Justin C. Touchon

AMPHIBIAN EMBRYO AND PARENTAL DEFENSES AND A LARVAL PREDATOR REDUCE EGG MORTALITY FROM WATER MOLD

Water molds attack aquatic eggs worldwide and have been associated with major mortality events in some cases, but typically only in association with additional stressors. We combined field observations and laboratory experiments to study egg stage defenses against pathogenic water mold in three temperate amphibians. Spotted salamanders (Ambystoma maculatum) wrap their eggs in a protective jelly layer that prevents mold from reaching the embryos. Wood frog (Rana sylvatica) egg masses have less jelly but are laid while ponds are still cold and mold growth is slow. American toad (Bufo americanus) eggs experience the highest infection levels. They are surrounded by thin jelly and are laid when ponds have warmed and mold grows rapidly. Eggs of all three species hatched early when infected, yielding smaller and less developed hatchlings. This response was strongest in B. americanus. Precocious hatching increased vulnerability of wood frog hatchlings to invertebrate predators. Finally, despite being potential toad hatchling predators, R. sylvatica tadpoles can have a positive effect on B. americanus eggs. They eat water mold off infected toad clutches, increasing their hatching success.

Reproductive mode plasticity: Aquatic and terrestrial oviposition in a treefrog

Diversification of reproductive mode is a major theme in animal evolution. Vertebrate reproduction began in water, and terrestrial eggs evolved multiple times in fishes and amphibians and in the amniote ancestor. Because oxygen uptake from water conflicts with water retention in air, egg adaptations to one environment typically preclude development in the other. Few animals have variable reproductive modes, and no vertebrates are known to lay eggs both in water and on land. We report phenotypic plasticity of reproduction with aquatic and terrestrial egg deposition by a frog. The treefrog Dendropsophus ebraccatus, known to lay eggs terrestrially, also lays eggs in water, both at the surface and fully submerged, and chooses its reproductive mode based on the shade above a pond. Under unshaded conditions, in a disturbed habitat and in experimental mesocosms, these frogs lay most of their egg masses aquatically. The same pairs also can lay eggs terrestrially, on vegetation over water, even during a single night. Eggs can survive in both aquatic and terrestrial environments, and variable mortality risks in each may make oviposition plasticity adaptive. Phylogenetically, D. ebraccatus branches from the basal node in a clade of terrestrially breeding species, nested within a larger lineage of aquatic-breeding frogs. Reproductive plasticity in D. ebraccatus may represent a retained ancestral state intermediate in the evolution of terrestrial reproduction.

Effects of plastic hatching timing carry over through metamorphosis in red‐eyed treefrogs

Environmentally cued plasticity in hatching timing is widespread in animals. As with later life-history switch points, plasticity in hatching timing may have carryover effects that affect subsequent interactions with predators and competitors. Moreover, the strength of such effects of hatching plasticity may be context dependent. We used red-eyed treefrogs, Agalychnis callidryas, to test for lasting effects of hatching timing (four or six days post-oviposition) under factorial combinations of resource levels (high or low) and predation risk (none, caged, or lethal Pantala flavescens dragonfly naiads). Tadpoles were raised in 400-L mesocosms in Gamboa, Panama, from hatching until all animals had metamorphosed or died, allowing assessment of effects across a nearly six-month period of metamorphosis. Hatching early reduced survival to metamorphosis, increased larval growth, and had context-dependent effects on metamorph phenotypes. Early during the period of metamorph emergence, early-hatched animals were la...

Negative synergism of rainfall patterns and predators affects frog egg survival

1. The importance of rainfall is recognized in arid habitats, but has rarely been explored in ecosystems not viewed as rainfall limited. In addition, most attempts to study how rainfall affects organismal survival have focused on long-term rainfall metrics (e.g. monthly or seasonal patterns) instead of short-term measures. For organisms that are short lived or are sensitive to desiccation, short-term patterns of rainfall may provide insight to understanding what determines survival in particular habitats. 2. We monitored daily rainfall and survival of arboreal eggs of the treefrog Dendropsophus ebraccatus at two ponds during the rainy season in central Panama. Desiccation and predation were the primary sources of egg mortality and their effects were not independent. Rainfall directly reduced desiccation mortality by hydrating and thickening the jelly surrounding eggs. In addition, rainfall reduced predation on egg clutches. 3. To elucidate the mechanism by which rainfall alters predation, we exposed experimentally hydrated and dehydrated egg clutches to the two D. ebraccatus egg predators most common at our site, ants and social wasps. Ants and wasps preferentially preyed on dehydrated clutches and ants consumed dehydrated eggs three times faster than hydrated eggs. 4. Rainfall patterns are expected to change and the responses of organisms that use rainfall as a reliable cue to reproduce may prove maladaptive. If rainfall becomes more sporadic, as is predicted to happen during this century, it may have negative consequences for desiccation-sensitive organisms.

