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Body size at metamorphosis is a key trait in species (such as many anurans) with biphasic life-histories. Experimental studies have shown that metamorph size is highly plastic, depending upon larval density and environmental conditions (e.g. temperature, food supply, water quality, chemical cues from conspecifics, predators and competitors). To test the hypothesis that this developmental plasticity is adaptive, or to determine if inducing plasticity can be used to control an invasive species, we need to know whether or not a metamorphosing anuran's body size influences its subsequent viability. For logistical reasons, there are few data on this topic under field conditions. We studied cane toads (Rhinella marina) within their invasive Australian range. Metamorph body size is highly plastic in this species, and our laboratory studies showed that larger metamorphs had better locomotor performance (both on land and in the water), and were more adept at catching and consuming prey. In mark-recapture trials in outdoor enclosures, larger body size enhanced metamorph survival and growth rate under some seasonal conditions. Larger metamorphs maintained their size advantage over smaller siblings for at least a month. Our data support the critical but rarely-tested assumption that all else being equal, larger body size at metamorphosis is likely to enhance an individual's long term viability. Thus, manipulations to reduce body size at metamorphosis in cane toads may help to reduce the ecological impact of this invasive species.

Biological invasions can favour rapid changes in intraspecific competitive mechanisms such as cannibalism by imposing novel evolutionary pressures. For example, cane toad ( Rhinella marina ) tadpoles are highly cannibalistic on eggs and hatchlings in their invasive range in Australia, but not in their native range in South America. Whether such changes in cannibalism occur in invasive populations of other amphibian species is unknown. To explore this question, we collected wild-laid egg clutches of Japanese common toads ( Bufo japonicus ) from native and invasive populations in Japan, and conducted laboratory experiments to examine cannibalism responses. Contrary to the Australian system, we found that invasion has been accompanied by reduced cannibalistic tendency of B. japonicus tadpoles. This reduction has occurred despite invasive-range B. japonicus eggs/hatchlings being more vulnerable than native-range B. japonicus eggs/hatchlings to cannibalism by native-range conspecific tadpoles, and to predation by native-range frog tadpoles. Our findings thus support the idea that biological invasions can generate rapid changes in rates of cannibalism, but also show that decreases as well as increases can occur. Future work could investigate the proximate cues and selective forces responsible for this rapid decrease in rates of cannibalism by tadpoles in an invasive B. japonicus population.

Hatchlings of invasive cane toads (Rhinella marina) in Australia respond facultatively to chemical cues of non‐feeding cannibalistic conspecific tadpoles by accelerating development, but consequently experience reduced growth, development and survival in the subsequent tadpole stage. Predation‐induced developmental acceleration of eggs or hatchlings is rare among amphibians, and the implications of and context‐dependent impacts of such developmental plasticity are poorly understood. For cane toads, the source and identity of the tadpole cue that induces this response are unknown. Additionally, it is unknown whether these carry‐over costs are due to accelerated early development per se or are specific to developmental acceleration induced by conspecific tadpole cues. Finally, it is unknown whether these costs can be mitigated by the availability of food resources at critical times during early development. We conducted laboratory experiments to investigate these issues. Our results show that (1) based on significant hatchling responses to skin swabs, the chemical that induces costly developmental plasticity is located in the skin of cannibalistic cane toad tadpoles, (2) carry‐over effects of early developmental acceleration are elicited only by cues from cannibal tadpoles because temperature‐induced developmental acceleration of hatchlings did not reduce subsequent growth, development or survival and (3) excess food availability during early development did not mitigate the carry‐over costs of exposure to cannibal tadpole cues. Thus, this developmental plasticity response, triggered by detection of chemicals exuded from the skin of conspecific tadpoles, causes unique negative carry‐over costs for younger larvae. However, we found that tadpole production of skin secretions is also plastic, with swabbed tadpoles inducing stronger responses in hatchlings than their unswabbed siblings. Finally, the carry‐over costs that follow cannibal exposure cannot be mitigated by favorable nutritional conditions. Hatchlings of invasive cane toads respond to the risk of cannibalism by accelerating development but experience significant negative carry‐over effects on fitness in the tadpole stage. Our experiments demonstrate that this hatchling response is induced by chemicals located in the skin of older cannibalistic tadpoles. This response is unique to cannibal tadpole cues, and once initiated, cannot be reversed by favourable conditions such as a high nutrient environment.

