvae from their Aristolochia (Aristolochiaceae) host plants. Studying the role of the chemical protection of the second and fifth instars of B. polydamas against potential predators, we found that the consumption of larvae by the carpenter ant Camponotus crassus Mayr and young chicks Gallus gallus domesticus was dependent on larval developmental stage. Second instars were more preyed upon than fifth instars; however, the assassin bug Montina confusa Stål was not deterred by chemical defences of the fifth instar B. polydamas. Laboratory bioassays with carpenter ants and young chicks using palatable baits topically treated with a pure commercial mixture of AAs I and AAs II in concentrations up to 100 times those previously found in B. polydamas larvae showed no activity. Similar results were found in field bioassays, where palatable baits treated as above were exposed to the guild of predators that attack B. polydamas larvae and were also consumed irrespective of the commercial AA concentration used. These results suggest that the mixture of AAs I and AAs II have no defensive role against predators, at least against those investigated in the present work. Other compounds present in Aristolochia host plants such as O-glycosylated AAs; benzylisoquinoline alkaloids; and mono-, sesqui-, di-, and triterpenes, which can be sequestered by Troidini, could act as deterrents against predators.     

No evidence for differential survival or predation between sympatric color morphs of an aposematic poison frog

Because variation in warning signals slows down the predator education process, aposematic theory predicts that animal warning signals should be monomorphic. Yet, warning color polytypisms are not uncommon in aposematic species. In cases where warning signal variants are separated geographically, adaptation to local predators could explain this variation. However, this cannot explain the persistence of sympatric polymorphisms in aposematic taxa. The strawberry poison frog (Oophaga pumilio) exhibits both allopatric and sympatric warning color variation in and around the Bocas del Toro archipelago of Panama. One explanation that has been proposed for the rapid diversification of O. pumilio coloration in this archipelago is low predation; if island populations have few predators, stabilizing selection would be relaxed opening the door for diversification via selection or genetic drift. Using a combination of mark-recapture and clay model studies, we tested for differences in survival and predation among sympatric red and yellow color morphs of O. pumilio from Bastimentos Island. We found no evidence for differential survival or predation in this population, despite the fact that one morph (red) is more common and widely distributed than the other (yellow). Even in an area of the island where the yellow morph is not found, predator attack rates were similar among morphs. Visual modeling suggests that yellow and red morphs are distinguishable and conspicuous against a variety of backgrounds and by viewers with different visual systems. Our results suggest that general avoidance by predators of red and yellow, both of which are typical warning colors used throughout the animal kingdom, may be contributing to the apparent stability of this polymorphism.     

Interacting selection diversifies warning signals in a polytypic frog: an examination with the strawberry poison frog

Aposematic signals represent one of the most accessible traits to evaluate the interaction of natural and sexual selection on signal evolution. Here we investigate the contributions of these two selective forces on the aposematic signal evolution of the highly polytypic strawberry poison frog, Oophaga pumilio, of Bocas del Toro, Panama. Previous research has shown that the brightness of O. pumilio warning coloration can inform predators of the toxicity levels associated with different populations of the archipelago. Other studies suggest that sexual selection may be influencing warning signal brightness within populations via female mate choice (Isla Solarte, Isla Bastimentos, and Aquacate Peninsula populations) and male–male competition (Isla Solarte). Here we present two non-exclusive scenarios for how natural and sexual selection interact to drive phenotypic variation across this archipelago: (1) predators impose a selective regime whereby populations above a toxicity-brightness threshold are at liberty to diversify via sexual selection and below which populations are constrained to maintain a stricter resemblance to a more cryptic population mean, and (2) synergistic/additive effects of inter- and intrasexual selection drive the evolution of brighter males within populations above this toxicity threshold. We investigate whether aposematic patterns of divergence across the archipelago relative to the common mainland phenotype meet these predictions using existing data on O. pumilio morph toxicity measures and overall conspicuousness estimates to an avian predator. Using standardized z-scores to evaluate the range of trait values, we find that indeed the population representative of the common mainland phenotype (Almirante) represents an intermediate level of both toxicity and conspicuousness, and that derived Bocas del Toro populations vary in each of those components in directions predicted by the proposed scenarios. Furthermore, we find greater divergence towards conspicuousness than crypsis, a pattern suggestive of sexual and natural selection acting synergistically in morphs with high toxicity.     

