Frogs

Leseur’s Frogs (Litoria lesueuri) emerge after dark from their diurnal concealment amongst leaf-litter on the dark-brown forest floor, where they elude the predatory appetites of a formidable avian oversight.

Like all members of the genus, Leseur’s have large finger and toe pads and horizontal pupils, however, males out-number females, perhaps as many as fifty-to-one. Adult Females, such as the one pictured, are three to four times the mass of males and much less gregarious and stream-bound (I get the impression that the greater mass of the female provides more liberty from running water).

It has been a tough year for wildlife sightings generally with the longest, coldest patch I have known since starting my nocturnal forays, fifteen years ago. The last couple of nights have been good for the primitive northern leaf-tailed and chameleon geckos, as well as the spectacular moth Lyssa Macleayi.

In the centre of the Daintree rainforest, Giant White-lipped Tree Frogs Litoria infrafrenata ordinarily call for mates at the beginning of September, but much cooler temperatures persisting until very recently, have delayed the unmistakable clatter of competing males.

They are world’s largest tree frog and have been known to grow to 139mm. Their colour is variable, from beige-brown during the day to bright-green at night. They are conspicuous with a white stripe around the lower jaw continuing through to the shoulder, as well as white stripes (salmon pink in breeding males) on the trailing edges of the lower legs.

In an attempt to mimic the sustained adhesiveness that tree-frogs’ toe-pads provide, a group of researchers in India have created a form of sticky coating that is both strong and reusable. More details can be found in this Scientific American article.

Confined to tropical Northern Queensland rainforests, between Cooktown and Townsville, Orange-thighed Tree Frogs (Litoria xanthomera) live in the upper-canopy and descend to the ground only to breed. Emerging only after heavy rains, these beautiful tree frogs can be conveniently observed only four or five nights of the year.

Courting rituals are very noisy and may be heard for hundreds of metres through dense rainforest. Only males vocalize and it would seem to be the smaller males that out-compete for the affections of the females through their higher frequency, more energy-efficient and sustained serenading.

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.

Northern Barred frogs (Mixophyes schevilli) from Cooper Creek Wilderness

According to Brendon O’Keefe of the Australian, two new north Queensland frog species have been discovered on or near the mountaintops of the Carbine Tableland. They have been identified as Barred Frogs; Mixophyes carbinensis and coggeri.

Conservation biologist Michael Mahony of the University of Newcastle, expressed concern that the frog(s) faced two associated threats in the form of global warming and also the frog-killing chytrid fungus, which would flourish in increased temperatures.

However, Nomination of Wet Tropical Rainforests of North-east Australia by the Government of Australia for inclusion in the World Heritage List, argued the Australian frog family, Myobatrachidae is believed to have had Gondwanan origins (Duellman & Trueb, 1986; White 1984), with primitive species within these families found in the Wet Tropics bioregion in the genera Mixophyes.

So, have the two identified species distinguished themselves from ancestral stock through recent speciation or have they persevered undetected to science from their Gondwanan origins. Surely the distinction would have implications for their survival prospects through climate variation.