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.
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A New South Wales Cane Toad, Rhinella marina. |
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)