Tadpole begging reveals high quality

Parents can benefit from allocating limited resources nonrandomly among offspring, and offspring solicitation (i.e. begging) is often hypothesized to evolve because it contains information valuable to choosy parents. We tested the predictions of three ‘honest begging’ hypotheses – Signal of Need, Signal of Quality and Signal of Hunger – in the tadpoles of a terrestrial frog (Oophaga pumilio). In this frog, mothers provision tadpoles with trophic eggs, and when mothers visit, tadpoles perform a putative begging signal by stiffening their bodies and vibrating rapidly. We assessed the information content of intense tadpole begging with an experimental manipulation of tadpole condition (need/quality) and food deprivation (hunger). This experiment revealed patterns consistent with the Signal of Quality hypothesis and directly counter to predictions of Signal of Need and Signal of Hunger. Begging effort and performance were higher in more developed and higher condition tadpoles and declined with food deprivation. Free‐living mothers were unlikely to feed tadpoles of a nonbegging species experimentally cross‐fostered with their own, and allocated larger meals to more developed tadpoles and those that vibrated at higher speed. Mother O. pumilio favour their high‐quality young, and because their concurrent offspring are reared in separate nurseries, must do so by making active allocation decisions. Our results suggest that these maternal choices are based at least in part on offspring signals, indicating that offspring solicitation can evolve to signal high quality.     

Interactions Between Competition and Predation Shape Early Growth and Survival of Two Neotropical Hylid Tadpoles

Experimental studies in temperate regions have revealed that competition and predation interact to shape aquatic communities. Predators typically reduce the effect of competition on growth and competitors provide alternative prey subjects, which may also alter predation. Here, we examine the independent and combined effects of competition and predation on the survival and growth of hatchling tadpoles of two widespread co‐occurring Neotropical hylid frogs (Agalychnis callidryas and Dendropsophus ebraccatus). Using 400 L mesocosms, we used a 2 × 3 factorial substitutive design, which crossed tadpole species composition with the presence or absence of a free‐roaming predator (Anax amazili dragonfly larva). Dragonflies were effective predators of both species, but had larger effects on A. callidryas survival. Both species had similar growth rates when alone, whereas A. callidryas grew 30 percent faster than D. ebraccatus when they co‐occurred, suggesting interspecific rather than intraspecific competition had relatively stronger effects on D. ebraccatus growth, while the opposite was true for A. callidryas. Predator presence dramatically reduced growth rates of both species and erased this asymmetry. Results suggest that the effects of predator induction (i.e., nonconsumptive effects) on growth were larger than both consumptive and competitive effects. Our study demonstrates that predators have strong effects on both survival and growth of prey, highlighting the potential importance of predators in shaping prey populations and tropical aquatic food web interactions. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Attract and deter: a dual role for pyrrolizidine alkaloids in plant–insect interactions

Pyrrolizidine alkaloids (PAs) are the major defense compounds of plants in the Senecio genus. Here I will review the effects of PAs in Senecio on the preference and performance of specialist and generalist insect herbivores. Specialist herbivores have evolved adaptation to PAs in their host plant. They can use the alkaloids as cue to find their host plant and often they sequester PAs for their own defense against predators. Generalists, on the other hand, can be deterred by PAs. PAs can also affect survival of generalist herbivores. Usually generalist insects avoid feeding on young Senecio leaves, which contain a high concentration of alkaloids. Structurally related PAs can differ in their effects on insect herbivores, some are more toxic than others. The differences in effects of PAs on specialist and generalists could lead to opposing selection on PAs, which may maintain the genetic diversity in PA concentration and composition in Senecio species.     