Inducible defences can improve survival in variable environments by allowing individuals to produce defences if they detect predators. These defences are often expressed as inter‐related developmental, morphological, and behavioural changes. However, producing defences can incur costs, which may be expressed immediately and/or during subsequent life stages. In Australia, waterborne cues of potentially cannibalistic conspecific tadpoles induce hatchlings of invasive cane toads to accelerate their developmental rate, thereby reducing their window of vulnerability. However, the mechanisms and costs of such accelerated development are poorly understood, and whether cane toad embryos show cannibal‐induced plasticity in other traits is unknown. Here, we found no evidence of altered time of hatching for embryos exposed to non‐feeding conspecific cannibal tadpole cues. Additionally, hatchling dispersal behaviours were not affected by exposure to these cues. However, developmental acceleration of hatchlings induced by exposure to tadpole cues was accompanied by reduced hatchling growth, indicating a trade‐off between these processes. At the conclusion of the hatchling stage, cannibal‐exposed individuals were smaller and morphologically distinct from control siblings. This size reduction affected performance during the subsequent tadpole stage: smaller cannibal‐exposed individuals were more likely to die, and initial size tended to be positively associated with subsequent tadpole growth and development across treatments (respectively, p = .07 and p = .06). However, even accounting for variation in initial size, there was an additional negative effect of cannibal exposure on tadpole growth and development, demonstrating that the fitness costs associated with developmental acceleration are not entirely attributable to size reductions. Hatchlings of invasive cane toads in Australia respond to risk of cannibalism by conspecific tadpoles by accelerating development. This occurs as a trade‐off with growth. The resulting size reduction predicts some, but not all, future fitness costs.

Many invasive species experience intense intraspecific competition, because they are abundant in anthropogenically disturbed habitats where few native species persist. Species‐specific competitive mechanisms that evolve in this context may offer novel, highly targeted means to control invasive taxa. We conducted laboratory experiments to evaluate the feasibility of this method of control, based on waterborne cues that are produced by tadpoles of the cane toad (Rhinella marina) to suppress the development of conspecific embryos. Our trials examined the nature and species‐specificity of the effect, the robustness of the cue to freezing and storage, and the amounts required to suppress toad embryos. Our results were encouraging. The cue appears to be chemical rather than a biological organism, and may well be species‐specific; the four species of native anurans that we tested were not influenced by toad larval cues. The cue retains its effectiveness after being frozen, but not after being dried, or after 7 d in water. It is effective at very low concentrations (the amount produced by three tadpoles within 750 L of water). Overall, the cane toad's suppressor pheromone may offer an effective new way to control invasive toads.

Chemical cues produced by late-stage embryos of the cane toad ( Rhinella marina ) attract older conspecific larvae, which are highly cannibalistic and can consume an entire clutch. To clarify the molecular basis of this attraction response, we presented captive tadpoles with components present in toad eggs. As previously reported, attractivity arises from the distinctive toxins (bufadienolides) produced by cane toads, with some toxins (e.g., bufagenins) much stronger attractants than others (e.g., bufotoxins). Extracts of frozen toad parotoid glands (rich in bufagenins) were more attractive than were fresh MeOH extracts of the parotoid secretion (rich in bufotoxins), and purified marinobufagin was more effective than marinobufotoxin. Cardenolide aglycones (e.g., digitoxigenin) were active attractors, whereas C-3 glycosides (e.g., digoxin, oubain) were far less effective. A structure–activity relationship study revealed that tadpole attractant potency strongly correlated with Na + /K + ATPase inhibitory activity, suggesting that tadpoles monitor and rapidly react to perturbations to Na + /K + ATPase activity.

