Thursday, 27 March 2014

Week 45: Common Frog (‘Rana temporaria’)

The Common Frog was @SpeciesofUK from 6th to 12th January, 2013.

The common frog is one of the UK’s native amphibians and is very widespread.

Common Frog
[Flickr Creative Commons ©  Isfugl]

The common frog is a ‘true’ frog.

There are about 4,800 species of frog worldwide in all, accounting for 85% of all amphibians.[1]

Autralian Green Tree Frog

The common frog, Rana temporaria, is a type of ‘true’ frog, the most widely distributed group of frogs in the world. There are over six hundred species of true frog.[2]

Given the wide variety of frog species, it’s not surprising that in other countries there are various other species that attract the name ‘common frog.’ The UK’s common frog can also be known as the ‘European common frog’ or ‘European common brown frog’ to help distinguish it.[3]

The common frog genus is Rana. There are about fifty species in this genus. They are characterised as largish frogs with ridges and wrinkled skin, and are excellent jumpers.[4]

Californian Red-legged Frog 'Rana draytonii'
[Wikimedia Commons © Chris Brown]

The common frog can be found right across Europe.

Common frogs exist as far west as Ireland, as far east as Siberia, south to northern Italy and Spain, and north to Scandinavia.[5]

Common Frog in Italy
[Flickr Creative Commons ©  Nicola Destefano]

In the UK, the common frog is widespread but declining. It's absent from Jersey, Scilly and some Scottish islands. It is present in Ireland although not thought to be native.[6]

You can find common frogs in most habitats, from highland to lowland, forest and field - as long as it’s damp enough.[7]

Common Frog
[Wikimedia Commons © Christian Fischer]

They can come in a wide range of colours.

The colouration of common frogs is highly variable. They can be olive green, brown, grey, yellowish, rufous, even black or albino.[8]

Pinkish Common Frog
[Flickr Creative Commons ©  Martin Olofsson]

Regardless of the colour, common frogs are covered in irregular dark blotches and they usually have a chevron shape at the back of their neck.[9]

Common Frog with Blotches and 'Chevron' Visible
[Wikimedia Commons © OhWeh]

The most consistent markings of all are strong barring on the hind limbs and a dark patch behind the eye.[10] This area behind a frog's eye is the 'tympanum'. It's the covering of the ear.

Common Frog Tympanum
[Flickr Creative Commons © Camponotus Vagus]

The underbelly of the common frog is white or yellow, sometimes more orange in females, and can be speckled with brown or orange.[11]

Female common frogs are usually slightly larger than males.[12] Male common frogs can be distinguished from females due to hard swellings called 'nuptial pads' on their first finger, which are used for gripping females during mating.[13]

During the mating season, the male common frog's throat often turns white and they go lighter-coloured and greyish all over.[14]

Male in Breeding Season showing White Throat
[Flickr Creative Commons © plumberjohn]

But how do you tell a frog from a toad?

The UK’s common frogs and common toads often breed in the same water bodies, so it’s useful to know how to tell them apart.

Common Frog and Common Toad
[Flickr Creative Commons © dhedwards]

Here’s three key differences:

1. The common toad is noticeably bigger than the common frog, 8-13cm long compared to 6-9cm.[15]

2. Common toads tend to walk, whereas common frogs tend to hop.[16]

3. The appearance. Common toads have a rounder snout, wartier skin, and lack the common frog's distinctive dark patch behind the eye.[17]

But don't worry, even frogs and toads sometimes get mixed up themselves!!! 

Common Frog and Common Toad
[Flickr Creative Commons © wolf 359]

Common frogs can walk but they prefer to jump.

The jumping abilities of the common frog are helped by the 'urostyle,' a stiff rod created by the lower segments of the backbone which are fused together.[18]

The common frog's urostyle and pelvic bones provide the firmness and strength to the rear of the body to support the muscles used for jumping.[19]

The National Geographic has a very cool video of frog jumps in slow motion (various species):

The common frog's powerful legs are not only used for jumping but for swimming as well.[20]

Common frogs ‘croak.’

The reason for this call is very simple: to attract a mate. It’s the males that do the croaking, often extremely noisily and in large numbers.[21]

This is the call of the common frog:

Frogs croak by squeezing their lungs with their nostrils and mouth shut. Air flows over their vocal chords and into their vocal sac (on the throat).[22]

Common Frog Croaking
[Flickr Creative Commons ©  Graham Gavaghan]

When common frogs spawn, it can get crowded!

Common frogs usually spawn (breed) in March, but occasionally as early as Jan depending on the weather, straight after waking up from hibernation. They return to water to spawn, in some numbers.[23]

Common Frogs Spawning
[Flickr Creative Commons © Greeeny]

If they can, common frogs often return to their pond of birth to spawn. It's thought they find it by following scents.[24]

There is heavy competition among male common frogs for females. Much wrestling takes place in the breeding pond and a lot of croaking![25]

Common Frog Breeding Pond
[Wikimedia Commons © Piet Spaans]

Male common frogs grab a female and remain clasped to her body for days (even weeks) before spawning takes place. This state is called 'amplexus'.[26]

Amplexus is a common process found in amphibian reproduction which ensures that the eggs and sperm are released in close proximity. Unlike in mammals no penetration occurs.[27]

Common Frog Amplexus
[Flickr Creative Commons © erikpaterson]

Common frog spawn is laid in clumps of eggs containing black embryos with a white spot.[28] Breeding ponds quickly get very full of spawn!

Breeding Pond Full of Frogspawn
[Wikimedia Commons © Piet Spaans]

New common frogs are born as tadpoles.

Common frog spawn hatches into tadpoles after 10-14 days. The tadpoles start to swim a few days later, then metamorphose after 10-15 weeks.[29]

Common Frog Tadpoles
[Flickr Creative Commons © Paul and Jill]

Common frog tadpoles are black at first, but soon become speckled brown making them distinguishable from the permanently black tadpoles of the Common Toad.[30]

The rear legs grow first, then the front legs.

Common Frog Tadpoles with Rear Legs Grown
[Flickr Creative Commons © Mark Stevens]

With Front and Rear Legs
[Flickr Creative Commons © Mark Stevens]
Eventually they become a little froglet!

Common Froglet
[Flickr Creative Commons ©  Rhithrogena22]

Strange but true...

The term ‘ribbit’ for the call of a frog is nothing do to with our own humble species. It originates from Hollywood - the Pacific treefrog is native there and makes that sound, which appeared as a common background sound in movies.[31]


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