Competing tadpoles: Australian native frogs affect invasive cane toads (Rhinella marina) in natural waterbodies

The cane toad (Rhinella marina) is one of the most successful invasive species worldwide, and has caused significant negative impacts on Australian fauna. Experimental work in the laboratory and in mesocosms has shown that tadpoles of native frogs can affect survival, size at metamorphosis and duration of larval period of cane toad tadpoles. To test if these effects occur in nature, we conducted a field experiment using two temporary ponds where we set up enclosures with tadpoles of native green tree frogs (Litoria caerulea) and cane toads in treatments with a range of densities and combinations. The presence of green tree frog tadpoles significantly decreased the growth rate of toad tadpoles and increased the duration of their larval period in both ponds; in one pond, frog tadpoles also significantly reduced the body length and mass of metamorph toads. Toad tadpoles did not have any significant negative effects on green tree frog tadpoles, but there was strong intraspecific competition within the latter species: increased frog tadpole density resulted in increased larval period and reduced survival, growth rate and size at metamorphosis for frogs at one or both ponds. Our results are encouraging for the possibility of using native frogs as one component of an integrated approach to the biological control of cane toads.     

Effects of induced variation in anuran larval development on postmetamorphic energy reserves and locomotion

Anuran larvae exhibit high levels of phenotypic plasticity in growth and developmental rates in response to variation in temperature and food availability. We tested the hypothesis that alteration of developmental pathways during the aquatic larval stage should affect the postmetamorphic performance of the Iberian painted frog (Discoglossus galganoi). We exposed tadpoles to different temperatures and food types (animal- vs. plant-based diets) to induce variation in the length of the larval period and body size at metamorphosis. In this species, larval period varied with temperature but was unaffected by diet composition. In contrast, size at metamorphosis was shaped by the interaction between food quality and temperature; tadpoles fed on an animal-based diet became bulkier metamorphs than those fed on plant-based food at high (22°C) but not at low (12°C) temperature. Body condition of newly metamorphosed frogs was unrelated to the temperature or food type experienced during the premetamorphic stage. Frogs maintained at high temperature during the larval period showed reduced jumping ability, especially when fed on the plant-based diet. However, when considering size-independent jumping ability, cold-reared individuals exhibited the lowest performance, and herbivores reared at 17°C the highest. Cold-reared (12°C) frogs accumulated larger amounts of energy reserves than individuals raised at 17°C or 22°C. This was still the case after correction for differences in body mass, thus indicating some size-independent effect of developmental temperature. Despite the higher lipid content of the carnivorous diet, the differences in energy reserves between herbivores and carnivores were relatively weak and associated with differences in body size. These results suggest that the consequences of environmental variation in the larval habitat can extend to the terrestrial phase and influence juvenile growth and survival.     

Influence of density and salinity on larval development of salt‐adapted and salt‐naïve frog populations

Environmental change and habitat fragmentation will affect population densities for many species. For those species that have locally adapted to persist in changed or stressful habitats, it is uncertain how density dependence will affect adaptive responses. Anurans (frogs and toads) are typically freshwater organisms, but some coastal populations of green treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands. Tadpoles from coastal populations metamorphose sooner and demonstrate faster growth rates than inland populations when reared solitarily. Although saltwater exposure has adaptively reduced the duration of the larval period for coastal populations, increases in densities during larval development typically increase time to metamorphosis and reduce rates of growth and survival. We test how combined stressors of density and salinity affect larval development between salt‐adapted (“coastal”) and nonsalt‐adapted (“inland”) populations by measuring various developmental and metamorphic phenotypes. We found that increased tadpole density strongly affected coastal and inland tadpole populations similarly. In high‐density treatments, both coastal and inland populations had reduced growth rates, greater exponential decay of growth, a smaller size at metamorphosis, took longer to reach metamorphosis, and had lower survivorship at metamorphosis. Salinity only exaggerated the effects of density on the time to reach metamorphosis and exponential decay of growth. Location of origin affected length at metamorphosis, with coastal tadpoles metamorphosing slightly longer than inland tadpoles across densities and salinities. These findings confirm that density has a strong and central influence on larval development even across divergent populations and habitat types and may mitigate the expression (and therefore detection) of locally adapted phenotypes.     