Clutch identity and predator-induced hatching affect behavior and development in a leaf-breeding treefrog

For species with complex life cycles, transitions between life stages result in niche shifts that are often associated with evolutionary trade-offs. When conditions across life stages are unpredictable, plasticity in niche shift timing may be adaptive; however, factors associated with clutch identity (e.g., genetic or maternal) may influence the effects of such plasticity. The red-eyed treefrog (Agalychnis callidryas) is an ideal organism for investigating the effects of genetics and life stage switch point timing because embryos exhibit adaptive phenotypic plasticity in hatching time. In this study, we evaluated the effects of experimentally manipulated hatching time and clutch identity on antipredator behavior of tadpoles and on developmental traits of metamorphs, including larval period, mass, SVL, and jumping ability. We found that in the presence of dragonfly nymph predator cues at 21 days post-oviposition, tadpoles reduced both their activity level and height in the water column. Furthermore, early-hatched tadpoles were less active than late-hatched tadpoles of the same age. This difference in behavior patterns of early- and late-hatched tadpoles may represent an adaptive response due to a longer period of susceptibility to odonate predators for early-hatched tadpoles, or it may be a carry-over effect mediated by early exposure to an environmental stressor (i.e., induction of early hatching). We also found that hatching time affected both behavioral traits and developmental traits, but its effect on developmental traits varied significantly among clutches. This study shows that a single early-life event may influence a suite of factors during subsequent life stages and that some of these effects appear to be dependent on clutch identity. This interaction may represent an evolutionary response to a complex life cycle and unpredictable environments, regardless of whether the clutch differences are due to additive genetic variance or maternal effects.     

Occurrence of theobromine synthase genes in purine alkaloid-free species of <Emphasis Type="Italic">Camellia</Emphasis> plants

Caffeine (1,3,7-trimethylxanthine) and theobromine (3,7-dimethylxanthine) are purine alkaloids that are present in high concentrations in plants of some species of Camellia. However, most members of the genus Camellia contain no purine alkaloids. Tracer experiments using [8-¹⁴C]adenine and [8-¹⁴C]theobromine showed that the purine alkaloid pathway is not fully functional in leaves of purine alkaloid-free species. In five species of purine alkaloid-free Camellia plants, sufficient evidence was obtained to show the occurrence of genes that are homologous to caffeine synthase. Recombinant enzymes derived from purine alkaloid-free species showed only theobromine synthase activity. Unlike the caffeine synthase gene, these genes were expressed more strongly in mature tissue than in young tissue. The nucleotide sequence data reported here have been deposited in the GenBank database under the accession numbers AB297451 (CjCS1), AB362882 (CgCS1), AB362883 (CgCS2), AB362884 (CkCS1), AB362885 (ClCS1), and AB362886 (CcCS2).     

Not all colors are equal: predation and color polytypism in the aposematic poison frog Oophaga pumilio

Aposematic organisms are not predicted to show high levels of warning signal diversity because they are expected to be under stabilizing selection to decrease costs of ‘educating’ predators about their unpalatability. However, systematic changes in warning signals (polytypism) can be expected if they represent adaptations to local predators. The aposematic strawberry poison frog (Oophaga pumilio) is red throughout its mainland distribution in Costa Rica and Panamá, but displays high levels of warning signal diversity in the Bocas del Toro Archipelago of Panamá. Both coloration and spot pattern vary in a polytypic sense. Sexual selection contributes to maintaining the polytypism, but little work has investigated the potential influence of predation. We used unspotted models of O. pumilio to determine if predation might help explain the color polytypism on Isla Colón in the Bocas del Toro Archipelago of Panamá. We tested whether attack rates differed among the red mainland morph, green/yellow Isla Colón morph, and the brown control. We found that frog color significantly predicted being attacked. The local green Isla Colón models were attacked more than foreign red or brown models. No difference in attack rate existed between red and brown control models. Our results suggest that the red mainland morph possesses a more effective warning signal, even when it is not the local morph. Honest signaling of unpalatability, neophobia, and the use of search images by local predators are potential explanations. Similarity of the brown model to other local poison frogs might explain the lower attack rate compared to previous work. The attack rate was lower on Isla Colón compared to mainland Costa Rica, which supports the hypothesis that less overall predation in the Bocas del Toro Archipelago may contribute to the overall warning signal diversity in O. pumilio there by relaxing selection for aposematic traits.     

