Friday, February 1, 2008

The Common Frog Rana temporaria

I am an amateur naturalist trying to find out what lives in my garden.

Have you spotted it yet?

Photo 1 (click to enlarge) taken in late summer, shows, squatting camouflaged against the undergrowth, my blog's first amphibian, (at (0.8,0.4) - see here).

Identification wasn't difficult. There are only two native British toads: the Common and the Natterjack. These have warty skin and an altogether more robust stature than my amphibian. There are also only two frogs: The Common and the Pool Frog. The latter is rare and in fact has only recently been recognised as an indigenous species. There are also a handful of introduced alien species of frog and toad in Britain, but again none is like mine. In all therefore, I've little heitation in indentifying my frog as The Common Frog (Rana temporaria).

To learn something about frogs I have been reading The British Amphibians and Reptiles (M. Smith, New Naturalist). Published in the 1950's, the book has a more 'robust' (a.k.a. cruel!) approach to studying nature than most amateurs would find acceptable today. I learn for example, that toads dropped into formalin will (surprise, surprise!) adopt a defensive posture; that Goldfinches injected with toad venom, die; and that a convenient way to mark frogs for identification is to cut one of their fingers off! Despite this, the book contains a lot of fascinating and surprising information: For example I hadn't previously known that, lacking a rib cage, frogs are unable to inflate their lungs as mammels do. Instead they absorb oxygen through their skin and by rhythmically 'gulping' air, which they can do up to 140 times a minute. Croaking is achieved by passing air backwards and forwards over the vocal chords. Since this does not require air to be expelled from the body, frogs can, and commonly do, croak underwater.

The skin colouring of the Common Frog is highly variable. Black patches (see photo 2) are due to accumulations of melanin. More melanin is produced in colder weather and during hibernation (typically mid October to early March) Common frogs may turn quite a dark brown. As they grow, frogs periodically moult a thin outer layer of their skin.

Male Common Frogs can be distinguished from females by the presence of thickened black nuptial pads on their hands. These help the male to grip onto the female's back during mating. As the female releases her eggs, the male fertilises them by releasing sperm into the water. Common Frogs can spawn suprisingly early in the year. March and April are most common, but the book describes records of spawning as early as mid-January and says that it is not uncommon to find spawn floating amongst the ice on freezing ponds.

Frogs have a 'homing instinct' and can find their way back to breeding pools from some distance. When the book above was written (the 1950's), how they achieved this wasn't known. Can anyone comment on whether this problem has been solved since?


Laura said...

Don't know about the homing instinct, sorry. I have, however, recently wondered and worried about sad-looking toads trying to find their way back to long gone ponds and what they do exactly when they can't make it. This bugs me.

In my partnership, it is me with the slightly rough and blackened hands. Must be all the slaving I do with the housework...

Henry Walloon said... a good scientist, I'd need to hear a second opinion from the pixie husband before accepting the veracity of this!