Evolutionary shifts in anti-predator responses of invasive cane toads (Rhinella marina)

Abstract

Abstract: A potential prey item’s response to encountering a predator depends on aspects of the predator (e.g., its locomotor capacity), the local environment (e.g., proximity to shelter) and the physiological state of the prey item, but in addition, anti-predator tactics also vary geographically among different populations within wide-ranging species. Using standardised trials, we tested responses of cane toads (Rhinella marina) to being placed on a laboratory runway and encouraged to flee. Overall, the toads least capable of rapid locomotion were the ones most likely to respond to simulated predator attack by exuding toxins rather than attempting to escape. A toad’s willingness to move down the runway and its propensity to exude toxin from the parotoid glands rather than fleeing were repeatable in successive trials and influenced by the animal’s location of origin, morphology, previous experience, and parentage. We found that specimens from Australia were more willing to flee than were those from the native range (French Guiana) or Hawai’i; larger toads, and those with relatively longer legs, were more willing to flee; captive-raised toads were less willing to flee and more willing to exude toxin. Captive-raised offspring resembled their wild-caught parents both in propensity to run and in propensity to exude toxin. Thus, geographic divergence among cane toad populations in anti-predator responses reflects a complex combination of processes, including both developmental plasticity and heritability. Significance statement: The process of colonising novel environments exposes an organism to a host of different predators that exert selection on morphology, behaviour and physiology. Over time, this can lead to the evolution of novel phenotypes that are adapted to local conditions. Anthropogenically facilitated biological invasions provide a unique opportunity to study how species respond to colonisation of new environments. Here, we studied geographic variation in cane toad (Rhinella marina) anti-predator behaviours from a range of locations (including invasive and native populations) using standardised trials. Our results suggest that invasive Australian cane toads have undergone a shift in their anti-predator response, becoming more willing to flee than are conspecifics from other areas. Location of origin, morphology, prior experience and parentage all played a role in shaping a toad’s anti-predator response, suggesting that the behaviour represents a combination of plastic and heritable variation.