Carry-over Effects of Size at Metamorphosis in Red-eyed Treefrogs: Higher Survival but Slower Growth of Larger Metamorphs

Rearing density and food variety impact growth, development, and survival of larvae in the declining amphibian, Pseudacris maculata

Boreal chorus frogs (Pseudacris maculata Agassiz 1850) are a widespread amphibian in North America, but several populations are in decline. Specifically, we are developing captive breeding and reintroduction methods for the Great Lakes/St. Lawrence-Canadian Shield population. Here we present the effects of tadpole density, food variety, and addition of supplemental minerals to rearing water on the growth, development, and survival during the larval and metamorph/juvenile stages. We conducted two experiments using a factorial design. We found that low tadpole density (1 vs. 2 tadpoles/L) and high food variety (five food items vs. three food items) significantly increased tadpole body length and Gosner stage after 2 weeks (p < .001), increased survival to metamorphosis (p < .001), decreased time to metamorphosis (p < .001), and increased weight after metamorphosis (p < .001). On average, tadpoles in the high density/low food treatment, compared to the low density/high food, were 25% smaller after 2 weeks, had 3.9× lower survival to metamorphosis, took 1.25× longer to reach metamorphosis, and weighed 1.5× less after metamorphosis. In contrast, neither density (0.5 vs. 1 tadpole/L) nor mineral supplemented water affected growth and development, but tadpole survival was higher at 1 tadpole/L. Our findings demonstrate the feasibility of rearing boreal chorus frogs in captivity and provide guidelines for rearing this and similar species in a laboratory environment.

Importance of Genetic-Fitness Correlations for the Conservation of Amphibians

Endangered animals suffer from isolation of their habitats. Isolation leads to a reduction in population size as well as a decrease in genetic diversity and a concomitant increase in the risk of extinction. Amphibians are the most endangered vertebrate class. Besides habitat loss, fragmentation and isolation, amphibians are threatened by emerging diseases e.g., chytrid fungus or Ranavirus. By employing experiments, researchers investigate whether changes in genetic diversity within or among isolated populations affect amphibian fitness. While genetic diversity estimates are based on molecular markers, typically microsatellites, fitness is mostly measured as tadpole performance in rearing experiments often under varying environmental conditions. Tadpole performances (e.g., body mass, growth rate and survival) have been found to be negatively affected by low genetic diversity, as several studies have found a positive association between genetic diversity and these fitness traits. Moreover, infection with pathogens also seems to be more likely in individuals or populations with lower genetic diversity. Overall, these genetic-fitness correlations seem to be more pronounced or detectable in smaller, declining populations but not in larger populations. Genomic studies, which sample a larger fraction of the genome, are still scarce in the conservation genetic literature on amphibians. These are likely to increase in upcoming years and may reveal adaptive variants that protect against dangerous pathogens or environmental changes. Altogether, genetic-fitness correlation studies should be a priority in order to develop effective management plans for the genetic rescue of isolated, imperilled amphibian populations.

Infection Causes Trade-Offs between Development and Growth in Larval Amphibians

AbstractTrade-offs between life history traits are context dependent; they vary depending on environment and life stage. Negative associations between development and growth often characterize larval life stages. Both growth and development consume large parts of the energy budget of young animals. The metabolic rate of animals should reflect differences in growth and developmental rates. Growth and development can also have negative associations with immune function because of their costs. We investigated how intraspecific variation in growth and development affected the metabolism of larval amphibians and whether intraspecific variation in growth, development, and metabolic rate could predict mortality and viral load in larvae infected with ranavirus. We also compared the relationship between growth and development before and after infection with ranavirus. We hypothesized that growth and development would affect metabolism and predicted that each would have a positive correlation with metabolic rate. We further hypothesized that allocation toward growth and development would increase ranavirus susceptibility and therefore predicted that larvae with faster growth, faster development, and higher metabolic rates would be more likely to die from ranavirus and have higher viral loads. Finally, we predicted that growth rate and developmental rate would have a negative association. Intraspecific variation in growth rate and developmental rate did not affect metabolism. Growth rate, developmental rate, and metabolism did not predict mortality from ranavirus or viral load. Larvae infected with ranavirus exhibited a trade-off between developmental rate and growth rate that was absent in uninfected larvae. Our results indicate a cost of ranavirus infection that is potentially due to both the infection-induced anorexia and the cost of infection altering priority rules for resource allocation.

