After introduction to a new environment, species may encounter abiotic and biotic factors with which they have not evolved; however, some introductions are remarkably successful. The ‘enemy release hypothesis’ (ERH) proposes that upon introduction, invasive species leave behind some of the natural pathogens and parasites with which they have co-evolved. This may then allow invaders to redirect defence resources towards characteristics that enable them to thrive in their new environment.

Studies on the 85-year old cane toad (Rhinella marina) invasion across Northern Australia have revealed rapid evolution of many phenotypic traits, for example immune function along the introduced range. ERH predicts that actively invading range-edge toads would be expected to down-regulate costly immune function, for example inflammation, when compared to those in the range-core.

Interestingly, our differential gene expression analysis revealed a curvilinear trend along the invasion gradient, with cane toads in the oldest and newest populations having up-regulated expression of inflammation-related genes, while individuals from intermediate populations exhibit down-regulation of inflammation genes.

Although these results appear to contradict ERH predictions, there is a possible confound. The higher levels of activity in range-edge versus range-core toads may also increase expression of inflammation genes, thereby causing an up-regulation of such genes in the range-edge toads relative to intermediate toads.


The project: “How do Activity Levels Affect Immune Function Gene Expression in Cane Toads?” is designed to disentangle the effects of ERH with those of activity levels. It aims to do so by investigating the relationship between activity levels in toads and their immune gene expression levels.

Student Benefits

This project will include a significant fieldwork component in the Northern Territory as well as genetics work in the laboratory. All students in our group are encouraged and supported to publish their research in international scientific journals.