Frogs and Snakes

When Stoney Creek treefrogs (Litoria lesueuri) mate, hundreds of males congregate around three or four females. In contrast to their normal olive drab, the much smaller and more numerous males display their state of excitement by becoming brilliant bright yellow.

I have never seen any any nocturnal snakes avail themselves of these veritable smorgasborgs; perhaps the treefrogs are poisonous. The Keelback or Freshwater Snake (Tropidonophis mairii) however, is diurnal and can tolerate the toxins of young cane toads (Bufo marinas) and also Stoney Creek treefrogs, if captured during the day.

In this media release from Sydney University, strategic behaviour of frogs and snakes reveals an unexpected sophistication:

When dinner is dangerous

Normally, when a snake meets a frog it is the frog that must fear for its life. But in the tropics of Australia there are several frog species that can turn the tables on their attacker.

The marbled frog produces a strong glue when bitten by a snake, making the frog difficult to handle. Dahl’s aquatic frog uses a different tactic, producing a potent poison that can kill a snake attempting to swallow it.

In a recent article in The American Naturalist Ben Phillips and Richards Shine from the University of Sydney show that one species of Australian snake has developed an ingenious trick for dealing with these dangerous prey items. The northern death adder is a highly venomous front-fanged snake native to the same floodplains inhabited by these dangerous frogs. By examining snake feeding behaviour, Phillips and Shine found that death adders not only know that these frog species are dangerous, but they recognize which species they are attacking and deal with them appropriately.

How do the snakes deal with these toxic prey? The answer is simple: by biting and then waiting. The adders simply bite then eat non-toxic frogs, but dangerous frogs are bitten, envenomated and then released. By waiting for the toxic frogs to die, and then waiting for the toxin to degrade, predatory snakes can effectively dodge the toxic frog bullet.

Intriguingly, the snakes recognize which kind of prey they are dealing with: the glue of marbled frogs takes about ten minutes to lose potency and so snakes wait about 12 minutes after biting this frog before eating it. The toxin in Dahl’s aquatic frog takes longer (about 30 minutes) to lose potency. Thus, adders delay swallowing for 30-40 minutes after biting and releasing these frogs.

In evolutionary terms, the snake’s strategy of “bite, release, and wait” is unbeatable by the frogs. Although prey often evolve ways of overcoming predator tactics, the frogs can’t do so in this case – because the snake’s strategy only becomes effective after the frog has died. Natural selection ceases to operate on an individual after that individual’s death, so frogs will probably never evolve toxins that last longer in response to the snake’s tactic. Thus, this waiting strategy is likely to be stable and unbeatable over evolutionary time.

“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. If dinner can kill you, you have to be careful,” said Dr Phillips.