Prey Responses to Predator Chemical Cues: Disentangling the Importance of the Number and Biomass of Prey Consumed

To effectively balance investment in predator defenses versus other traits, organisms must accurately assess predation risk. Chemical cues caused by predation events are indicators of risk for prey in a wide variety of systems, but the relationship between how prey perceive risk in relation to the amount of prey consumed by predators is poorly understood. While per capita predation rate is often used as the metric of relative risk, studies aimed at quantifying predator-induced defenses commonly control biomass of prey consumed as the metric of risk. However, biomass consumed can change by altering either the number or size of prey consumed. In this study we determine whether phenotypic plasticity to predator chemical cues depends upon prey biomass consumed, prey number consumed, or both. We examine the growth response of red-eyed treefrog tadpoles (Agalychnis callidryas) to cues from a larval dragonfly (Anax amazili). Biomass consumed was manipulated by either increasing the number of prey while holding individual prey size constant, or by holding the number of prey constant and varying individual prey size. We address two questions. (i) Do prey reduce growth rate in response to chemical cues in a dose dependent manner? (ii) Does the magnitude of the response depend on whether prey consumption increases via number or size of prey? We find that the phenotypic response of prey is an asymptotic function of prey biomass consumed. However, the asymptotic response is higher when more prey are consumed. Our findings have important implications for evaluating past studies and how future experiments should be designed. A stronger response to predation cues generated by more individual prey deaths is consistent with models that predict prey sensitivity to per capita risk, providing a more direct link between empirical and theoretical studies which are often focused on changes in population sizes not individual biomass.

Thermally contingent plasticity: temperature alters expression of predator‐induced colour and morphology in a Neotropical treefrog tadpole

1. Behavioural, morphological and coloration plasticity are common responses of prey to predation risk. Theory predicts that prey should respond to the relative magnitude of risk, rather than a single level of response to any risk level. In addition to conspecific and predator densities, prey growth and differentiation rates affect the duration of vulnerability to size- and stage-limited predators and therefore the relative value of defences. 2. We reared tadpoles of the Neotropical treefrog Dendropsophus ebraccatus with or without cues from a predator (Belostoma sp.) in ecologically relevant warm or cool temperatures. To track phenotypic changes, we measured morphology, tail coloration and developmental stage at three points during the larval period. 3. Cues from predators interacted with growth conditions causing tadpoles to alter their phenotype, changing only tail colour in response to predators in warm water, but both morphology and colour in cool growth conditions. Tadpoles with predators in warm water altered coloration early but converged on the morphology of predator-free controls. Water temperature alone had no effect on tadpole phenotype. 4. We demonstrate that seemingly small variation in abiotic environmental conditions can alter the expression of phenotypic plasticity, consistent with predictions about how growth rate affects risk. Predator-induced tadpole phenotypes depended on temperature, with strong expression only in temperatures that slow development. Thermal modulation of plastic responses to predators may be broadly relevant to poikilotherm development. It is important to include a range of realistic growth conditions in experiments to more fully understand the ecological and evolutionary significance of plasticity.

Genetic variation in pathogen-induced early hatching of toad embryos

Accelerated hatching is one of few defences available to embryos, and is effective against many egg‐stage risks. We present the first analysis of genetic variation in hatching plasticity, examining premature hatching of American toad embryos in response to pathogenic water moulds. We reared eggs from half‐ and full‐sib families in the presence and absence of water mould. Hatching age and hatchling size showed low cross‐environment genetic correlations, suggesting that early‐induced hatching can evolve largely independently of spontaneous hatching. We found less phenotypic and additive genetic variation for early‐induced hatching than spontaneous hatching, and a stronger correlation between egg and induced hatchling sizes. Directional selection by the pathogen may have eroded variation in early‐induced hatching, pushing it against the constraint of hatching gland development. Later hatching has a second, muscular component. This pattern of variation may characterize defences based on developmental transitions, although other inducible defences show more variation in induced phenotypes.

Behavioral plasticity mitigates risk across environments and predators during anuran metamorphosis.

Most animals metamorphose, changing morphology, physiology, behavior and ecological interactions. Size- and habitat-dependent mortality risk is thought to affect the evolution and plastic expression of metamorphic timing, and high predation during the morphological transition is posited as a critical selective force shaping complex life cycles. Nonetheless, empirical data on how risk changes across metamorphosis and stage-specific habitats, or how that varies with size, are rare. We examined predator–prey interactions of red-eyed treefrogs, Agalychnis callidryas, with an aquatic predator (giant water bug, Belostoma) and a semi-terrestrial predator (fishing spider, Thaumasia) across metamorphosis. We manipulated tadpole density to generate variation in metamorph size and conducted predation trials at multiple developmental stages. We quantified how frog behavior (activity) changes across metamorphic development, habitats, and predator presence or absence. In aquatic trials with water bugs, frog mortality increased with forelimb emergence, as hypothesized. In semi-terrestrial trials, contrary to predictions, predation by spiders increased, not decreased, with tail resorption. In neither case did frog size affect mortality. Frogs reduced activity upon forelimb emergence in the water, and further with emergence into air, then increased activity with tail resorption. Longer-tailed metamorphs were captured more often in spider attacks, but attacked less, as most attacks followed prey movements. Metamorphs behaviorally compensated for poor escape performance more effectively on land than in water, thus emergence timing may critically affect mortality. The developmental timing of the ecological transition between environments that select for different larval and juvenile phenotypes is an important, neglected variable in studies of complex life cycles.

