Geographic divergence in dispersal-related behaviour in cane toads from range-front versus range-core populations in Australia

Abstract

In invasive species, geographically variable evolutionary and ecological pressures can cause the rapid evolution of divergent behavioural phenotypes. Studies on invasive cane toads (Rhinella marina) in tropical Australia have revealed strong (and heritable) shifts in physiological traits related to dispersal rate. Behavioural phenotypes may have evolved in similar ways. We used standardised arena trials to test field-collected adult female toads from three populations: a range-core area in Queensland (ca. 76 years post-colonisation), a range-front population in Western Australia (<5 years post-colonisation) and an intermediate Northern Territory population (11 years post-colonisation). As predicted, toads from the range-front population were more exploratory and more likely to take risks in a novel arena environment than were conspecifics from the range-core population. We suggest that differential selection on behavioural responses to novel conditions in range-core versus range-front populations has produced a distinctive behavioural phenotype at the range-front that retains a high propensity for exploration and risk-taking (enhancing the ability of range-front toads to locate food and shelter) even when faced with novel environments. In contrast, at the range core where the locations of resources are known, a decrease in exploration and risk-taking in response to a novel environment may be favoured as it assists toads in evading threats. Ongoing biological invasions provide an ideal opportunity to examine which phenotypic traits drive establishment, range-expansion and invasion success. Furthermore, ongoing invasions allow us to investigate if variation in evolutionary and ecological pressures across an invasion range leads to geographical divergence in phenotypic traits. Dispersal ability is a key factor in invasion success. Behavioural traits such as exploration and a propensity to take risks enhance dispersal as individuals with these traits rapidly move out of their existing range and exploit new habitats and resources. We studied geographic divergence of dispersal-related behavioural traits across the Australian invasion range of cane toads (Rhinella marina) using standardised laboratory trials. We found that range-front toads were more exploratory and more likely to take risks than were conspecifics from range-core areas. Our results suggest that dispersal-enhancing behavioural traits may be important drivers of invasion success in cane toads.