Developmental Plasticity in Anurans: Meta-analysis Reveals Effects of Larval Environments on Size at Metamorphosis And Timing of Metamorphosis

Many anuran amphibians (frogs and toads) rely on aquatic habitats during their larval stage. The quality of this environment can significantly impact lifetime fitness and population dynamics. Over 450 studies have been published on environmental impacts on anuran developmental plasticity, yet we lack a synthesis of these effects across different environments. We conducted a meta-analysis and used a comparative approach to understand whether developmental plasticity in response to different larval environments produces predictable changes in metamorphic phenotypes. We analyzed data from 124 studies spanning 80 anuran species and six larval environments and showed that intraspecific variation in mass at metamorphosis and the duration of the larval period is partly explained by the type of environment experienced during the larval period. Changes in larval environments tended to reduce mass at metamorphosis relative to control conditions, with the degree of change depending on the identity and severity of environmental change. Higher temperatures and lower water levels shortened the duration of the larval period, whereas less food and higher densities increased the duration of the larval period. Phylogenetic relationships among species were not associated with interspecific variation in mass at metamorphosis plasticity or duration of the larval period plasticity. Our results provide a foundation for future studies on developmental plasticity, especially in response to global changes. This study provides motivation for additional work that links developmental plasticity with fitness consequences within and across life stages, as well as how the outcomes described here are altered in compounding environments.

Bigger and bolder: Widespread agricultural pollutant 17β-trenbolone increases growth and alters behaviour in tadpoles (Litoria ewingii)

Endocrine-disrupting chemicals-compounds that directly interfere with the endocrine system of exposed animals-are insidious environmental pollutants that can disrupt hormone function, even at very low concentrations. The dramatic impacts that some endocrine-disrupting chemicals can have on the reproductive development of wildlife are well documented. However, the potential of endocrine-disrupting chemicals to disrupt animal behaviour has received far less attention, despite the important links between behavioural processes and population-level fitness. Accordingly, we investigated the impacts of 14 and 21-day exposure to two environmentally realistic levels of 17β-trenbolone (4.6 and 11.2 ng/L), a potent endocrine-disrupting steroid and agricultural pollutant, on growth and behaviour in tadpoles of an anuran amphibian, the southern brown tree frog (Litoria ewingii). We found that 17β-trenbolone altered morphology, baseline activity and responses to a predatory threat, but did not affect anxiety-like behaviours in a scototaxis assay. Specifically, we found that tadpoles exposed to our high-17β-trenbolone treatment were significantly longer and heavier at 14 and 21 days. We also found that tadpoles exposed to 17β-trenbolone showed higher levels of baseline activity, and significantly reduced their activity following a simulated predator strike. These results provide insights into the wider repercussions of agricultural pollutants on key developmental and behavioural traits in aquatic species, and demonstrate the importance of behavioural studies in the ecotoxicological field.

Carry-Over Effects of Desiccation Stress on the Oxidative Status of Fasting Anuran Juveniles

