Tuesday, May 22, 2007

New camera - new photos!

New toy arrived today, and after the infuriating initial battery charge, I managed to get out in the last few minutes of sunlight to have a quick play. Nothing special. But I'm a very happy chappy!

The new camera is the Pentax K10D by the way. First impression is that I'll have a lot of fun with it!

Monday, May 21, 2007

Bizzare File Snakes and the Runoff

We all know about the seasons in the wet-dry tropics, right? There's the Wet, and there's the Dry. True, but there's also Buildup (aka suicide season - I've not experienced it but it's apparently hot, humid, yet infuriatingly it doesn't rain), and when the rain has finished, Runoff. It's called that for an obvious reason - there's so much water around from the amount of rain that has fallen, and it's all got to go somewhere... creeks, rivers, dams etcetera all continue to flow for quite some time following the last of the rains.

As the water recedes, many aquatic organisms risk becoming trapped, and desperately try to travel upstream to permanent water. At a nearby creek, just such an event has been occurring recently. In addition to the millions of fish, shrimp and crayfish, the most exciting inhabitants of the waterway are the entirely aquatic file snakes occurring there in amazing numbers, trying to cross the culvert under the road where the water flows swiftly.

File snakes are a very small family - the Acrochordidae, comprising three species. The species around here is Acrochordus arafurae, the Arafura file snake. The family gets its name from the characteristic rasp like skin - though to aid in capturing the slippery fish that these snakes prey upon. From this skin, which is loose and baggy, to its bull-dog face; its thin bifurcated tongue to the way it can hardly move out of the water (the classic analogy compares this snake with a wet sock) - this is a snake that is simply unlike any other I've ever come across.

They're almost completely harmless. Non-venomous and they don't bite. I say almost, though. Apparently, these snakes may eat catfish, and the venomous spines of the latter may pierce the snake's skin and pose a danger when handling them! I've heard that they adopt a 'fishing pose' - the tail is anchored in some tree or pandanus roots while the body and head lolls in the current, hoping to detect a fish as it brushes by seeking shelter. The low metabolism of the species means it doesn't need to surface very frequently to breathe.

Trivia: they're supposedly quite a tasty dish in indigenous culture!

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!