A Simple Behavioral Assay for Testing Visual Function in Xenopus laevis

Measurement of the visual function in the tadpoles of the frog, Xenopus laevis, allows screening for blindness in live animals. The optokinetic response is a vision-based, reflexive behavior that has been observed in all vertebrates tested. Tadpole eyes are small so the tail flip response was used as alternative measure, which requires a trained technician to record the subtle response. We developed an alternative behavior assay based on the fact that tadpoles prefer to swim on the white side of a tank when placed in a tank with both black and white sides. The assay presented here is an inexpensive, simple alternative that creates a response that is easily measured. The setup consists of a tripod, webcam and nested testing tanks, readily available in most Xenopus laboratories. This article includes a movie showing the behavior of tadpoles, before and after severing the optic nerve. In order to test the function of one eye, we also include representative results of a tadpole in which each eye underwent retinal axotomy on consecutive days. Future studies could develop an automated version of this assay for testing the vision of many tadpoles at once.     

Foraging behaviour in tadpoles of the bronze frog<Emphasis Type="Italic">Rana temporalis</Emphasis>: Experimental evidence for the ideal free distribution

The ability of bronze frogRana temporalis tadpoles (pure or mixed parental lines) to assess the profitability of food habitats and distribute themselves accordingly was tested experimentally using a rectangular choice tank with a non-continuous input design. Food (boiled spinach) was placed at two opposite ends of the choice tank in a desired ratio (1:1, 1:2 or 1:4) to create habitat A and B. The tadpoles in Gosner stage 28–33, pre-starved for 24 h, were introduced in an open ended mesh cylinder placed in the center of the choice tank, held for 4 min (for acclimation) and then released to allow free movement and habitat selection. The number of tadpoles foraging at each habitat was recorded at 10, 15, 20, 25 and 30 min time intervals. The actual suitability,S _i (the food available in a habitat after colonization of tadpoles) of each habitat was obtained from the equationS _i =B _i−f _i (d _i) whereB _i is basic suitability (amount of food provided at each habitat before release of tadpoles),f _i is the rate of depletion of food (lowering effect) with introduction of each tadpole, andd _i is the density of tadpoles in habitati. The expected number of tadpoles at each habitat was derived from the actual suitability. With no food in the choice tank, movement of the tadpoles in the test arena was random indicating no bias towards any end of the choice tank or the procedure. In tests with a 1:1 food ratio, the observed ratio of tadpoles (11.71: 12.28) was comparable with the expected 12:12 ratio. The observed number of tadpoles in the habitats with a 1:2 food ratio was 8.71:15.29 and 7.87:16.13 for pure and mixed parental lines respectively. In both cases, the observed ratios were close to the expected values (7:17). Likewise, in experiments with a 1:4 food ratio, the observed number of tadpoles in the two habitats (10.78:37.22) did not differ significantly from the expected ratio of 7:41. In all tests, the number ofR. temporalis tadpoles matched ideally with habitat profitability (undermatching indexK ≜ 1. The study shows that tadpoles of the bronze frog exhibit an ideal free distribution while foraging regardless of whether they are siblings or non-siblings in a group, which correlates well with their group living strategy in nature.     

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.     

Predator-prey interactions between two amphibian species: effects of insecticide exposure

The presence of environmental contaminants may alter predator-prey interactions among aquatic species by altering activity levels of predators or prey, or by altering predator avoidance behavior. The outcome of a predatory encounter may be dependent upon whether both species are exposed to a contaminant simultaneously, or whether exposure occurs only in one of the species. In a laboratory experiment, I used the insecticide carbaryl to examine predation of southern leopard frog tadpoles (Rana sphenocephala) by adult red-spotted newts (Notophthalmus viridescens) under four conditions: both tadpoles and newts exposed, neither tadpoles nor newts exposed, and either newts or tadpoles only exposed. After one hour, exposed newts consumed half as many tadpoles as non-exposed newts. Carbaryl potentially affected newt activity enough to reduce time spent searching for prey, or may have altered the speed and coordination necessary to capture tadpoles. After six hours, non-exposed and exposed newts consumed similar numbers of tadpoles, most likely indicating recovery from exposure. After 24 h, predation rates were lowest when both newts and tadpoles were simultaneously either exposed or not exposed, and were greatest when newts and tadpoles were not exposed simultaneously. This study suggests that when tadpoles and newts are exposed to a sublethal level of a contaminant simultaneously, that predation rates do not differ from those observed under natural conditions, but exposure of either predator or prey at different times can disrupt predator-prey dynamics. This revised version was published online in August 2006 with corrections to the Cover Date.     

