Frogs can be pretty or not, but few people know that they can be extremely toxic. Most frogs you see are highly venomous (with one exception: the common frogs of the Rana genus). Like the cute little tree frogs.
Still, they have their predators. Some snakes have 'a preference' for them, even venomous frogs, like the Australian northern death adder, and can feed on them without getting intoxicated. Some of the adder's preys have skins releasing powerful venoms or sticky foams that can kill the snake.
The adder is highly venomous and in the case of non-poisonous frogs, the snake swallows immediately its prey. This is not the case with the toxic frogs, which are bitten and immediately released. Then the snake waits the venom to have its effect, till the poison or foam in the dead frog's skin degrades to benign chemicals.
"The frog usually hops some distance before dying, but the snake can easily track the frog down using its forked tongue and sensitive olfactory system," said co-author Ben Phillips of the University of Sydney.
The team was amazed to find that the snake tuned its behavior according with the frog species: in the case of the Australian marbled frog (Lymnodinastes), the foam dries in 10 minutes following the death of the amphibian, while the adder starts to eat the frog 12 minutes following the bite, but in the case of Dahl's aquatic frog (Litoria), the skin poison turns inactive in 30 minutes after the death and the adder will start eating the corpse 40 minutes after poisoning the animal.
How does the snake adjust its behavior? It appears that frog toxins from the snake's poisoning bite persists in the snake's mouth in non-dangerous, low levels.
"So they seem to wait until they can't taste the toxin before attempting to eat the frog," Phillips told LiveScience.
"Often they go and taste, then drop the prey before waiting a further five minutes or so and trying again."
As the snake kills before the frog, natural selection cannot operate in frogs for longer-lasting toxins.
"The common assumption is that snakes are pretty stupid, and to them a frog is a frog. But here we see a snake that effectively discriminates between frog species and then deals with each species in an appropriate manner," said Phillips.