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

Abstract

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.

Observation records of the Bangon Monitor Lizard, Varanus bangonorum (Squamata, Varanidae), with emphasis on behaviour and local threats, from the Municipality of Abra de Ilog, Occidental Mindoro, Philippines

“…material 3). When threatened, Cane Toads release toxins from their parotid glands that may be lethal to some Varanidae species (Pettit et al 2020(Pettit et al , 2021aHarvey et al 2022), but genetic testing suggests that four varanids of the subgenera Soterosaurus are physiologically resistant to toad toxins (Pettit et al 2021b).…”

Shifts in the foraging tactics of crocodiles following invasion by toxic prey

“…Research on captive freshwater crocodiles reported rapid aversion learning in response to an initial non-fatal encounter with cane toads as prey 26 . Studies of other vulnerable predators (e.g., red-bellied blacksnakes, Pseudechis porphyriacus ; common planigales, Planigale maculata ; yellow-spotted monitors, Varanus panoptes ) have shown that toad colonisation can induce a rapid and long-sustained aversion to their consumption as prey 1 , 27 , 28 . At least two mechanisms may underpin the elimination of cane toads from diets of these predators: behavioural plasticity (conditioned taste aversion) and natural selection (higher mortality of individuals with a genetically based propensity to consume toads 1 , 9 ).…”

Novel habitat associations and seasonality in threatened Mitchell’s water monitors (Varanus mitchelli): Implications for conservation

“…2020) and chicken necks (Pettit et al . 2020) have been used successfully to improve detections of other Australian varanids. In light of our observations, we suggest this, in combination with active searches by experts (but see Ward‐Fear et al .…”

“…For this reason, species-specific survey methodologies, which account for the species' ecology and do not rely on the skill level of the observer, should be designed for V. mitchelli. Remote camera traps baited with sardines (Moore et al 2020) and chicken necks (Pettit et al 2020) have been used successfully to improve detections of other Australian varanids. In light of our observations, we suggest this, in combination with active searches by experts (but see Ward-Fear et al 2019), as the most appropriate survey methodology for this species.…”