Proximate mechanisms underlying the rapid modification of phenotypic traits in cane toads (Rhinella marina) across their invasive range within Australia

Biological invasions often involve rapid modification of phenotypic traits. This is presumably in response to the novel pressures to which an invader is exposed, but the proximate basis for those changes remains unclear. Phenotypic changes may be generated by environmental factors (E), genetic factors (G) or the interaction between these two processes (GxE). To explore this issue, we obtained eight clutches of the cane toad (Rhinella marina) from three regions across its range of invasion within Australia, and raised the offspring under standard conditions (diet and/or exercise level manipulations) to clarify the proximate underpinnings of geographical divergence in phenotypic traits. Our results demonstrate that phenotypic variation among Australian cane toad populations is affected by genetics and environment, and an interaction between these two processes. Some traits (e.g. sprint distance) differed among populations, suggesting a heritable basis. Other traits (e.g. relative heart mass) were affected by experimental treatments but not by population. Intriguingly, other traits (e.g. body length) were affected by interactions between population of origin and experimental treatments. The relative importance of G, E and G x E differs among traits, but all three mechanisms have contributed to the rapid phenotypic divergence observed across the Australian range of invasive cane toads.