Invasion of cane toads (Rhinella marina) affects the problem-solving performance of vulnerable predators (monitor lizards, Varanus varius)

Abstract

Variation in morphological, genetic, or behavioural traits within and among native species can modify vulnerability to impacts from an invasive species. If an individual’s vulnerability depends upon its cognitive performance, we may see adaptive shifts in cognitive traits post-invasion. Commonly, animals with enhanced cognitive abilities perform better in novel tasks, often by prioritising decision accuracy over decision speed. In eastern Australia, giant monitor lizards (Varanus varius) are fatally poisoned if they ingest invasive cane toads (Rhinella marina), but vulnerability is lower for individuals that carefully evaluate the novel prey type before swallowing it. To test if toad-imposed selection for neophobia and caution affected cognitive performance, we tested free-ranging monitors with a device that required lizards to manipulate the apparatus in order to obtain food. Success at accomplishing that task, and the speed of that success, was lower and slower in lizards from long-colonised sites than from uninvaded sites. Our results suggest that toad invasion has modified cognitive phenotypes within populations of this apex predator, a change that might have substantial effects on other species. Many studies of the impacts of biological invasions focus on the numerical effect of an invader on the abundances of native taxa, neglecting other types of impacts. Colonising taxa can also impose selection on behavioural traits of native species, generating shifts in behaviour as native taxa adapt to intruders. Such shifts in behaviour are interesting not only in their own right but also because such shifts (especially in apex predators) may influence other taxa within food webs. Importantly, the nature and magnitude of such shifts may change over time post-invasion.