Interactions of bullfrog tadpole predators and an insecticide: predation release and facilitation

The effect of a contaminant on a community may not be easily predicted, given that complex changes in food resources and predator-prey dynamics may result. The objectives of our study were to determine the interactive effects of the insecticide carbaryl and predators on body size, development, survival, and activity of tadpoles of the bullfrog (Rana catesbeiana). We conducted the study in cattle tank mesocosm ponds exposed to 0, 3.5, or 7.0 mg/l carbaryl, and no predators or two red-spotted newts (Notophthalmus viridescens), bluegill sunfish (Lepomis macrochirus), or crayfish (Orconectes sp.). Carbaryl negatively affected predator survival by eliminating crayfish from all ponds, and by eliminating bluegill sunfish from ponds exposed to the highest concentration of carbaryl; carbaryl exposure did not effect survival of red-spotted newts. Because crayfish were eliminated by carbaryl, bullfrogs were released from predation and survival was near that of predator controls at low concentrations of carbaryl exposure. High concentrations of carbaryl reduced tadpole survival regardless of whether predators survived carbaryl exposure or not. Presence of crayfish and newts reduced tadpole survival, while bluegill sunfish appeared to facilitate bullfrog tadpole survival. Presence of carbaryl stimulated bullfrog tadpole mass and development. Our study demonstrates that the presence of a contaminant stress can alter community regulation by releasing prey from predators that are vulnerable to contaminants in some exposure scenarios.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Influence of density and predators on metamorphic size in Rhinella schneideri tadpoles raised in mesocosm conditions

Amphibians exhibit extreme plasticity in the timing of metamorphosis, and several species respond to water availability, accelerating metamorphosis when their ponds dry. We analyzed the plasticity of the developmental response to water volume in Rhinella schneideri tadpoles. We raised tadpoles in mesocosm. Covariation between body size at metamorphosis and timing of development was positive. Nevertheless, the first approximately 53% of the metamorphoses finishing the cycle required between 34 and 56 days, and the covariation between body size at metamorphosis and timing of development was negative. For these tadpoles, the larval density and the presence of predators did not significantly affect their mass to metamorphosis. Nevertheless, predators affected time to metamorphosis. For the remainder of the tadpoles that reached metamorphosis at > 56 days, the relationship between body size at metamorphosis and timing of development was positive. For these tadpoles, larval density was important for mass at metamorphosis and presence of predators was also important for time to metamorphosis. Two dominant features were observed: (i) approximately 53% of metamorphs had morphological features similar to individuals developing in desiccating ponds, and (ii) the other individuals had morphological characteristics comparable to metamorphs developing in an unchanging environment.     

Mechanisms and Consequences of Developmental Acceleration in Tadpoles Responding to Pond Drying

Many amphibian species exploit temporary or even ephemeral aquatic habitats for reproduction by maximising larval growth under benign conditions but accelerating development to rapidly undergo metamorphosis when at risk of desiccation from pond drying. Here we determine mechanisms enabling developmental acceleration in response to decreased water levels in western spadefoot toad tadpoles (Pelobates cultripes), a species with long larval periods and large size at metamorphosis but with a high degree of developmental plasticity. We found that P. cultripes tadpoles can shorten their larval period by an average of 30% in response to reduced water levels. We show that such developmental acceleration was achieved via increased endogenous levels of corticosterone and thyroid hormone, which act synergistically to achieve metamorphosis, and also by increased expression of the thyroid hormone receptor TRΒ, which increases tissue sensitivity and responsivity to thyroid hormone. However, developmental acceleration had morphological and physiological consequences. In addition to resulting in smaller juveniles with proportionately shorter limbs, tadpoles exposed to decreased water levels incurred oxidative stress, indicated by increased activity of the antioxidant enzymes catalase, superoxide dismutase, and gluthatione peroxidase. Such increases were apparently sufficient to neutralise the oxidative damage caused by presumed increased metabolic activity. Thus, developmental acceleration allows spadefoot toad tadpoles to evade drying ponds, but it comes at the expense of reduced size at metamorphosis and increased oxidative stress.     

