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

Invasive species pose a significant threat to native biodiversity globally, often causing population declines or extinctions. However, some native species demonstrate resilience and coexist with invaders. Understanding the mechanisms facilitating this coexistence is crucial for conservation. Three potential mechanisms include behavioral avoidance, physiological adaptation (e.g., toxin tolerance), and morphological changes. The cane toad (*Rhinella marina*) invasion in Australia provides a valuable case study. Previous research suggests that native species, such as black snakes, exhibit all three mechanisms in response to cane toads. However, the situation is less clear for monitor lizards (*Varanus spp.*), which are fatally poisoned by toad consumption. While some studies indicate that monitor lizards in toad-infested areas avoid toads, evidence for a widespread, long-term taste aversion remains limited. This study aims to address this gap by conducting a large-scale investigation of monitor lizard feeding behavior in areas with varying exposure to cane toads across Australia, testing the hypothesis that lizards in areas with toads would avoid consuming them.

The impact of invasive species on native biodiversity is well-documented, with many leading to extinctions. However, some native species recover and coexist with invaders. Mechanisms for increased tolerance to invaders include behavioral avoidance, physiological adaptation, and morphological changes. The cane toad invasion in Australia provides a case study. Black snakes, for instance, have shown reduced likelihood of consuming toads, increased toxin tolerance, and altered morphology in response to toads. The response of monitor lizards is less clear. While studies have shown toad avoidance in toad-invaded areas, large-scale evidence for taste aversion as a key mechanism for persistence is lacking. Existing studies have mainly focused on local comparisons, potentially overlooking broader patterns. This paper aims to investigate taste aversion in monitor lizards across a wider spatial and temporal scale.

The study investigated behavioral responses of two monitor lizard species, *Varanus panoptes* and *V. varius*, to cane toad legs and non-toxic controls (chicken necks, eggs, sardines) across 1300 and 2500 km transects in Australia. The transects covered areas with varying durations of cane toad invasion, including sites yet to be invaded. Ex situ trials were conducted at 41 sites (15 for lace monitors, 24 for yellow-spotted monitors). For lace monitors, trials involved presenting a chicken egg and a toad leg in a tray, monitored using remote cameras. For yellow-spotted monitors, bait units with non-consumable sardines, a chicken egg, and a toad leg were deployed, monitored by remote cameras. Lizards were identified individually based on morphological characteristics. Data were analyzed using Generalized Linear Models (GLM) with a binomial error distribution and Logit link function, using Firth Adjusted Maximum Likelihood estimations to address quasi-complete separation. Animal ethics protocols were followed and approvals obtained from relevant committees and agencies.

Of the 28 lace monitor trials, all consumed chicken necks. 60% of those from toad-free sites consumed toad legs, while none from toad-present sites did. In yellow-spotted monitor trials (43 trials), 40 lizards consumed non-toxic baits, but only those from toad-free areas consumed toad legs. Statistical analysis confirmed a significant difference in toad leg consumption between toad-free and toad-present sites for both species (lace monitor χ² = 14.69, P < 0.0007; yellow-spotted monitor, similar significant results). The duration of toad presence did not significantly affect the consumption of non-toxic baits, indicating the continued willingness to consume alternative food sources. Although no toad leg consumption was observed in invaded areas, one yellow-spotted monitor was observed carrying a road-killed toad in a toad-present area.

The results strongly support the hypothesis that taste aversion is a key mechanism allowing monitor lizards to coexist with cane toads. The consistent avoidance of toad legs in invaded areas, even after long periods of toad presence (up to 80 years), demonstrates the effectiveness of this learned behavior. The observation of some toad avoidance even in toad-naive lizards (around 40%) suggests an innate aversion or rapid learning. The study's extensive sampling and long-term perspective provide robust evidence for taste aversion as a major factor in shaping the interaction between monitor lizards and cane toads, highlighting behavioral plasticity as an important mechanism in species' responses to invasion. These findings contradict the hypothesis that monitor lizards have evolved physiological resistance or morphological changes to cope with the toxic threat. The lack of evolved resistance underscores that behavioral avoidance acts as the primary defense mechanism.

This study provides compelling evidence that taste aversion is a critical mechanism enabling monitor lizards to survive the cane toad invasion. The consistent avoidance of toxic prey across a wide geographical area and long time frame supports this conclusion. The study highlights the importance of behavioral plasticity as a key driver of species persistence in the face of invasive species. Future research could focus on investigating the genetic basis of individual variation in learning and aversion, examining the role of other sensory cues in toad avoidance, and exploring the generality of taste aversion in other predator-prey systems.

The study relies on observational data, limiting the ability to definitively determine the proximate mechanisms underlying toad avoidance. While strong correlations were found, it is not possible to conclusively rule out other factors contributing to toad avoidance behavior. The ex situ trials might not perfectly replicate natural foraging conditions, although efforts were made to maintain ecological realism. The study focuses on two monitor lizard species and may not generalize to all species or invasive species.