An inducible offense: carnivore morph tadpoles induced by tadpole carnivory

Phenotypic plasticity is commonplace, and plasticity theory predicts that organisms should often evolve mechanisms to detect and respond to environmental cues that accurately predict future environmental conditions. Here, we test this prediction in tadpoles of spadefoot toads, Spea multiplicata. These tadpoles develop into either an omnivore ecomorph, which is a dietary generalist, or a carnivore ecomorph, which specializes on anostracan shrimp and other tadpoles. We investigated a novel proximate cue – ingestion of Scaphiopus tadpoles – and its propensity to produce carnivores by rearing tadpoles on different diets. We found that diets containing tadpoles from the genus Scaphiopus produced more carnivores than diets without Scaphiopus tadpoles. We discuss why Scaphiopus tadpoles are an excellent food source and why it is therefore advantageous for S. multiplicata tadpoles to produce an inducible offense that allows them to better utilize this resource. In general, such inducible offenses provide an excellent setting for investigating the proximate and evolutionary basis of phenotypic plasticity.     

Responses of bullfrog tadpoles to hypoxia and predators

Low dissolved oxygen concentrations present numerous challenges for non-air-breathing aquatic organisms. Amphibian larvae and their predators can respond to oxygen levels by altering their behavior and physiology, but the ecological consequences of these responses are generally unknown. We conducted two laboratory experiments to study the effects of dissolved oxygen on respiratory behavior and susceptibility to predation of larval bullfrogs (Rana catesbeiana). In the first, we exposed small, lungless tadpoles to a predatory salamander larva (Ambystoma tigrinum) under high and low oxygen conditions. More tadpoles were consumed in high oxygen tanks than in low ones, presumably because salamanders remained near the surface in the low oxygen tanks while most tadpoles rested on the bottom. Tadpole activity depended on both oxygen and predator presence: swimming decreased after addition of salamanders under high oxygen, but increased under low oxygen. In the second experiment, we examined the effect of predator chemical cues on the air-breathing rate of large tadpoles with well-developed lungs under low oxygen conditions. In the presence of chemical cues produced by dragonfly larvae consuming bullfrog tadpoles, air-breathing and swimming were significantly reduced relative to controls. These experiments demonstrate the potential impact of dissolved oxygen on predator-prey interactions, and suggest that outcomes depend on the respiratory ecology of both predator and prey.     

Feeding by omnivores increases food available to consumers

Consumers are usually thought of as negatively affecting producers, but they can affect them positively by releasing nutrients (nutrient regeneration). The net effects of consumers on producers should depend on the balance between the effects of consumption and nutrient regeneration. In aquatic habitats, nutrient regeneration by consumers may increase microbial activity on leaf detritus as well as algal production, which in turn may stimulate further nutrient release and benefit herbivores or detritivores by increasing food quantity or quality. Omnivores can regenerate nutrients from animals, algae and detritus, creating diverse nutrient pathways. Many tadpoles are omnivores, and their nutrient regeneration may be important in aquatic food webs. To reveal the nutrient pathways created by tadpoles and examine whether omnivorous tadpoles can have positive effects on producers and consumers, we experimentally examined the effects of nutrient regeneration by three densities of tadpoles on primary producers, leaf litter, and other consumers in tank mesocosms. Tadpole exclosures were placed inside each mesocosm, allowing us to separate direct consumption effects from indirect nutrient regeneration effects. Nutrient regeneration caused by the herbivorous and carnivorous feeding activities of tadpoles positively affected rates of production of benthic algae, phytoplankton, and herbivorous benthic chironomid larvae, and rates of mineralization of leaf litter. The increased production of benthic algae and chironomid larvae was consumed by the tadpoles themselves, leaving no net change in the standing biomass of these resources. Our experiment thus demonstrated that omnivores created complicated nutrient pathways and accelerated rates of primary production and growth rates of other consumers, leading to increased rates of food availability to the omnivores themselves. Interactions of this nature may be common in many systems and could strongly moderate the effects of consumers on their resources and each other.     