Amphibians are sensitive to deteriorating environmental conditions, especially during transition to a terrestrial environment which is full of uncertainties. Harsh conditions, such as desiccation during earlier stages, affect different larval traits with possible carry-over effects on juvenile and adult life histories. The first consequences of the effects can be seen in juveniles in the challenges to find food and the ability to survive without it in a terrestrial habitat. Body size and the internal energy reserves acquired during the larval phase play an important role in this period. Herein, we tested how different water regimes (low water availability, desiccation and constant high-water availability) during larval development reflect on the oxidative status and ability of yellow belly toad (Bombina variegata) juveniles to endure short-term fasting. The desiccation regime significantly reduced the body size of metamorphs. The same was observed after 2 weeks of fasting, while the feeding treatment reduced differences mostly in the body mass of individuals from different water regimes. This was the result of a greater gain in mass in juveniles pre-exposed to desiccation. Pre-exposure to desiccation also modified the parameters of the antioxidant system (AOS) under feeding conditions, leading to higher values of superoxide dismutase, glutathione reductase and glutathione S-transferase, glutathione and sulfhydryl group concentrations, and lower glutathione peroxidase in comparison to juveniles reared under constant water. The increase in the AOS of juveniles can be considered as a physiological carry-over effect of desiccation, probably as the result of compensatory growth and/or earlier exposure to chronic stress. However, water levels during larval development did not exert significant effects on the oxidative status of juveniles subjected to food unavailability. Fasting juveniles, both control and desiccated, were exposed to oxidative stress, significantly higher lipid peroxide concentrations, lower superoxide dismutase, glutathione peroxidase, glutathione S-transferase, glutathione and sulfhydryl group values in comparison to feeding individuals. The lack of food in juvenile anurans activated the AOS response in the same manner, regardless of body size and stress pre-exposure, suggesting that the generally accepted hypothesis about the influence of metamorphic body size on the fitness of the postmetamorphic stage should be tested further.

Growing up in a new world: trait divergence between rural, urban, and invasive populations of an amphibian urban invader

Cities are focal points of introduction for invasive species. Urban evolution might facilitate the success of invasive species in recipient urban habitats. Here we test this hypothesis by rearing tadpoles of a successful amphibian urban coloniser and invader in a common garden environment. We compared growth rate, morphological traits, swimming performance, and developmental rate of guttural toad tadpoles (Sclerophrys gutturalis) from native rural, native urban, and non-native urban habitats. By measuring these traits across ontogeny, we were also able to compare divergence across different origins as the tadpoles develop. The tadpoles of non-native urban origin showed significantly slower developmental rate (e.g., the proportion of tadpoles reaching Gosner stage 31 or higher was lower at age 40 days) than tadpoles of native urban origin. Yet, tadpoles did not differ in growth rate or any morphological or performance trait examined, and none of these traits showed divergent ontogenetic changes between tadpoles of different origin. These findings suggest that prior adaptation to urban habitats in larval traits likely does not play an important role in facilitating the invasion success of guttural toads into other urban habitats. Instead, we suggest that evolutionary changes in larval traits after colonization (e.g., developmental rate), together with decoupling of other traits and phenotypic plasticity might explain how this species succeeded in colonising extra-limital urban habitats.

Frogs Respond to Commercial Formulations of the Biopesticide Bacillus thuringiensis var. israelensis, Especially Their Intestine Microbiota

It is generally believed that Bacillus thuringiensis var. israelensis (Bti) biopesticides are harmless to non-target organisms; however, new research shows controversial results. We exposed acutely and chronicallyLithobates sylvaticusandAnaxyrus americanus tadpoles until metamorphic climax to VectoBac 200G (granules) and VectoBac 1200L (aqueous suspension) at 300-20,000 ITU/L covering field-relevant concentrations and higher. The data show that the exposure parameters tested did not affect significantly the survival, total length, total weight, hepatosomatic index, gonadosomatic index, the expression of genes of interest (i.e., related to xenobiotic exposure, oxidative stress, and metamorphosis), and the intestine tissue layer detachment ofL. sylvaticusandA. americanus in a concentration-response pattern. In contrast, VectoBac 200G significantly increased the median time to metamorphosis ofL. sylvaticus tadpoles by up to 3.5 days and decreased the median by up to 1 day inA. americanus. VectoBac 1200L significantly increased the median time to metamorphosis ofL. sylvaticusandA. americanustadpoles by up to 4.5 days. Also, the exposure to VectoBac 200G and 1200L altered the intestine bacterial community composition inA. americanus at application rates recommended by the manufacturer, which led to an increase in the relative abundance of Verrucomicrobia, Firmicutes, Bacteroidetes, and Actinobacteria. Changes in the intestine microbiota might impact the fitness of individuals, including the susceptibility to parasitic infections. Our results indicate that the effect of Bti commercial products is limited; however, we recommend that Bti-spraying activities in amphibian-rich ecosystems should be kept minimal until there is more conclusive research to assess if the changes in the time to metamorphosis and microbiota can lead to negative outcomes in amphibian populations and, eventually, the functioning of ecosystems.