Lateralized Turning Biases in Two Neotropical Tadpoles

Lateralized turning behavior in startle responses and upon descent after surfacing for a breath of air has been documented for tadpoles in several anuran species. A left‐handed preference is most common and was previously thought to be linked to the asymmetry in spiracle location. Here, we investigate the presence of behavioral asymmetries in tadpoles of Agalychnis callidryas and Leptodactylus melanonotus in explosive turns after air‐breathing. Data were also collected on lateralized posture in the orientation of tails of embryonic A. callidryas within the egg case, as well as the startle response of free‐swimming tadpoles exposed to mechanical stimulation. A left‐curled tail bias was found among several clutches from Costa Rica, but this was not the case among clutches examined from Panama. Free‐living tadpoles of L. melanonotus displayed a distinct right‐handed preference during explosive turns. While some tadpoles of A. callidryas were at stages too early to detect any explosive turns when breathing, later‐staged individuals did display a left‐handed bias in startle response to mechanical stimulation. Additionally, it appears that the expression of behavioral lateralization of larvae (but not embryos) may predict whether or not the adults exhibit similar lateralization. Findings herein provide insight into the ontogeny and evolutionary origins of lateralized behavior in anurans.     

Effects of size and size structure on predation and inter-cohort competition in red-eyed treefrog tadpoles

Individual and relative body size are key determinants of ecological performance, shaping the strength and types of interactions within and among species. Size-dependent performance is particularly important for iteroparous species with overlapping cohorts, determining the ability of new cohorts to invade habitats with older, larger conspecifics. We conducted two mesocosm experiments to examine the role of size and size structure in shaping growth and survival in tadpoles of the red-eyed treefrog (Agalychnis callidryas), a tropical species with a prolonged breeding season. First, we used a response surface design to quantify the competitive effect and response of two tadpole size classes across three competitive environments. Large tadpoles were superior per capita effect competitors, increasing the size difference between cohorts through time at high resource availability. Hatchlings were better per biomass response competitors, and maintained the size difference between cohorts when resource availability was low. However, in contrast to previous studies, small tadpoles never closed the size gap with large tadpoles. Second, we examine the relationship between body size, size structure, and predation by dragonfly nymphs (Anax amazili) on tadpole survival and growth. Hatchlings were more vulnerable to predation; predator and large competitor presence interacted to reduce hatchling growth. Again, the size gap between cohorts increased over time, but increased marginally more with predators present. These findings have implications for understanding how variation in resources and predation over the breeding season will shape population size structure through time and the ability of new cohorts to invade habitats with older conspecifics.     

Is tadpole coloration adaptive against bird predation?

Birds are visually oriented predators, and some are known or supposed to prey upon tadpoles. Neotropical tadpoles exhibit several colorations that could potentially improve survivorship through camouflage, aposematism, or mimetism. In this study, we tested tadpoles of two hylid species for protective effects of their colorations, one potentially aposematic (Bokermannohyla martinsi) and another (Ololygon machadoi) previously hypothesized as disruptive and/or mimetic to a naucorid insect (Limnocoris porphyros). We conducted in situ and ex situ experiments and observed that the uniformly black B. martinsi tadpoles were more intensely preyed upon by birds than those of O. machadoi, regardless of background color, which may have happened because the disruptive coloration of O. machadoi hampered its detection. Alternatively, the duration of the experiments may have been too short for birds to learn about a possible toxicity of B. martinsi tadpoles, which are more active and thus more visible. Birds that had a previous experience with L. porphyros tended to attack tadpoles of O. machadoi less than naive birds. On the other hand, we did not observe any change in predation levels upon B. martinsi tadpoles whether presented to birds before or after L. porphyros. Our results indicate that the predation by birds, although apparently occasional, is likely to be influenced by tadpole coloration.     

Differential efficacy of toxic pederin in deterring potential arthropod predators of Paederus (Coleoptera: Staphylinidae) offspring

This study investigates the effects of pederin, a hemolymph toxin that is accumulated in the eggs of most Paederus females, on potential arthropod predators of the offspring of P. fuscipes and P. riparius. Insects generally do not respond to pederin present in the prey. Paederus larvae are sufficiently agile to escape from these predators by running away, and the eggs are hidden by the females. Unlike insects, (wolf) spiders are deterred by prey with pederin. They turn away from larvae they have already captured and exhibit cleansing behavior. Larvae containing pederin survive the attacks of spiders without damage, whereas larvae descended from females that do not transfer pederin into their eggs are often killed and eaten. In the case of sudden attacks by spiders, the larvae have no chance of escape. Their survival thus depends on chemical defense. These investigations show for the first time why pederin might be of considerable importance for Paederus in the field.     