COMPETITION AMONG TADPOLES OF COEXISTING HEMICLONES OF HYBRIDOGENETIC RANA ESCULENTA: SUPPORT FOR THE FROZEN NICHE VARIATION MODEL

Vertebrate animals reproducing without genetic recombination typically are hybrids, which have large ranges, are locally abundant, and live in disturbed or harsh habitats. This holds for the hemiclonal hybridogenetic frog Rana esculenta: it is widespread in Europe and commonly is found in disturbed habitats such as gravel pits. We hypothesize that its widespread occurrence may either be the result of natural selection for a single hemiclone acting as a broadly adapted “general-purpose” genotype, or of interclonal selection, which maintains multiple hemiclones that each are relatively narrowly adapted and perform differently across environments, that is, the Frozen Niche Variation model. We tested these competing hypotheses using 1000-L outdoor artificial ponds to rear tadpoles of the parental species (Rana lessonae [LL] and Rana ridibunda [RR]) alone, and each of three hemiclones of Rana esculenta (GUT1, GUT2, GUT3) alone, and in mixed hemiclonal populations from hatching to metamorphosis. Tadpoles of three coexisting hemiclones from a single natural population (near Gütighausen, Switzerland) were reared in both two- and three-way mixtures in equal total numbers at high and low density. For each species and hemiclone, the proportion of tadpoles metamorphosing decreased as the density of tadpoles increased, with the three hemiclones spanning the range of values exhibited by the two parental species. LL and GUT1 tadpoles produced the highest proportion of metamorphs, whereas tadpoles of RR produced the fewest metamorphs at both densities. GUT1 tadpoles also produced the largest metamorphs at low density, GUT2 and GUT3 tadpoles produced smaller metamorphs than did GUT1 tadpoles at the low density, but the three hemiclones did not differ from each other at high density. The parental species (LL and RR) were intermediate in metamorphic size to the hemiclones at low density, but all genotypes converged on a similar size at high density. Length of the larval period also was affected by density, but its effect was dependent on genotype. GUT1 tadpoles had the shortest larval period at the low density, but larval period was longer and not different between GUT1, GUT3, and LL at high density. RR tadpoles had the longest larval period at both densities. The most dramatic results were that three genotypes (GUT1, GUT2, and RR) maintained rank order and increased days to metamorphosis from low to high density, whereas two genotypes (GUT3 and LL) changed rank order and decreased days to metamorphosis from low to high density. Mixtures of hemiclones in two- and three-way combinations facilitated the proportion of tadpoles metamorphosing for GUT1 and GUT2 at both densities, but only at the low density for GUT3 tadpoles. Results from this experiment are incompatible with the General-Purpose Genotype model as a global explanation of hybrid abundance in these frogs. Alternatively, the Frozen Niche Variation prediction of general performance superiority of clonal mixtures relative to single clone populations is strongly supported. The data confirm that fitness advantages of hemiclones change, depending on the environment, such that in temporally and spatially heterogeneous habitats like ponds, frequency-dependent selection among hemiclones may promote coexistence in hemiclonal assemblages. Yet, differential dispersal or colonization ability and historical factors affecting hemiclone distribution may also be important in shaping patterns of clonal coexistence.     

Canopy Closure,Competition, and the Endangered Dusky Gopher Frog

Abstract: A major challenge facing wildlife biologists is understanding why some species go extinct while others persist in the same habitat. To address this question, we investigated whether tree canopy closure over ponds affects growth and survival of rare and common tadpoles within ponds and mediates competitive interactions among species. We conducted 2 experiments to test whether canopy closure and competition may have contributed to the decline of the endangered dusky gopher frog (Rana sevosa), but allowed the persistence of the southern leopard frog (R. sphenocephala). We explored the response of both species to canopy closure in single-species and mixed- (1:1) species treatments of identical total tadpole density. An experiment using aquatic enclosures in temporary ponds showed that canopy closure reduced tadpole growth approximately 20% for both species. Survival of dusky gopher frog tadpoles was higher in mixed-species enclosures than in single-species enclosures. In a complementary experiment using artificial ponds, dusky gopher frogs had lower survival to metamorphosis, reduced size at metamorphosis, and produced a lower total biomass of metamorphosed juveniles in shaded ponds. Southern leopard frogs exhibited reduced body size at metamorphosis only when shaded. These studies suggest that pond canopy closure, not larval competition, may be contributing to the decline of the dusky gopher frog. The different responses to canopy closure suggest a potential mechanism for the loss of dusky gopher frogs and the persistence of southern leopard frogs. Removal of trees from historically open-canopy ponds may help facilitate the recovery of dusky gopher frogs and benefit similar species.     