Stress hormones mediate developmental plasticity in vertebrates with complex life cycles

The environment experienced by developing organisms can shape the timing and character of developmental processes, generating different phenotypes from the same genotype, each with different probabilities of survival and performance as adults. Chordates have two basic modes of development, indirect and direct. Species with indirect development, which includes most fishes and amphibians, have a complex life cycle with a free-swimming larva that is typically a growth stage, followed by a metamorphosis into the adult form. Species with direct development, which is an evolutionarily derived developmental mode, develop directly from embryo to the juvenile without an intervening larval stage. Among the best studied species with complex life cycles are the amphibians, especially the anurans (frogs and toads). Amphibian tadpoles are exposed to diverse biotic and abiotic factors in their developmental habitat. They have extensive capacity for developmental plasticity, which can lead to the expression of different, adaptive morphologies as tadpoles (polyphenism), variation in the timing of and size at metamorphosis, and carry-over effects on the phenotype of the juvenile/adult. The neuroendocrine stress axis plays a pivotal role in mediating environmental effects on amphibian development. Before initiating metamorphosis, if tadpoles are exposed to predators they upregulate production of the stress hormone corticosterone (CORT), which acts directly on the tail to cause it to grow, thereby increasing escape performance. When tadpoles reach a minimum body size to initiate metamorphosis they can vary the timing of transformation in relation to growth opportunity or mortality risk in the larval habitat. They do this by modulating the production of thyroid hormone (TH), the primary inducer of metamorphosis, and CORT, which synergizes with TH to promote tissue transformation. Hypophysiotropic neurons that release the stress neurohormone corticotropin-releasing factor (CRF) are activated in response to environmental stress (e.g., pond drying, food restriction, etc.), and CRF accelerates metamorphosis by directly inducing secretion of pituitary thyrotropin and corticotropin, thereby increasing secretion of TH and CORT. Although activation of the neuroendocrine stress axis promotes immediate survival in a deteriorating larval habitat, costs may be incurred such as reduced tadpole growth and size at metamorphosis. Small size at transformation can impair performance of the adult, reducing probability of survival in the terrestrial habitat, or fecundity. Furthermore, elevations in CORT in the tadpole caused by environmental stressors cause long term, stable changes in neuroendocrine function, behavior and physiology of the adult, which can affect fitness. Comparative studies show that the roles of stress hormones in developmental plasticity are conserved across vertebrate taxa including humans.

Geographical differences in competitive hierarchy in a native–invasive system

Biological invasions can create novel competitive interactions and force ecological shifts in both native and invasive species. Anuran tadpoles are able to modify their behaviour, morphology, growth and development to cope with competitive pressure. This plasticity is a good target for natural selection and can drive rapid evolutionary changes in response to novel interactions. Here, we explore changes in plastic responses and fitness of competing invasive and native tadpoles by exposing tadpoles from different locations with contrasting evolutionary histories to the same set of varied competitive conditions. Eggs were collected from one site near the first introduction of the invasive frog (~110 years of coexistence) and from a second site that was invaded recently. We hypothesized less favourable outcomes for the invasive species in long-coexisting populations, where the native competitor might have developed adaptive responses. Most results support the hypothesis. Where the invasion was older, invasive tadpoles exposed to native competitors grew less, developed more slowly and displayed morphologies linked to competitive stress, whereas the developmental stability and canalization of native tadpoles increased. On the whole, the asymmetric competitive relationship thus appeared to approach symmetry after ~35 generations, highlighting a noteworthy example of rapid adaptation after an invasion.