Competition within and among species can play a key role in structuring the assemblages of anuran tadpoles. Previous studies have reported that tadpoles of the invasive cane toad ( Rhinella marina ) are more strongly disadvantaged by the presence of native frog tadpoles than by the same number of conspecific toad tadpoles. That effect might arise from a lack of coevolution of the invasive toad with its competitors; and/or from a generalized superiority of frog tadpoles over toad tadpoles. To clarify those possibilities, we conducted experimental trials using the larvae of a native rather than invasive toad ( Bufo japonicus formosus in Japan) exposed to larvae of native anurans (the sympatric frogs Rana japonica and Rana ornativentris and the parapatric toad Bufo japonicus japonicus ). In intraspecific competition trials, higher densities of B. j. formosus prolonged the larval period and reduced size at metamorphosis, but did not affect survival. In interspecific competition trials, the effects of the other anuran species on B. j. formosus were similar to the effects of the same number of conspecific larvae. This similarity in impact of interspecific versus intraspecific competition argues against any overall competitive superiority of frog larvae over toad larvae. Instead, the vulnerability of larval cane toads to frog tadpoles may result from a lack of coevolutionary history.

Although animals of many species kill and consume conspecifics, most such cases probably involve serendipitous encounters between the individuals concerned. In some taxa, however, cannibalism is an active process, with predatory individuals searching out and consuming specific types of conspecific prey items. Although anuran tadpoles often have been reported to consume conspecific eggs, this behaviour has been interpreted as a by-product of usual foraging behaviours rather than a result of targeted searching. Our field and laboratory studies in tropical Australia show that the tadpoles of invasive cane toads Bufo marinus are strongly attracted to chemical cues from conspecific eggs; the effective cues are released late in embryonic development, as the jelly coat breaks down. Tadpoles of native Australian frog species were attracted to the eggs of toads only rarely. If deployed as bait in traps, chemical cues from toad eggs could provide a way to selectively remove toad larvae from waterbodies.

Cane Toads (Rhinella marina) are invasive pests in many parts of the world, including the Japanese island of Ishigaki. Extensive research in Australia has identified promising new methods for control, but also has shown that toads exhibit geographic variation in many traits (suggesting that methods developed in one location may not work in another). Can the approaches developed in Australia play a useful role for controlling this invasive species in Japan? Our experimental trials on Ishigaki Island suggest that these new methods can be successfully applied to Japan. First, Cane Toad embryos exposed to chemical cues of conspecific tadpoles exhibited a reduction in viability (subsequent growth and development). This response appears to be species-specific, with native frog embryos not being affected by exposure to cues from toad tadpoles, and Cane Toad embryos not being affected by exposure to cues from native frog tadpoles. Second, Cane Toad tadpoles were attracted to traps containing water from conspecific eggs, and toxin from adult conspecifics. Third, adult Cane Toads were attracted to acoustic cues of calling males, with sex differences in rates of attraction to specific versions of a synthetic call (males were attracted to choruses whereas females were attracted to low-frequency calls). Our results suggest that the methods developed by Australian researchers are applicable to controlling invasive Cane Toads in Japan.

We conducted a quantitative and qualitative chemical analysis of cane toad bufadienolides--the cardioactive steroids that are believed to be the principal cane toad toxins. We found complex shifts in toxin composition through toad ontogeny: (1) eggs contain at least 28 dominant bufadienolides, 17 of which are not detected in any other ontogenetic stage; (2) tadpoles present a simpler chemical profile with two to eight dominant bufadienolides; and (3) toxin diversity decreases during tadpole life but increases again after metamorphosis (larger metamorph/juvenile toads display five major bufadienolides). Total bufadienolide concentrations are highest in eggs (2.64 ± 0.56 μmol/mg), decreasing during tadpole life stages (0.084 ± 0.060 μmol/mg) before rising again after metamorphosis (2.35 ± 0.45 μmol/mg). These variations in total bufadienolide levels correlate with toxicity to Australian frog species. For example, consumption of cane toad eggs killed tadpoles of two Australian frog species (Limnodynastes convexiusculus and Litoria rothii), whereas no tadpoles died after consuming late-stage cane toad tadpoles or small metamorphs. The high toxicity of toad eggs reflects components in the egg itself, not the surrounding jelly coat. Our results suggest a dramatic ontogenetic shift in the danger that toads pose to native predators, reflecting rapid changes in the types and amounts of toxins during toad development.