Post-metamorphic change in activity metabolism of anurans in relation to life history

Summary Newly-metamorphosed individuals of some species of frogs and toads differ from adults in behavior, ecology, and physiology. These differences may be related to broader patterns of the life histories of different species of frogs. In particular, the length of larval life and the size of a frog at metamorphosis appear to be significant factors in post-metamorphic ontogenetic change. These changes in performance are associated with rapid post-metamorphic increases in oxygen transport capacity. Bufo americanus (American toads) and Rana sylvatica (wood frogs) spend only 2–3 months as tadpoles and metamorphose at body masses of 0.25 g or less. Individuals of these species improve endurance and aerobic capacity rapidly during the predispersal period immediately following metamorphosis. Increases in hematocrit, hemoglobin concentration, and heart mass relative to body mass are associated with this improvement in organismal performance. Rana clamitans (green frogs) spend from 3 to 10 months as larvae and weigh 3 g at metamorphosis. Green frogs did not show immediate post-metamorphic increases in performance. Rana palustris (pickerel frogs) are intermediate to wood frogs and green frogs in length of larval life and in size at metamorphosis, and they are intermediate also in their post-metamorphic physiological changes.American toads and wood frogs appear to delay dispersal from their natal ponds while they undergo rapid post-metamorphic growth and development, whereas green frogs disperse as soon as they leave the water, even before they have fully absorbed their tails. The very small body sizes of newly metamorphosed toads and wood frogs appear to limit the scope of their behaviors. The brief larval periods of these species permit them to exploit transient aquatic habitats, but impose costs in the form of a period of post-metamorphic life in which their activities are restricted in time and space compared to those of adults.     

Predator‐mediated phenotypic plasticity in tadpoles of the striped marsh frog,Limnodynastes peronii

Abstract We tested the phenotypic responses of larval striped marsh frogs (Limnodynastes peronii) to the odonate nymph predator, Aeshna brevistyla. When reared in the presence of dragonfly nymphs feeding upon conspecifics of L. peronii larvae the tadpoles showed a strong change in morphology. Morphological changes included an increase in total tail height, but also an unexpected marked change in head‐body shape. In addition, we examined how tadpole development, as well as mass and length at metamorphosis, was affected by exposure to dragonfly nymphs. Larval development of L. peronii was strongly influenced by exposure to the predatory behaviour of dragonfly nymphs. Predator‐induced tadpoles had significantly slower developmental rates than control larvae. Although metamorphs of non‐exposed L. peronii were approximately 33% lighter than predator‐exposed metamorphs and possessed lower jump distances, after adjusting for mass there was no difference in jump distance. The newly described morphological response may assist in more accurately relating morphological plasticity to fitness.     

Predation risk and competition effects on the life-history characteristics of larval Oregon spotted frog and larval red-legged frog

We conducted an artificial pond experiment to test hypotheses about the effects of competition and non-lethal predator cues on metamorphic characteristics of sympatric Oregon spotted frogs (Rana pretiosa) and red-legged frogs (Rana aurora) in southwestern British Columbia. Tadpoles were exposed to the presence or absence of one another, two density levels and to the presence or absence of predacious odonate larvae (Aeshna palmata) isolated in enclosures. In the artificial pond study, R. aurora were significantly larger at metamorphosis (12%) and exhibited only slightly longer larval periods when exposed to Aeshna. In the presence of R. pretiosa, they significantly decreased time to metamorphosis, and were significantly larger at metamorphosis (12%) than those reared alone. Rana pretiosa in treatments with R. aurora were somewhat larger at metamorphosis when a non-lethal predator was present, and in treatments where R. pretiosa were alone with a predator tadpole mass at metamorphosis was smaller than those in the absence of Aeshna, but these results were not statistically significant. Both species reduced activity and moved away from the predator in the presence of an enclosed dragonfly larva in the laboratory. Most tadpole mesocosm experiments have found that the trade-off between size and timing of metamorphosis is extremely important to amphibians, but we suggest that the trade-off discussed in traditional amphibian models may not apply to species like R. pretiosa that are exposed to the same gape-limited predators upon reaching metamorphosis.     

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.     

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.     

The embryonic development of Xenopus laevis under a low frequency electric field

The aim of this study was to determine the effects of a low frequency electric field on the early embryonic development of frogs. The embryos of African clawed toads, Xenopus laevis, were exposed to a 20-μA electric current during the cleavage stages. The developmental processes of embryos during and after electric field exposure were monitored for teratogenic effects. All the embryos continuously exposed to the electric field died without undergoing any developmental processes. However, when the embryos were exposed to the electric field for 20-min periods (four times/over 2 d), the embryos developed into both normal tadpoles (70 %) and malformed tadpoles with light edema, reduced pigmentation, or axial anomalies, such as crooked tails. After exposure, the control embryos were at development stage 35.5 (2 d 2 h), while the normal embryos of the assay group were at developmental stage 41(3 d 4 h). There was a 1 d 2 h difference between the two developmental stages, revealing the importance of that time period for embryogenesis. In conclusion, the effects of electric current on Xenopus embryos are dependent on the initial developmental stage and the duration of exposure.     