Carryover effects and the evolution of polyphenism

An individual’s early-life environment and phenotype often influence its traits and performance as an adult. We investigated whether such ‘carryover effects’ are associated with alternative, environmentally-induced phenotypes (‘polyphenism’), and, if so, whether they influence the evolution of polyphenism. To do so, we studied Mexican spadefoot toads, Spea multiplicata, which have evolved a polyphenism consisting of two, dramatically different forms: a carnivore morph and an omnivore morph. We sampled both morphs from a fast-drying and a slow-drying pond and reared them to sexual maturity. Larval environment (pond) strongly influenced survival as well as age and size at metamorphosis and sexual maturity; i.e. environment-dependent carryover effects were present. By contrast, larval phenotype (morph) did not affect life-history traits at sexual maturity; i.e. phenotype-dependent carryover effects were absent. These results are consistent with theory, which suggests that by amplifying selective trade-offs in heterogenous environments, environment-dependent carryover effects might foster the evolution of polyphenism. At the same time, by freeing selection to refine a novel phenotype without altering the existing form, the absence of phenotype-dependent carryover effects might enable polyphenism to evolve in the first place. Generally, carryover effects might play an underappreciated role in the evolution of polyphenism.

How to recover from a bad start: size at metamorphosis affects growth and survival in a tropical amphibian

Background

In species with complex life cycles, size at metamorphosis is a key life-history trait which reflects the complex interactions between costs and benefits of life in the aquatic and terrestrial environments. Whereas the effects of a deteriorating larval habitat (e.g. pond desiccation) on triggering an early metamorphosis have been extensively investigated in amphibians, the consequences of the resulting reduced size at metamorphosis on fitness in the post-metamorphic terrestrial stage remain poorly understood. We tested the hypothesis that a smaller size at metamorphosis negatively affects performance and survival in the ensuing terrestrial stage. Using as model a tropical amphibian (Ceratophrys stolzmanni) showing a large phenotypic plasticity in metamorphosing traits, we evaluated the effects of size at metamorphosis on fitness-related trophic and locomotor performance traits, as well as on growth and survival rates.

Results

Our results support the hypothesis that a larger size at metamorphosis is correlated with better survival and performance. The survival rate of large metamorphosing individuals was 95%, compared to 60% for those completing metamorphosis at a small size. Locomotor performance and gape size were positively correlated with body size, larger animals being more mobile and capable to ingest larger prey. However, smaller individuals achieved higher growth rates, thus reducing the size gap.

Conclusions

Overall, size at metamorphosis affected profoundly the chances of survival in the short term, but smaller surviving individuals partly compensated their initial disadvantages by increasing growth rates.

Evaluating adaptive, carry-over, and plastic antipredator responses across a temporal gradient in Pacific chorus frogs

The development of antipredator traits is dependent on the frequency and intensity of predator exposure over evolutionary and ecological time. We hypothesized that prey species would respond with increasing accuracy when exposed to predators across generational, ontogenetic, and immediate time scales. We assessed larval Pacific chorus frog (PSRE; Pseudacris regilla) individuals that varied in population sympatry, embryonic conditioning, and immediate exposure to stocked populations of rainbow trout (Oncorhynchus mykiss). Using PSRE populations from sites with and without resident rainbow trout, we conditioned embryos to trout odor, PSRE alarm cues, trout odor in combination with alarm cues, or control water. After being hatched and reared in control water, individuals were exposed to the four predator cue treatments using a fully factorial design. Tadpoles from populations with resident rainbow trout did not behave or develop differently than tadpoles originating from fishless sites. However, we found evidence that PSRE reduced predation risk with a combination of carry-over effect (i.e., transfer of information across life history stages) and within-life stage phenotypically plastic mechanisms. We found both developmental and behavioral carry-over effects: tadpoles conditioned with trout odor as embryos grew more slowly and took refuge more often than control animals. Within-life-stage behavioral plasticity was observed in tadpoles from all treatment groups, responding to predator cues with increased refuge use. Potentially additive effects of predator exposure on prey response should be considered when predicting the ability of prey to recognize novel threats.