Kin recognition in Bufo scaber tadpoles: ontogenetic changes and mechanism

Ontogenetic changes in kin-recognition behavior, effect of social environment on kin-recognition ability, and use of visual and chemical cues in kin recognition have been studied in tadpoles of Bufo scaber after rearing them with kin, in mixed groups, or in isolation from Gosner stage 12 (gastrula). By use of a rectangular choice tank the tadpoles were tested for their ability to choose between (a) familiar siblings and unfamiliar non-siblings, (b) unfamiliar siblings and familiar non-siblings, and (c) unfamiliar siblings and unfamiliar non-siblings. When tested without any stimulus groups in the end compartments of the tank, random distribution was observed for the tadpoles and no bias for the apparatus or the procedure. In the presence of kin and non-kin in the end compartments, significantly more tadpoles spent most of their time near kin (familiar or unfamiliar) rather than near non-kin during early larval stages, up to stage 37. After stage 37 (characterized by the differentiation of toes), test tadpoles showed no preference to associate with kin, suggesting an ontogenetic shift in the kin-recognition ability in B. scaber. In experiments involving selective blockade of visual or chemical cues the test tadpoles preferentially associated near their kin on the basis of chemical rather than visual cues. These findings suggest that familiarity with siblings is not necessary for kin recognition and that kin-recognition ability is not modified after exposure to non-kin by mixed rearing. The findings for B. scaber indicate a self referent phenotype matching mechanism of kin recognition which is predominantly aided by chemical rather than visual cues.     

Ecological constraints on amphibian metamorphosis: interactions of temperature and larval density with responses to changing food level

Phenotypic plasticity is adaptive for an organism inhabiting a variable environment if the optimal phenotype of a trait that affects fitness varies with environmental conditions, and if the organism can perceive environmental conditions and respond appropriately. Wilbur and Collins have proposed that amphibian larvae might respond adaptively to changes in their resource environment. If conditions for growth in the aquatic environment deteriorate, then a tadpole should metamorphose earlier and smaller than a tadpole under constant high growth conditions. Several experiments on a variety of species have tested this prediction, but only one demonstrated such a response. That experiment involved Couch's spadefoot toads (Scaphiopus couchii) and employed a gradual decrease in food level, whereas the others all used an abrupt switch from high to low food. The purpose of the present experiment was to examine the response of S. couchii to an abrupt change in food level, and to determine if the response depended on the level of two other factors, density and temperature, that also affect larval development. The average effects of the abrupt change in food level were similar to those seen in studies on other species: age at metamorphosis was primarily determined by the early food regime, and size at metamorphosis was determined by food level late in the larval period, suggesting that the effect of decreased food depends on how the food change is done. However, the response to even an abrupt food change depended on interactions with other environmental factors. At high temperature, high initial food, and low density, development was very rapid and tadpoles switched from high to low food metamorphosed at about the same time and size as those at constant high food. In contrast, under high temperature and high initial food conditions, but at high density, tadpoles switched to low food metamorphosed somewhat earlier and smaller, on average, than tadpoles kept at high food. At low temperature, the direction of response depended on density: tadpoles metamorphosed much smaller and slightly, but significantly, earlier at low density, but smaller and later at high density. The developmental response to increased food also varied with temperature. Larvae at high temperature metamorphosed earlier and larger than those at constant low food. At low temperature, larvae metamorphosed larger, but at nearly the same time as their counterparts at constant low food. The combination of high density and constant low food prevented any tadpoles from metamorphosing at high temperature, and allowed relatively few metamorphs at low temperature. Under conditions which impose either very rapid or retarded development, the opportunity to respond to altered food level may be limited. Interactions among environmental factors, therefore, may constrain responses to changing conditions, and may even prevent completion of development. Received: 3 February 1997 / Accepted: 2 October 1997     

