The enduring toxicity of road-killed cane toads (Rhinella marina)

Taking the bait: Developing a bait delivery system to target free‐ranging crocodiles and varanid lizards with a novel conservation strategy

In tropical Australia, conditioned taste aversion (CTA) can buffer vulnerable native predators from the invasion of a toxic prey species (cane toads, Rhinella marina). Thus, we need to develop methods to deploy aversion‐inducing baits in the field, in ways that maximize uptake by vulnerable species (but not other taxa). We constructed and field‐tested baiting devices, in situ with wild animals. Apparatus were set next to waterbodies and baited concurrently at multiple locations (over water, water's edge, and on the bank). Baits were checked and replaced twice daily during the trial; remote cameras recorded visitation by native predators. Bait longevity was compared at sun‐exposed and shaded locations over 12 h. The strength required to remove baits from apparatus was measured in varanids and crocodiles. The device promoted high rates of bait uptake by freshwater crocodiles (47% baits consumed), varanid lizards (19% baits consumed), and non‐target taxa (34% baits consumed). Targeting specific predators can be achieved by manipulating bait location and time of deployment, as well as the force required to dislodge the bait. Crocodiles were best targeted with over‐water baits, whereas varanid lizards preferred baits located at the edges of waterbodies. When testing bait longevity in ambient conditions, during the daytime baits desiccated fully within 12 h, and faster in the sun than in the shade. Based on studies using captive animals, the “pulling force” strength of reptilian predators scaled with body size and was greater in crocodiles than in varanid lizards. We present the first conservation baiting protocol designed specifically for reptiles. Our results demonstrate the feasibility of widespread and taxon‐specific deployment of aversion‐inducing baits to buffer the impacts of invasive cane toads, and our methods are applicable (with modification) to other research and management programs globally.

Choosy cannibals: Targeted consumption of conspecific hatchlings by larval cane toads is triggered by species‐specific defensive toxins

In many species, cannibalism is uncommon and involves nonselective consumption of conspecifics as well as heterospecifics. However, within their invasive Australian range, cane toad larvae (Rhinella marina) specifically target and voraciously consume the eggs and hatchlings of conspecifics, often extirpating entire clutches. In contrast, toad larvae rarely consume the eggs and hatchlings of native frogs. Here, we use laboratory studies to demonstrate that this selective consumption is triggered by species‐specific chemical cues: maternally‐invested bufadienolide toxins that otherwise defend cane toad eggs and hatchlings against predators. We find that these cues stimulate feeding behaviors in toad tadpoles, such that the addition of bufadienolide toxins to the water column increases predation on eggs, not only of conspecifics, but also of native anuran species that are otherwise usually ignored. In contrast, we find that cannibalism rates on conspecific hatchlings are high and unaffected by the addition of bufadienolide cues. The maternally‐invested toxins present in conspecific eggs may therefore be more easily detected post‐hatching, at which point tadpole feeding behaviors are induced whether or not additional toxin cues are present. As bufadienolide cues have previously been found to attract toad tadpoles to vulnerable hatchlings, our present findings demonstrate that the same toxin cues that attract cannibalistic tadpoles also induce them to feed, thereby facilitating cannibalism through multiple behavioral effects. Because native fauna do not produce bufadienolide toxins, the species specificity of these chemical cues in the Australian landscape may have facilitated the evolution of targeted (species‐specific) cannibalism in invasive cane toad populations. Thus, these bufadienolide toxins confer cost (increased vulnerability to cannibalism in early life‐stages) as well as benefit (reduced vulnerability to predation by other taxa).

Choose your meals carefully if you need to coexist with a toxic invader

Vulnerable native species may survive the impact of a lethally toxic invader by changes in behaviour, physiology and/or morphology. The roles of such mechanisms can be clarified by standardised testing. We recorded behavioural responses of monitor lizards (Varanus panoptes and V. varius) to legs of poisonous cane toads (Rhinella marina) and non-toxic control meals (chicken necks or chicken eggs and sardines) along 1300 and 2500 km transects, encompassing the toad's 85-year invasion trajectory across Australia as well as yet-to-be-invaded sites to the west and south of the currently colonised area. Patterns were identical in the two varanid species. Of monitors that consumed at least one prey type, 96% took control baits whereas toad legs were eaten by 60% of lizards in toad-free sites but 0% from toad-invaded sites. Our survey confirms that the ability to recognise and reject toads as prey enables monitor lizards to coexist with cane toads. As toxic invaders continue to impact ecosystems globally, it is vital to understand the mechanisms that allow some taxa to persist over long time-scales.

Strain by the train: Patterns of toad fatalities on a Brazilian Amazonian railroad

Transportation infrastructures are directly responsible for killing billions of animals worldwide. Although the understanding about road impacts have recently increased, the impact of railroads on wildlife has received less attention. The current knowledge concerning the impacts of railroads focuses mainly on large mammals although amphibians might be affected. Our study aims to unravel temporal and spatial patterns of Rhinella toad fatalities on a Brazilian Amazonian railroad, to comprehend how toads are killed and to estimate the magnitude of toad fatalities. Data collection was carried out on foot on an 871-km stretch of the Estrada de Ferro Carajás from 2013 to 2017. We identified different potential causes for fatalities: being run over, desiccated or with barotrauma signs. We estimated a surprisingly high carcass persistence probability of about 38 days. After correcting for the bias from carcass detection and removal, we estimated that approximately 10,000 toads are killed per year (≈ 11 fatalities/km/year). A GLM model showed that toads were more likely to be killed in the dry to wet transition. We identified critical zones of fatalities and prioritized them according to their intensity. The highly critical segments encompass >10% of all fatalities although they cover only 1.5% of the railroad. Our study is the first one to address carcass detection and persistence on railroads and to unravel patterns of fatalities of an amphibian species in a tropical climate. A better understanding of the patterns of animal fatality on railroads is of fundamental importance to manage and mitigate this impact.

Exploiting intraspecific competitive mechanisms to control invasive cane toads (Rhinella marina)

If invasive species use chemical weapons to suppress the viability of conspecifics, we may be able to exploit those species-specific chemical cues for selective control of the invader. Cane toads (Rhinella marina) are spreading through tropical Australia, with negative effects on native species. The tadpoles of cane toads eliminate intraspecific competitors by locating and consuming newly laid eggs. Our laboratory trials show that tadpoles find those eggs by searching for the powerful bufadienolide toxins (especially, bufogenins) that toads use to deter predators. Using those toxins as bait, funnel-traps placed in natural waterbodies achieved near-complete eradication of cane toad tadpoles with minimal collateral damage (because most native (non-target) species are repelled by the toads' toxins). More generally, communication systems that have evolved for intraspecific conflict provide novel opportunities for invasive-species control.

Embryonic exposure to conspecific chemicals suppresses cane toad growth and survival

Adaptations to suppress the viability of conspecifics may provide novel ways to control invasive taxa. The spread of cane toads (Rhinella marina) through tropical Australia has had severe ecological impacts, stimulating a search for biocontrol. Our experiments show that cane toad tadpoles produce waterborne chemical cues that suppress the viability of conspecifics encountering those cues during embryonic development. Brief (72 h) exposure to these cues in the egg and post-hatching phases massively reduced rates of survival and growth of larvae. Body sizes at metamorphosis (about three weeks later) were almost twice as great in control larvae as in tadpole-exposed larvae. The waterborne cue responsible for these effects might provide a weapon to reduce toad recruitment within the species' invaded range.