Endocrine Disruption Alters Developmental Energy Allocation and Performance in Rana temporaria

Environmental change exposes wildlife to a wide array of environmental stressors that arise from both anthropogenic and natural sources. Many environmental stressors with the ability to alter endocrine function are known as endocrine disruptors, which may impair the hypothalamus-pituitary-thyroid axis resulting in physiological consequences to wildlife. In this study, we investigated how the alteration of thyroid hormone (TH) levels due to exposure to the environmentally relevant endocrine disruptor sodium perchlorate (SP; inhibitory) and exogenous L-thyroxin (T4; stimulatory) affects metabolic costs and energy allocation during and after metamorphosis in a common amphibian (Rana temporaria). We further tested for possible carry-over effects of endocrine disruption during larval stage on juvenile performance. Energy allocated to development was negatively related to metabolic rate and thus, tadpoles exposed to T4 could allocate 24% less energy to development during metamorphic climax than control animals. Therefore, the energy available for metamorphosis was reduced in tadpoles with increased TH level by exposure to T4. We suggest that differences in metabolic rate caused by altered TH levels during metamorphic climax and energy allocation to maintenance costs might have contributed to a reduced energetic efficiency in tadpoles with high TH levels. Differences in size and energetics persisted beyond the metamorphic boundary and impacted on juvenile performance. Performance differences are mainly related to strong size-effects, as altered TH levels by exposure to T4 and SP significantly affected growth and developmental rate. Nevertheless, we assume that juvenile performance is influenced by a size-independent effect of achieved TH. Energetic efficiency varied between treatments due to differences in size allocation of internal macronutrient stores. Altered TH levels as caused by several environmental stressors lead to persisting effects on metamorphic traits and energetics and, thus, caused carry-over effects on performance of froglets. We demonstrate the mechanisms through which alterations in abiotic and biotic environmental factors can alter phenotypes at metamorphosis and reduce lifetime fitness in these and likely other amphibians.

The Costs of Living Together: Immune Responses to the Microbiota and Chronic Gut Inflammation

While the vertebrate microbiota is critical to the normal function of many host traits, hosts may expend a large amount of energy to constrain and interface with their microbiota via their immune system to avoid the high fitness costs associated with gut dysbiosis, pathobionts, and opportunistic pathogens. All jawed vertebrates share mucosal immunity dedicated to isolating the microbiota, and a breakdown of this system can result in chronic gut inflammation. In humans, chronic gut inflammation negatively affects growth and development. There is little information available on the prevalence of chronic gut inflammation in wild animals, but given that animals with different life histories emphasize different immune responses, it follows that wild animals may vary in their susceptibility to chronic gut inflammation, and most animals will experience signaling that can lead to this state. These can be top-down signals originating from sources like the central nervous system or bottom-up signals originating from changes in the gut microbiota. The sources of these signals might include stress, developmental transitions, food restriction, and dietary shifts. Here, we briefly discuss host-microbiota interactions from the perspective of life history theory and ecoimmunology, focusing on the mucosal immune system and chronic gut inflammation. We also include future directions for research and the tools necessary to investigate them.

Artificial light at night decreases metamorphic duration and juvenile growth in a widespread amphibian

Artificial light at night (ALAN) affects over 20% of the earth's surface and is estimated to increase 6% per year. Most studies of ALAN have focused on a single mechanism or life stage. We tested for indirect and direct ALAN effects that occurred by altering American toads' (Anaxyrus americanus) ecological interactions or by altering toad development and growth, respectively. We conducted an experiment over two life stages using outdoor mesocosms and indoor terraria. In the first phase, the presence of ALAN reduced metamorphic duration and periphyton biomass. The effects of ALAN appeared to be mediated through direct effects on toad development, and we found no evidence for indirect effects of ALAN acting through altered ecological interactions or colonization. In the second phase, post-metamorphic toad growth was reduced by 15% in the ALAN treatment. Juvenile-stage ALAN also affected toad activity: in natural light, toads retreated into leaf litter at night whereas ALAN toads did not change behaviour. Carry-over effects of ALAN were also present; juvenile toads that had been exposed to larval ALAN exhibited marginally increased activity. In this time frame and system, our experiments suggested ALAN's effects act primarily through direct effects, rather than indirect effects, and can persist across life stages.