Predaceous diving beetle, Dytiscus sharpi sharpi (Coleoptera: Dytiscidae) larvae avoid cannibalism by recognizing prey

Larvae of diving beetles such as the various Dytiscus species (Coleoptera: Dytiscidae) are carnivorous and usually prey on other aquatic animals. Cannibalism among larvae of Dytiscus sharpi sharpi (Wehncke) was observed to begin when they were starved for more than two days under artificial breeding conditions. However, the 2-day starved larvae did not show cannibalism in the presence of intact, motionless, frozen tadpoles, or frozen shrimps. The beetle larvae attacked and captured intact tadpoles faster (15 sec) than other motionless and frozen tadpoles (120 sec), indicating that prey movement was an important factor in stimulating feeding behavior in larvae. Prey density does not have an effect on larval cannibalism. In cases in which preys are present at lower densities than that of larvae, a group of beetle larvae frequently fed on single prey. This feeding behavior, therefore, provides direct evidence of self-other recognition at the species level. Using two traps in one aquarium that allows the larvae to detect only prey smell, one containing tadpoles and another empty, the beetle larvae were attracted to the trap with tadpoles at high frequency, but not to the empty trap. In another experiment, the beetle larvae were not attracted to the trap containing a beetle larva. These results suggest that the larvae of D. sharpi sharpi are capable of recognizing prey scent, which enables the promotion of foraging behavior and the prevention of cannibalism.     

Phenotypic lability and the evolution of predator-induced plasticity in tadpoles

The hypothesis that predator-induced defenses in anuran larvae are maintained by divergent selection across multiple predation environments has not been fully supported by empirical results. One reason may be that traits that respond slowly to environmental variation experience a fitness cost not incorporated in the standard adaptive model, due to a time lag between detecting the state of the environment and expressing the phenotypic response. I measured the rate at which behavior and morphology of Rana temporaria tadpoles change when confronted with a switch in the predation environment at two points in development. Hatchling tadpoles that had been exposed during the egg stage to Aeshna dragonfly larvae were not phenotypically different from those exposed as eggs to predator-free conditions, and both responded similarly to post-hatching predator treatments. When 25-day-old tadpoles from treatments with and without dragonflies were subjected to a switch in the environment, their activity budgets reversed completely within 24-36 h, and their body and tail shape began changing significantly within 4 days. The behavioral response was conservative: Tadpoles switched from high-risk to predator-free treatments were slower to adjust their activity. The study confirmed that behavioral traits are relatively labile and exhibit strong plasticity, but it did not reveal such a pattern at the level of individual traits: Morphological traits that developed slowly did not show the least plasticity. Thus, I found that differences in lability of traits were useful for predicting the magnitude of plasticity only for fundamentally different kinds of characters.     

Microhabitat selection by tadpoles of <Emphasis Type="Italic">Buergeria japonica</Emphasis> inhabiting the coastal area

Microhabitat selection is particularly important to increase the survival rate and reproductive success of animals inhabiting heterogeneous environments. I investigated microhabitat selection of Buergeria japonica tadpoles inhabiting the stream in a coastal area to reveal how animals select their appropriate habitat under heterogeneous and unstable environments on the subtropical Okinawa Island of Japan. Tadpoles are sensitive to subtle environmental changes, and the mouths of streams in coastal areas that have intense environmental fluctuations such as desiccation and sudden changes in current velocity would be risky habitat for tadpoles. Tadpoles of B. japonica can inhabit both lotic and lentic systems. Field observations showed that, among six physical factors (water depth, water temperature, salinity, pH, current velocity, and substrate), current velocity and water temperature were key factors in microhabitat selection by tadpoles. It is likely that tadpoles stay at low current velocity sites to reduce the probability of being washed out to the sea by a sudden squall and that selection of warmer sites would accelerate development of tadpoles so as to escape the heterogeneous aquatic habitat sooner.     