Tadpoles of the bronze frog (Rana temporalis) assess predation risk before evoking antipredator defense behavior

Predation threat-associated behavioral response was studied in Rana temporalis tadpoles to discover the importance of predators’ visual and chemical cues (kairomones and diet-derived metabolites of consumed prey) in evoking antipredator behavior. The caged predators (dragonfly larvae) fed on prey tadpoles or insects (Notonecta spp.) and water conditioned with the predators provided the threat stimuli to the tadpole prey. The predators’ visual cues were ineffective in evoking antipredator behaviors in the tadpole prey. However, exposure to caged tadpole-fed predators or water conditioned with tadpole-fed predators elicited predator avoidance behavior in the tadpoles; they stayed away from the predators, significantly reduced swimming activity (swimming time and distance traveled), and increased burst speed. Interestingly, exposure to water conditioned with starved predators did not elicit any antipredator behavior in the prey. Further, the antipredator responses of predator-experienced tadpoles were significantly greater than those exhibited by predator-na?ve tadpoles. The study shows that R. temporalis tadpoles assess predation threat based exclusively on chemical cues emanating from the predators’ dietary metabolites and that the inclusion of conspecific prey items in the diet of the predators is perceived as a threat. The study also shows that antipredator behavior in these tadpoles is innate and is enhanced during subsequent encounters with the predators.     

Body size and group size of Cuban tree frog (<Emphasis Type="Italic">Osteopilus septentrionalis</Emphasis>) tadpoles influence their escape behaviour

Tadpoles risk attack from both aquatic and aerial predators. We investigated how body size and group size influenced the behaviour of tadpoles before and during a predatory attack from above to test the predictions of the theoretical economic escape model. We examined escape (swimming) response of small and large Cuban tree frog (Osteopilus septentrionalis) tadpoles kept under three density treatments and predicted that increased group size, body size and depth in the water column would all reduce perceived risk and, therefore, escape responses to simulated predation. Compared with the lower density groups, tadpoles in higher density groups moved shorter distances, and many individuals did not even move away in response to being touched. Contrary to our predictions based on the economic escape model, smaller tadpoles (which should be more vulnerable to a greater suite of predators) were less reactive than larger tadpoles, and this result may reflect different costs of escape. Finally, although tadpoles might be exposed to a wider range of predator species (aerial as well as aquatic predators), we found no effect of initial depth on escape responses. In conclusion, it appears that the main benefit of increased group density in O. septentrionalis tadpoles is likely to be predator dilution, and that variation in densities of tadpoles influences the escape behaviour of individual tadpoles, regardless of tadpole size.     

Trunk structural traits explain habitat use of a tree-dwelling spider (Selenopidae) in a tropical forest

Habitat selection by spiders may be strongly influenced by biotic, climatic, and physical factors. However, it has been shown that the selection of habitats by generalist predators (like spiders) is regulated more by the physical structure of the habitat than by prey availability. Yet, the preferences of spiders in relation to plants or plant traits remain poorly explored. In a remnant of the Atlantic forest in Brazil, the spider Selenops cocheleti is frequently detected on the trunks of plants from the Myrtaceae family. Here, we investigated quantitatively and experimentally whether the colonization of trees by S. cocheleti is related to plant species or the presence of specific structures on trunks. We found that S. cocheleti preferentially occurred on plants of the family Myrtaceae. This spider was also strongly associated with trees that have smooth trunks and/or exfoliating bark. Non-Myrtaceae plants that were occupied by this species have exfoliating bark (e.g., Piptadenia gonoacantha) or deep fissures on the trunk (e.g., the exotic species Pinus elliottii). Our results indicate that the selection of host plants by S. cocheleti is not species-specific, but based on the structural characteristics of plants. Trunks with exfoliating bark may benefit spiders by providing shelter against predators and harsh climatic conditions. Smooth surfaces might allow rapid movements, facilitating both attacks on preys and escape from predators. Our study emphasizes the importance of the physical structure of the habitat on spider's distribution. Future studies investigating how specific plant characteristics influence prey acquisition and predator avoidance would improve our understanding of habitat selection by these animals.     

Chemical defense of toad tadpoles under risk by four predator species

Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.