Oviposition site choice under conflicting risks demonstrates that aquatic predators drive terrestrial egg-laying

Laying eggs out of water was crucial to the transition to land and has evolved repeatedly in multiple animal phyla. However, testing hypotheses about this transition has been difficult because extant species only breed in one environment. The pantless treefrog, Dendropsophus ebraccatus, makes such tests possible because they lay both aquatic and arboreal eggs. Here, we test the oviposition site choices of D. ebraccatus under conflicting risks of arboreal egg desiccation and aquatic egg predation, thereby estimating the relative importance of each selective agent on reproduction. We also measured discrimination between habitats with and without predators and development of naturally laid aquatic and arboreal eggs. Aquatic embryos in nature developed faster than arboreal embryos, implying no cost to aquatic egg laying. In choice tests, D. ebraccatus avoided habitats with fish, showing that they can detect aquatic egg predators. Most importantly, D. ebraccatus laid most eggs in the water when faced with only desiccation risk, but switched to laying eggs arboreally when desiccation risk and aquatic predators were both present. This provides the first experimental evidence to our knowledge that aquatic predation risk influences non-aquatic oviposition and strongly supports the hypothesis that it was a driver of the evolution of terrestrial reproduction.

Variation in Abundance and Efficacy of Tadpole Predators in a Neotropical Pond Community

Abstract Variation in predation risk plays an important role in shaping prey behavior, morphology, life history, population dynamics, and community structure in freshwater systems. Anuran larvae are important prey in freshwater communities and spatiotemporal variation in risk can arise from changes in the number and identity of predators; however, our understanding of variation in abundance, identity, and foraging rates for natural predator assemblages in tropical pond communities is limited. We surveyed ponds near Gamboa, Panama in 2004 and 2010 to estimate variation in predator communities of tadpoles over space and time. We also conducted short-term predation trials with the 10 most common predators using hatchling tadpoles of two widespread Neotropical frog species, Red-Eyed Treefrogs (Agalychnis callidryas) and Pantless Treefrogs (Dendropsophus ebraccatus). Predator abundance varied nearly threefold across ponds within a single year and as much as 19-fold within a pond across years. Dominant taxa also varied, with backswimmers (Notonectidae), poeciliid fish, or libellulid dragonfly naiads being the most common depending upon pond and year. Predation trials revealed that prey-specific predation rates differed among predator taxa. Some presumed predators did not consume hatchlings, whereas others consumed >90% of prey. The smaller D. ebraccatus hatchlings generally experienced higher predation rates; however, large invertebrate predators like aeshnid dragonfly naiads, giant water bugs, and fishing spiders consumed more A. callidryas. These results suggest that strong but variable larval-stage risk may be an important selective factor shaping tadpole communities and phenotypes in Neotropical ponds. Resumen Variación en el riesgo de depredación tiene una función importante sobre el comportamiento, morfología, historia de vida, dinámicos de poblaciones y la estructura de comunidades en sistemas acuáticos. Las larvas de ranas, renacuajos, son presas importantes en las comunidades acuáticas y la variación espacio-temporal en el riesgo puede deberse a cambios en el número y la identidad de los depredadores. Sin embargo, nuestra comprensión de la variación en la abundancia, la identidad, y las tasas de forrajeo de los depredadores de las comunidades de estanques tropicales es limitado. Encuestamos a cerca de los estanques de Gamboa, Panamá, en 2004 y 2010 para estimar la variación en las comunidades de depredadores de renacuajos en el espacio y el tiempo. También realizamos pruebas de depredación a corta plazo con los 10 depredadores más comunes utilizando los renacuajos de dos especies de ranas neotropicales con ámplios rangos, ranas arborícolas de ojos rojos (Agalychnis callidryas) y sin pantalones (Dendropsophus ebraccatus). La abundancia de los depredadores varió casi tres veces a través de estanques dentro de un solo año y tanto como 19 veces en un estanque a través de años. Las especies dominantes también variaron, con Notonecta, peces poeciliidos y náyades siendo el más común, dependiendo de la charca y año. Las pruebas de depredación revelaron que la depredación de cada especie de presa diferían entre los taxones depredadores. Algunos depredadores presuntos no consumieron renacuajos, mientras que otros consumieron >90% de las presas. El renacuajo más pequeño, D. ebraccatus, generalmente sufrido tasas de depredación mas altas, pero los depredadores mas grandes, como las ninfas de libélulas de la familia Aeshnidae, Belostoma y arañas de pesca, consumieron más A. callidryas. Estos resultados sugieren que el riesgo fuerte pero variable durante la etapa de renacuajo puede ser un factor selectivo importante para la formación de comunidades de renacuajos y fenotipos en estanques neotropicales.