Cane Toads, Keelbacks and Elapids

Since the introduction of the Cane Toad (Rhinella marina, formerly Bufo marinus) into Australia in 1935 it was feared that many native predators would suffer from eating this toxic toad, and many have. Some species have been able to keep the toads out of their diets. One native natracid snake, the Australia Keelback (Tropidonophis mairii) showed some resistance to the toad's toxins. But, Llewelyn et al. (2010a) have shown that the Keelback avoids the toad and eats native frogs instead. The snake's preferences are expressed in both the laboratory and field. If the snakes are force-fed toads they frequently regurgitate them, this is not true when the snakes were force-fed frogs. The toads are not lethal to the snakes, but they do have sub-lethal consequences, making the snakes sluggish and unable to respond normally to their environments. Previous work showed the toads are poor in nutrients. The authors note that while the snakes are not harmed by the toads, they do not benefit from a potentially new prey. In a second paper (Llewelyn et al. (2010b) the authors asked the questions: Is the Keelback’s ability to coexist with toads the result of its ancestral Asian origins, or a consequence of rapid adaptation since cane toads arrived in Australia? And does the snake’s feeding preference for frogs rather than toads reflect an innate or learned behavior? By comparing populations of snakes that had been exposed to toads for longer times, to those only exposed to toads recently, the researchers were able to establish that the Keelback's resistance to toads is indeed innate, as is their preference for frogs over toads. The Keelback had an Asian natracid ancestor that was undoubtedly exposed to many species of toxic bufonids, while the Australian elapids were phylogenetically naive to bufonid toxins and are unable to consume them without serious ill-effects or death.

Llewelyn, J., L. Schwarzkopf, R. Alford, R. Shine. 2010. Something different for dinner? Responses of a native Australian predator (the keelback snake) to an invasive prey species (the cane toad). Biological Invasions 12:1045-1051.

Llewelyn, J., Phillips, B.L., Brown, G.P., Schwarzkopf, L., Alford, R., Shine, R. Adaptation or preadaptation: why are keelback snakes (Tropidonophis mairii) less vulnerable to invasive cane toads (Bufo marinus) than are other Australian snakes? Evolutionary Ecology (in press – accepted Feb 1, 2010)