Embryonic learning and developmental carry-over effects in an invasive anuran

Carry-over effects influence trait responses in later life stages as a result of early experience with environmental cues. Predation risk is an influential stressor and selection exists for early recognition of threats. In particular, invasive species may benefit from carry-over effects by preemptively recognizing and responding to novel predators via latent developmental changes and embryonic learning. In a factorial experiment, we conditioned invasive American bullfrog embryos (Lithobates catesbeianus) to the odor of a novel fish predator, largemouth bass (Micropterus salmoides) alone or in combination with injured conspecific cues. We quantified developmental carryover in the larval life stage and found that individuals conditioned to the highest risk (fish and injured conspecific cues) grew into longer bodied larvae relative to larvae from lower risk treatments. We also assessed embryonic learning, a behavioral carry-over effect, and found an interaction between embryonic conditioning and larval exposure. Behavioral responses were only found in scenarios when predation risk varied in intensity across life history stages, thus requiring a more flexible antipredator strategy. This indicates a potential trade-off between the two strategies in larval growth and development rates, and time until metamorphosis. Our results suggest that early predator exposure and carry-over effects have significant impacts on life history trajectories for American bullfrogs. This research contributes to our understanding of a potentially important invasion mechanism in an anuran species of conservation concern.

Exogenous stress hormones alter energetic and nutrient costs of development and metamorphosis

Variation in environmental conditions during larval life stages can shape development during critical windows and have lasting effects on the adult organism. Changes in larval developmental rates in response to environmental conditions, for example, can trade-off with growth to determine body size and condition at metamorphosis, which can affect adult survival and fecundity. However, it is unclear how use of energy and nutrients shape trade-offs across life stage transitions because no studies have quantified these costs of larval development and metamorphosis. We used an experimental approach to manipulate physiological stress in larval amphibians, along with respirometry and 13C-breath testing to quantify the energetic and nutritional costs of development and metamorphosis. Central to larval developmental responses to environmental conditions is the hypothalamus pituitary-adrenal/interrenal (HPA/I) axis, which regulates development, as well as energy homeostasis and stress responses across many taxa. Given these pleiotropic effects of HPA/I activity, manipulation of the HPA/I may provide insight into costs of metamorphosis. We measured the energetic and nutritional costs across the entire larval period and metamorphosis in a larval amphibian exposed to exogenous glucocorticoid (GC) hormones- the primary hormone secreted by the HPA/I axis. We measured metabolic rates and dry mass across larval ontogeny, and quantified lipid stores and nutrient oxidation via 13C-breath testing during metamorphosis, under control and GC-exposed conditions. Changes in dry mass match metamorphic states previously reported in the literature, but dynamics of metabolism were influenced by the transition from aquatic to terrestrial respiration. GC-treated larvae had lower dry mass, fat stores, and higher oxygen consumption during stages where controls were conserving energy. GC-treated larvae also oxidized greater amounts of 13C-labelled protein stores. These results provide evidence for a proximate cause of the physiological trade-off between larval growth and development, and provide insight into the energetic and nutrient costs that shape fitness trade-offs across life stages.