Evolutionary reduction of developmental plasticity in desert spadefoot toads

Organisms vary their rates of growth and development in response to environmental inputs. Such developmental plasticity may be adaptive and positively correlate with environmental heterogeneity. However, the evolution of developmental plasticity among closely related taxa is not well understood. To determine the evolutionary pattern of plasticity, we compared plasticity in time to and size at metamorphosis in response to water desiccation in tadpoles among spadefoot species that differ in breeding pond and larval period durations. Like most tadpoles, spadefoot tadpoles possess the remarkable ability to accelerate development in response to pond drying to avoid desiccation. Here, we hypothesize that desert spadefoot tadpoles have evolved reduced plasticity to avoid desiccation in ephemeral desert pools compared to their nondesert relatives that breed in long-duration ponds. We recorded time to and size at metamorphosis following experimental manipulation of water levels and found that desert-adapted species had much less plasticity in larval period and size at metamorphosis than nondesert species, which retain the hypothetical ancestral state of plasticity. Furthermore, we observed a correlation between degree of plasticity and fat body content that may provide mechanistic insights into the evolution of developmental plasticity in amphibians.     

The impact of larval predators and competitors on the morphology and fitness of juvenile treefrogs

Studies of phenotypic plasticity typically focus on traits in single ontogenetic stages. However, plastic responses can be induced in multiple ontogenetic stages and traits induced early in ontogeny may have lasting effects. We examined how gray treefrog larvae altered their morphology in four different larval environments and whether different larval environments affected the survival, growth, development, and morphology of juvenile frogs at metamorphosis. We then reared these juveniles in terrestrial environments under high and low intraspecific competition to determine whether the initial differences in traits at metamorphosis affected subsequent survival and growth, whether the initial phenotypic differences converged over time, and whether competition in the terrestrial environment induced further phenotypic changes. Larval and juvenile environments both affected treefrog traits. Larval predators induced relatively deep tail fins and short bodies, but there was no impact on larval development. In contrast, larval competitors induced relatively short tails and long bodies, reduced larval growth, and slowed larval development. At metamorphosis, larval predators had no effect on juvenile growth or relative morphology while larval competitors produced juveniles that were smaller and possessed relatively shorter limbs and shorter bodies. After 1 month of terrestrial competition among the juvenile frogs, the initial differences in juvenile morphology did not converge. There were no differences in growth due to larval treatment but there were differences in survival. Individuals that experienced low competition as tadpoles experienced near perfect survival as juvenile frogs but individuals that experienced high competition as tadpoles suffered an 18% decrease in survival as juvenile frogs. There were also morphological responses to juvenile competition, but these changes appear to be due, at least in part, to allometric effects. Collectively, these results demonstrate that larval environments can have profound impacts on the traits and fitness of organisms later in ontogeny.     

Developmental alterations and osmoregulatory physiology of a larval anuran under osmotic stress

Water salinity represents an environmental stress for many species. Amphibians are particularly sensitive because they are generally poor osmoregulators, and most species are completely absent from brackish and saline environments. We experimentally examined the effect of different salinity levels on larvae of the toad Bufo calamita L., a species that occupies freshwater ponds but can also breed in brackish ponds. Two independent experiments are reported here. In both experiments, tadpoles under saline conditions (ranging between 85 and 200 mOsm) showed a slower developmental rate, metamorphosing between 4 and 9 d later than the controls. Bufo calamita tadpoles reared in brackish water increased their osmolality and solute concentration (mainly sodium and chloride), decreased their levels of glucose, and decreased the total protein content, all measured from whole-animal extracts. Although most larval anurans are strictly ammoniotelic until the completion of metamorphosis, a few species exposed to dehydrating environments have evolved the ability to use urea as an osmolyte during the larval phase. The data presented here reveal that although B. calamita seems to be yet another exception to the rule of larval strict ammoniotelism, the tadpoles are not able to use urea as an osmolyte and rely on sodium-chloride balance instead. Preliminary immunoassays of thyroid hormone content suggest a possible decrease in hormone levels induced in water salinity conditions that correlate with a decreased developmental rate.     