Cannibalistic Interactions Resulting from Indiscriminate Predatory Behavior in Tadpoles of Poison Frogs (Anura: Dendrobatidae)1

Poison frogs in the genus Dendrobates have very small clutch sizes (2–6 eggs among species for which there are data) and typically transport their tadpoles singly to small phytotelmata, such as bromeliad tanks, leaf axils, fallen fruit capsules, and treeholes. Tadpoles of many species are predaceous, consuming larvae of insects that use the same microhabitat for breeding, such as giant damselflies and mosquitoes. Previous studies and observations on the behavior of poison frog tadpoles led us to question whether tadpoles might be cannibalistic. We studied a population of Dendrobates castaneoticus in lowland rainforest in Pará, Brazil; additional data were collected on Dendrobates auratus in Nicaragua. At the study site in Brazil, we established a grid of 40 Brazil nut capsules, the microhabitat used by D. castaneoticus for tadpole deposition. Of 42 tadpoles deposited during the 55 days of the study, 20 were killed or died; 16 of these were presumably killed by conspecific tadpoles. Growth rate and time to metamorphosis was higher among tadpoles that consumed three or more tadpoles or relatively large larvae of the mosquito Trichoprosopon digitatum, a colonist of newly opened Brazil nut capsules. We propose that selection has favored the development of predatory behavior in poison frog tadpoles primarily as a mechanism to eliminate predators from the small phytotelmata in which they develop and that cannibalism is a secondary outcome of this behavior. Predatory behavior also provides tadpoles with a source of food, which is frequently limited in these microhabitats. Additional studies of the biology of tadpoles of other species of Dendrobates are needed to determine the evolution of predatory and cannibalistic behavior in the clade.     

Effects of food-deprivation on migratory restlessness and diurnal activity in the garden warbler Sylvia borin

Summary The effects of tadpole body size, tadpole sibship, and fish body size on predation of gray treefrog tadpoles, Hyla chrysoscelis, were studied in laboratory and artificial pond experiments. Tadpole body size had a significantly positive effect on the survival of tadpoles in all experiments. The relationship between tadpole biomass eaten and biomass available suggested that fish were not satiated when consuming the largest tadpoles. Large tadpoles were probably better able to evade predators. A difference in survival among full sib families of tadpoles was only present in one family, suggesting that genetic differences in predator avoidance behavior or palatability were probably secondarily important to body size per se. Fish body size had a significantly negative effect on the survival of tadpoles. Larger fish consumed a larger number and proportion of tadpoles as well as greater biomass. These results indicate that environmental factors affecting the growth rate of tadpoles cand dramatically alter their vulnerability to gape-limited predators.     

The role of environmental factors in the conidiation of the predacious rotiferovorous fungus Zoophagus insidians (Zoopagomycota)

Compared to other groups of fungi, the knowledge of freshwater predacious fungi that feed on trapped rotifers and tardigrades is very limited. They are known to spread and survive under adverse conditions by releasing asexual spores (conidia), but the environmental factors that induce their conidiation are unclear. In this study, we investigated the conidiation of the rotiferovorous fungus Zoophagus insidians isolated from activated sludge and maintained under laboratory conditions in spring water (medium). We found that its conidiation can undergo significant changes in response to various environmental factors, such as medium exchange, presence or absence of prey, lighting conditions, and their combination. Our results revealed a surprisingly high flexibility of this obligate predacious fungus, which being constantly exposed to unpredictable availability of prey in an unstable environment is still able to survive and disperse.     

Foraging Responses of the Larvae of Invasive Bullfrogs(Lithobates catesbeianus): Possible Implications for Bullfrog Control and Ecological Impact in China

The predatory behavior of invasive species can affect their ecological impact, and offer opportunities for targeted control. In Australia, tadpoles of invasive cane toads(Rhinella marina) do not consume eggs of native anurans, but are strongly attracted to(and consume) newly-laid eggs of conspecifics; chemical cues from such eggs(or adult secretions) thus can be used to attract toad tadpoles to traps. Do other invasive anurans show similar selectivity? Our laboratory trials on a Chinese population of invasive American bullfrogs(Lithobates catesbeianus) revealed similar behaviors as exhibited by Australian cane toads. Bullfrog tadpoles rarely consumed the eggs of native anurans, but were attracted to both bullfrog eggs and bullfrog skin secretions. Although the attraction response was less intense in bullfrogs than in cane toads, it might nonetheless enable selective removal of bullfrog tadpoles from invaded sites.     