Additive effects of mean temperature, temperature variability, and chlorothalonil to red-eyed treefrog (Agalychnis callidryas) larvae

Amphibian populations are declining globally, and multiple anthropogenic stressors, including contamination by pesticides and shifting climates, are driving these declines. Climate change may increase average temperatures or increase temperature variability, either of which may affect the susceptibility of nontarget organisms to contaminants. Eight-day ecotoxicological assays were conducted with red-eyed treefrog (Agalychnis callidryas) larvae to test for additive and interactive effects of exposure to the fungicide chlorothalonil, average temperature, and temperature variability on tadpole growth and survival. Egg masses were collected from seasonal ponds at La Selva Biological Station in Costa Rica, and tadpoles were exposed to a series of chlorothalonil concentrations across a range of ecologically relevant mean temperatures (23.4-27.3 °C) and daily temperature fluctuations (1.1-9.9 °C). Survival was measured each day, and tadpole growth was measured at the end of each trial. Concentrations of chlorothalonil ≥60 µg/L reduced survival, although survival was not affected by mean temperature or daily temperature range, and there were no synergistic interactions between chlorothalonil and temperature regime on survival. Chlorothalonil suppressed tadpole growth at relatively low concentrations (∼15 µg/L). There were impacts of both average temperature and daily temperature range on tadpole growth, although there were no synergistic interactions between temperature regimes and chlorothalonil. The results should inform efforts to manage ecosystems impacted by multiple large-scale anthropogenic stressors as well as methods for the design of ecologically appropriate toxicology trials. Environ Toxicol Chem 2016;35:2998-3004. © 2016 SETAC.

Effects of Water Loss on New Mexico Spadefoot Toad (Spea multiplicata) Development, Spleen Cellularity, and Corticosterone Levels

Amphibian metamorphosis is complex and larval morphology and physiology are completely restructured during this time. Amphibians that live in unpredictable environments are often exposed to stressors that can directly and indirectly alter physiological systems during development, with subsequent consequences (carryover effects) later in life. In this study, we investigated the effects of water level reduction on development rate, spleen size and cellularity, and examined the role of corticosterone levels in premetamorphic, metamorphic, and postmetamorphic New Mexico spadefoot toads (Spea multiplicata). Based on previous studies, we hypothesized that declining water level would increase tadpole developmental rate, but with the trade-off of increasing corticosterone to a level that would subsequently affect spleen size and cellularity, thus prolonging potential immunological suppression. Declining water levels increased developmental rate by 3 days; however, there were no significant body size effects. Corticosterone (CORT) was negatively correlated with total length, snout vent length, body weight, and spleen weight at metamorphosis, suggesting that size at metamorphosis and the immune system may be affected by excessive CORT levels. When compared to other studies, our results support the view that multiple factors may be acting as stressors in the field affecting amphibian responses, and simple pathways as tested in this study may not adequately represent field conditions.

Alternative competition-induced digestive strategies yield equal growth, but constrain compensatory growth in red-eyed treefrog larvae

Compensatory growth is well documented across taxa and provides a fitness advantage to animals who would otherwise reach a smaller reproductive size. We investigated the role of competition-induced gut plasticity in facilitating a compensatory response in red-eyed treefrog larvae. We reared larvae at low, medium, and high densities with different per capita resources, environments known to produce individuals with long and short guts. We then transferred larvae to competitively equal environments to determine if longer guts provided an advantage when resources became available. We predicted that larvae from higher densities with longer guts would exhibit hyperphagia and compensatory growth. We measured growth over 1-week, as well as the time to and size at metamorphosis. To assess mechanisms underlying the growth response, we measured diet transit time and intake. Growth, development, and metamorph snout-vent length did not differ between larvae with long and short guts. Instead, different gut lengths were associated with dramatically different feeding strategies. Medium- and high-density larvae fed at rates far below what their guts could accommodate. However, the combination of low intake and longer guts extended diet transit times, presumably increasing digestibility. This unexpected strategy achieved the same results as that of low-density larvae, which ate twice as much food, but passed it more quickly through a shorter gut. The lack of a compensatory response may be attributed to the costs of accelerated growth and weak seasonal time constraints in the tropics. This suggests that although compensatory growth is widespread among animals, expression of the response may vary with environmental context. J. Exp. Zool. 323A: 778-788, 2015. © 2015 Wiley Periodicals, Inc.