Experimental analysis of the evolutionary potential of hybridization in leopard frogs (Anura: Ranidae)

Artificial crossing using Rana blairi and R. sphenocephala frogs produced conspecific, interspecific and F₁ backcross hybrid genotypes. Although hybrid males used in the crosses were sterile, crosses using hybrid females produced viable larvae. The larval performance of resultant parental and hybrid genotypes was measured in experimental ponds at two densities. Density significantly affected survival, body mass at metamorphosis, larval period length and metamorphosis for all genotypes. Survival was the same among genotypes, but decreased with increasing density. Body mass at metamorphosis was the same among genotypes, but decreased with increasing density. Larval period increased with increasing density. Among genotypes, larvae from the conspecific R. sphenocephala cross had the shortest larval period while larvae from the conspecific R. blairi cross had the longest larval period. All hybrid genotypes had larval periods longer than R. sphenocephala, but shorter than R. blairi. The percentage of individuals metamorphosing was highest for R. sphenocephala ponds and lowest for R. blairi ponds across densities. Ponds with hybrid larvae produced a greater proportion of metamorphs than those with R. blairi larvae, but a smaller proportion than R. sphenocephala ponds. Equivalent or increased relative larval performance of hybrid genotypes under the conditions of our experiment suggests that hybrid genotypes may possess similar or higher fitnesses relative to their progenitors in some environments. Reduced fertility of adult hybrid males is a powerful selective force against natural hybridization. Nevertheless, because of the successful reproduction by female hybrids, natural hybridization has the potential to serve as a mechanism for the introgression of novel genetic variation that can benefit both R. blairi and R. sphenocephala in fluctuating and unpredictable larval environments. Experimental determination of the fitness of parental and hybrid genotypes is crucial for a comprehensive understanding of the effects of hybridization on organismal evolution.     

Effects of density and predation on Scaphiopus couchi tadpoles in desert ponds

Summary The effects of density on growth and development of Scaphiopus couchi tadpoles in desert ponds were investigated, and sources of mortality over a three-year period were documented. In 16 of the 82 ponds monitored, predation was the principal cause of death, demonstrating that tadpoles in desert ponds may be exposed to high levels of predation, although the overall importance of predation is less here than in more mesic areas. Desiccation was the primary cause of mortality in 49 ponds. Growth and development were extremely slow in most high density ponds and as a result most tadpoles were unable to metamorphose before ponds dried. Only 8 ponds produced metamorphs, and mortality was high even in these. Food-supplementation resulted in some metamorphosis in high density ponds, although the effect was diminished by the extreme crowding in most ponds. In low density ponds, S. couchi tadpoles can develop very quickly and metamorphose. High mortality due to desiccation is largely a consequence of high density: tadpoles rarely completed development in high density ponds, regardless of pond duration.     

Simultaneous activation of genes encoding urea cycle enzymes and gluconeogenetic enzymes coincides with a corticosterone surge period before metamorphosis in Xenopus laevis

Amphibian tadpoles are postulated to excrete ammonia as nitrogen metabolites but to shift from ammonotelism to ureotelism during metamorphosis. However, it is unknown whether ureagenesis occurs or plays a functional role before metamorphosis. Here, the mRNA-expression levels of two urea cycle enzymes (carbamoyl phosphate synthetase I [CPSI] and ornithine transcarbamylase [OTC]) were measured beginning with stage-47 Xenopus tadpoles at 5 days post-fertilization (dpf), between the onset of feeding (stage 45, 4 dpf) and metamorphosis (stage 55, 32 dpf). CPSI and OTC expression levels increased significantly from stage 49 (12 dpf). Urea excretion was also detected at stage 47. A transient corticosterone surge peaking at stage 48 was previously reported, supporting the hypothesis that corticosterone can induce CPSI expression in tadpoles, as found in adult frogs and mammals. Stage-46 tadpoles were exposed to a synthetic glucocorticoid, dexamethasone (Dex, 10–500 nM) for 3 days. CPSI mRNA expression was significantly higher in tadpoles exposed to Dex than in tadpoles exposed to the vehicle control. Furthermore, glucocorticoid receptor mRNA expression increased during the pre-metamorphic period. In addition to CPSI and OTC mRNA upregulation, the expression levels of three gluconeogenic enzyme genes (glucose 6-phosphatase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase 1) increased with the onset of urea synthesis and excretion. These results suggest that simultaneous induction of the urea cycle and gluconeogenic enzymes coincided with a corticosterone surge occurring prior to metamorphosis. These metabolic changes preceding metamorphosis may be closely related to the onset of feeding and nutrient accumulation required for metamorphosis.