Mechanism of food detection in the tadpoles of the bronze frog <Emphasis Type="Italic">Rana temporalis</Emphasis>

Mechanism of food detection in Rana temporalis tadpoles was studied using a rectangular choice tank with end compartments (stimulus zones) providing exclusively visual and/or chemical food cues. Boiled spinach served as the food. The test tadpoles were starved for 24 h before use. They were released from the center of the choice tank (n=24) after 5 min of acclimation to test for end bias and food-detecting mechanism. The number of tadpoles in the two stimulus zones was recorded at 5-min intervals from 10 to 30 min. In the end-bias tests (without food cues) tadpole distribution was comparable at all times in the two compartments of the choice tank, exhibiting no end bias. In tests with the visual food cues provided in one of the stimulus zones, the tadpole distribution was also random. On the other hand, in experiments involving chemical cues emanating from food the tadpoles preferentially associated with the food source in significantly greater numbers compared to the zone lacking food or providing only a visual cue. The experiments with individual test tadpoles also revealed that they detect food based on chemical cues and ruled out copycat behavior. These findings on R. temporalis tadpoles reveal that chemical senses predominate over the visual senses in detection of food and foraging.     

Flexible architecture of inducible morphological plasticity

1. Predator-induced morphological defences are produced in response to an emergent predator regime. In natural systems, prey organisms usually experience temporal shifting of the composition of the predator assemblage and of the intensity of predation risk from each predator species. Although, a repetitive morphological change in response to a sequential shift of the predator regime such as alteration of the predator species or diminution of the predation risk may be adaptive, such flexible inducible morphological defences are not ubiquitous. 2. We experimentally addressed whether a flexible inducible morphological defence is accomplished in response to serial changes in the predation regime, using a model prey species which adopt different defensive morphological phenotypes in response to different predator species. Rana pirica (Matsui) tadpoles increased body depth and tail depth against the predatory larval salamander Hynobius retardatus (Dunn); on the other hand, they only increased tail depth against the predatory larval dragonfly Aeshna nigroflava (Martin). 3. Rana pirica tadpoles with the predator-specific phenotypes were subjected to removal or exchange of the predator species. After removal of the predator species, tadpoles with each predator-specific phenotype changed their phenotype to the nondefensive basic one, suggesting that both predator-specific phenotypes are costly to maintain. After an exchange of the predator species, tadpoles with each predator-specific phenotype reciprocally, flexibly shifted their phenotype to the now more suitable predator-specific one only by modifying their body part. The partial modification can effectively reduce time and energy expenditures involved in repetitive morphological changes, and therefore suggest that the costs of the flexible morphological changes are reduced.     

Echinostome metacercariae cyst elimination in Rana clamitans (green frog) tadpoles is age-dependent

Host response to parasite exposure is an important determinant of the outcome of host-parasite interactions. Factors such as host body condition and age can strongly influence host response to parasites and infection. This study followed Echinostoma revolutum infection levels in larval green frogs (Rana clamitans) exposed at 2 different ages. Tadpoles at early developmental stages are more susceptible to the adverse effects of echinostomes. Green frog tadpoles approximately 2 wk apart in age and of the same developmental stage can exhibit dramatically different responses to echinostome infection, with the younger tadpoles having high rates of mortality and the older tadpoles experiencing no mortality. The goal of the present study was to begin to explore the mechanism underlying the striking age-dependent response of tadpoles to echinostomes. I conducted 2 controlled infection laboratory experiments in which tadpoles were preserved at 6 time points ranging from 4 hr to 1 wk postinfection (PI). Tadpoles infected at the younger age did not eliminate echinostome metacercariae. However, tadpoles that were 13 days older at the time of echinostome exposure steadily eliminated metacercariae during the hours and days PI. The absence of echinostome cyst elimination in the younger tadpoles likely contributes to their elevated, infection-induced mortality rates.     

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.