Evolution of physiological traits in the invasive cane toad (Rhinella marina)

Abstract

Cane toads are native to warm humid landscapes of the Americas, but since its introduction to Australia, the rapid geographic spread has exposed toads to climatic conditions well outside those of the native range, with many Australian sites hotter and seasonally drier than those in which the species evolved. I investigated the impacts of temperature and hydration on aspects of dispersal behaviour and physiological traits of toads, to clarify the ways in which toads may have changed as a result of encountering novel environments within their invaded range, by comparing populations within the native range (Brazil) to ones introduced to Hawai’i and Australia. When stimulated to run at high or low temperatures, or when dehydrated, Australian toads exhibited greater performance endurance than either Brazilian or Hawai’ian toads. When raised under the same conditions, toads whose parents were collected from the climatic challenging area of Western Australia exhibited better performance than the offspring of Queensland toads. Rates of evaporative water loss, skin resistance and rehydration showed that toads from eastern Australia had higher skin resistance to water loss whereas Brazilian toads had faster rehydration rates, suggesting a shift in water balance strategy after toads were introduced to Australia. Histological examination of the skin structure showed that Australian toads have thinner skin than do native-range conspecifics. Laboratory trials revealed that toads from Townsville heated faster than toads from northwestern Australian populations, especially when deprived of water. When exposed to extreme heat or desiccation, Brazilian toads exhibited higher levels of corticosterone and of heat-shock protein 70 than Australian toads, suggesting a more reactive stress response. My studies clarify some of the mechanisms that have allowed cane toads to colonize diverse areas that impose climatic challenges not present within the species’ native range.