Field trials of chemical suppression of embryonic cane toads (Rhinella marina) by older conspecifics

Abstract

Laboratory experiments have shown that the viability of embryos of the invasive cane toad (Rhinella marina) can be reduced by exposure to chemical cues from older conspecific larvae. These effects (very strong in laboratory trials) may offer an exciting new approach to controlling this problematic invasive species in Australia. However, the degree to which the method works in natural environments has yet to be assessed. Our experiments in the laboratory and in seminatural outdoor waterbodies show that chemical cues from tadpoles do indeed suppress the growth, development, and survival of conspecific larvae that are exposed as embryos and do so in a dose‐dependent manner; higher tadpole densities cause greater suppression of embryos. In seminatural outdoor waterbodies, suppressor‐exposed tadpoles were less than half as likely to survive to metamorphosis as were controls, and were much smaller when they did so and hence, less likely to survive the metamorph stage. Additionally, female cane toads were less likely to oviposit in a waterbody containing free‐ranging (but not cage‐enclosed) tadpoles, suggesting that the presence of tadpoles (rather than the chemical cues they produce) may discourage oviposition. Broadly, our results suggest that the suppression effect documented in laboratory studies does indeed occur in the field also, and hence that we may be able to translate that approach to develop new and more effective ways to reduce rates of recruitment of peri‐urban populations of cane toads in their invasive range.

Intergenerational effects of manipulating DNA methylation in the early life of an iconic invader

“…To determine whether exposure to +A or +Z affected the potency of the chemical cues produced by cane toad tadpoles, we exposed the tadpoles from each treatment to conspecific hatchlings. Conspecific hatchlings exhibit clear, dose-dependent responses to the cues produced by cane toad tadpoles [26][27][28]. These effects are associated with the induction of defence against cannibalism by older conspecifics, which is a major source of mortality for cane toad eggs and hatchlings.…”

“…To defend against this threat, hatchlings accelerate their development upon detecting cues produced by conspecific tadpoles [27]. The magnitude of the subsequent effects depends upon the level of risk perceived, such that stronger cues lead to larger reductions in viability during the tadpole stage [28]. Although stronger cues may generally indicate greater risk (i.e.…”

virToad: simulating the spatiotemporal population dynamics and management of a global invader

“…Both male and female cane toads attempt to select spawning sites that optimise the survival of their offspring; and the prospect of cannibalism or chemical suppression of their offspring by older tadpoles will influence spawning site selection (Hearnden 1991;Clarke et al 2015). The best spawning sites are those with no or few tadpoles, and with high densities of calling adult males (Hearnden 1991;Clarke et al 2019;McCann et al 2020a). Once a spawning site has been selected, simulated breeding females mate with a randomly selected breeding male, and then spawn a clutch of eggs (but see Bowcock et al 2013;Yasumiba et al 2015).…”

“…Trap placement frequency was associated with a specified level of effort. As is the case in the wild, trapped simulated tadpoles could not be detected by breeding adults prospecting for optimal spawning sites (McCann et al 2020a). Moreover, trapped simulated tadpoles were only removed from the simulation after they metamorphosed, or when the virtual traps were removed from the simulation, which allowed them to continue producing pheromones that suppress the growth and development of nearby eggs.…”

Is developmental plasticity triggered by DNA methylation changes in the invasive cane toad (Rhinella marina)?

“…First, hatchlings are affected by ‘cannibal cues’ associated with the approach of older, cannibalistic tadpoles (Clarke, Crossland, Shilton, Shine, & Rohr, 2015; Crossland & Shine, 2011, 2012; DeVore, Crossland, & Shine, 2021). Exposure to cannibal cues causes hatchlings to accelerate development, with significant carry-over effects during the subsequent tadpole stage: decreased tadpole survival, decreased body mass and body size (i.e., growth), reduced tooth row keratinization, increased swimming behaviour and repression of feeding behaviour (Clarke et al, 2015; Crossland & Shine, 2012; DeVore, Crossland, & Shine, 2021; DeVore, Crossland, Shine, & Ducatez, 2021; McCann, Crossland, Greenlees, & Shine, 2020). Second, injured tadpoles release an ‘alarm cue’, reflecting a predation risk, that elicits immediate avoidance by conspecifics (Hagman & Shine, 2008).…”