Saturday, May 12, 2007

Litoria dahlii, Dahl's Aquatic Frog

Visitors to Fogg Dam that know a bit about herps generally try to see two animals that are almost icons of the place. One of these is the Water Python, Liasis mackloti. These snakes occur at incredibly high densities in the floodplains of this region, feasting on the staggeringly abundant Dusky Rats (Rattus colletti). The biomass of these two species is said to exceed that of an equal area of the Serengeti plains. But I digress.

The other animal that this area is 'famous' for is the subject of this post, Litoria dahlii, Dahl's Aquatic Frog. Again, this species is one of the most conspicuous animals of the floodplains. On wet nights the Arnhem Highway can be absolutely covered in them. To me, this frog is one of the more interesting in the area, there are some aspects of its ecology that are quite unique.

But firstly, who was Dahl? A bit of googling turned up a Knut Dahl, a native of Norway, who did some exploring and collecting in this part of Australia, including Arnhem land, in the late 19th century. Not sure of his connection to the species' describer, George Boulenger (who seems to have been quite a clever character), though I suppose that it's possible Knut was the collector of the species.

The other part of this species' common name is also quite correct; one of the peculiar things about this species is that it's largely aquatic. Although they do seem to travel considerable distances over land by night, their main place of habitation tends to be in water around the edges of dams and ponds, where they are often spotted clinging to or sitting on top of aquatic vegetation such as lilly pads or algal mats on the surface. Their aquatic nature goes further - they are one of the few species of frog in Australia that are known to feed on underwater items. I've seen them eat things whilst both they and their prey are floating on the surface by lunging forward by means of a thrust of those back legs with their webbed toes.

You might recognise some similarities to other Australian frogs - the 'complex' of frogs that this species belongs to is the Bell Frogs, containing things like Sydney's endangered Green and Golden Bell frog (Litoria aurea) and the Motorbike frog of WA (Litoria moorei). All the frogs in this rough group seem to be rather aquatic, though if I'm not mistaken L. dahlii is the most so. A further trait of the group is some daytime activity, and this is true of L. dahlii too - I've seen them basking in the afternoon sun out at Fogg dam.

Another interesting thing about the species is its diet. The menu includes frogs, either of the same species or another. At places where the species is abundant, the scream of a young frog as it's eaten by a larger individual is a sound that may be heard occasionally. I suspect that small fish are taken as they doze in the shallows at night. Tadpoles are definitely eaten too. There's even an account in the literature of a frog trying to eat a roadkilled death adder! That's one reference I'll have to try to track down!

Apologies for the nearly month-long absence of posts, I'll try to post something else tomorrow!


Evan said...

The reference you are looking for is:

Willams, D. E. (2004). Observation of a hylid frog Litoria dahlii (Boulenger, 1896) feeding on a road-killed Death Adder. 34(1) p 59.

This is a frog that I would really like to see. Good to hear they are common.

A question from a previous thread that you seemed to have missed. Are Litoria microbelos common?



David Nelson said...

G'day Evan,
L. microbelos is pretty common around here. In the wet season, they call from roadside gutters and when driving along one regularly hears a cacophony of calls of the species.


Alunfoto said...

Hey David!

Thanks for the article! Interesting read. I'll see if I can find something on that Knut Dahl figure for you. Love the shot with the damselfly resting on the frog's back.


tony baulch said...

Has there been any more research into this frog eating young cane toads and tadpoles? I would much rather have these native Aussie frogs hopping around Queensland than cane toads. I think it could be a good strategy to introduce them combined with the all male gene cane toads being developed at the Uni. of Queensland.