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Tadpoles and using them to understand brains

Life Cycles

From frog's spawn to tadpoles to frogs


Life-cycle of the common frog

Below is the life-cycle of the common frog. The Latin name for this frog is Rana temporaria. The life-cycle of the American bullfrog (Latin name: Lithobates catesbeiana) is very similar. Click here for a picture of the American bullfrog.


Detailed life story of the common frog

Mum and dad frogs go to ponds in the winter. This picture shows how they mate in spring and the mum lays big clumps of eggs.


Frogs eggs are called frogs spawn. Each round black egg 1 mm across sits in a blob of jelly. Click here to see toad spawn.


After a few days the eggs grow into tiny tadpoles inside the jelly.


Then the tadpoles hatch. They are about 5 mm long and can’t swim. They can bend their body from side to side.

Hatched tadpole flexes body after hatching 23 Feb 2016

When the tail is big enough, they swim off into the pond to start to feed. At first they have gills so can breath under water like fish. Young tadpoles feed by scraping the surface of pond weeds and also eating tiny floating plants called algaeClick here to play a tadpole feeding game called Taddypole!Rana-tadpoles-SRSaf


Later they develop lungs and swim up to the surface to breath. The gills are absorbed and eyes develop. Older tadpoles are then able to feed on small animals like young insects.


The next stage in development is to grow back legs. Tadpoles like the one below need to eat meat at this stage, so if you are looking after them, be careful as they will eat each other if you do not give them meat! Click here to learn how to look after tadpoles as pets.


Finally, tadpoles grow front legs and the tail shrinks and nearly disappears. This is when they move to the side of the pond and climb out onto dry land. A small frog like the one below is called a froglet.


The little froglets will stay by the pond for some time and slowly grow over the summer, eating small insects and worms. They will hibernate just like other adult frogs in damp places near ponds from autumn until the next spring. 

The new frogs will become adults ready to mate after four years and will begin the cycle again. Below is an adult female common frog.OLYMPUS DIGITAL CAMERA

Activities and resources

pencil22small_foka.tkLife-cycle diary

Click here or on the link below to print a life-cycle diary to fill out as your tadpoles grow:


pencil22small_foka.tkFrog colour-in

Click here or on the link below to print a frog colour-in activity:


Life-cycle links

Watch this amazing video by Nature North of Wood Frogs developing into froglets in just 7 weeks.

Watch the Singing Zoologist perform his frog life-cycle song “Metamorphosis”. Or click here to visit the website for more info.

Click below to watch an animation of a frog life-cycle in an Australian wetland from Life for Beginners. Or click here to find out more or download the animation.

Media credits: frogs mating- Piet Spaans; Two tadpoles Markus@Thinkaholic; woodfrog with back legs-NatureNorth; froglet-Carols Cornwall.

Tadpole swims when touched at *

The details of swimming movements which hatchling Xenopus tadpoles make in response to touch with a fine hair  have been studied by making high speed videos at 200 fps. In these examples touch on the left (*) leads to a bend to the right followed by swimming. Waves of bending travel from the head to tail (at ~ 14 cm per second) and increase in amplitude as they travel along the body. They move the tadpole in the direction shown by the arrows. Swimming speeds at ~ 20 oC range from 4 to 6 cm per second.hatchling tadpole swims when touched at *

Kahn J.A., Roberts A. & Kashin S. (1982) The neuromuscular basis of swimming movements in embryos of the amphibian Xenopus laevis. J. exp. Biol.  99, 175‑184. http://jeb.biologists.org/cgi/reprint/99/1/175

Adult South Africal Clawed toad Xenopus laevis


         Xenopus laevis

What Tadpoles Look Like

Tadpoles can start swimming spontaneously or when they are stimulated but it is just as important that they can stop. This normally happens when their head and cement gland bumps into the surface of the water or some other solid tadpoles swimmingobject like a plant or the side of a dish. This kind of stimulus and the tension in the mucus strand when the tadpole is hanging attached have an inhibitory effect on the tadpole. While hanging, it never moves spontaneously and is much less responsive to stimulation. This ability to keep still may make it more difficult for predators to detect and eat tadpoles. 

Types of Neurons

There are different types of neurons in a nervous system and they are named depending on their function.


Interneuron from Xenopus laevis tadpole

Broadly there are 3 main types:

  1. Sensory neurons
  2. Motor neurons
  3. Interneurons 





Flexion behaviour of hatchling tadpole in response to skin stimulation (represented by arrow).

When the skin of Xenopus laevis hatchling is touched, sensory neurons are activated, passing on exitation to sensory pathway neurons (interneurons) in the spinal cord, which in turn excite motor neurons, causing flexion behaviour

For more info on research into flexion behaviour click here. 

Synaptic transmission (blank diagram)

pencil22small_foka.tkPrint and fill in the blank diagram with the key steps in the process of synaptic transmission:



Resting potential and action potential confusion!



The terms resting potential and action potential can be confusing, as they seem to suggest that one is an active process and the other not.


Action potentials are actually produced by a passive process- sodium ions diffusing into the axon, causing depolarisation. 

Resting potentials are generated by an active process, which needs ATP. The sodium-potassium pump carries out active transport of ions in and out of the axon to generate a potential difference across the cell and a voltage of -60/70 mV inside the axon.

So even though the axon is said to be at “rest”, an active process involving energy in the form of ATP is actually going on. And even though the action potential sounds like it needs energy, it is actually a passive process.

Make sure you are clear on this!

Axon, membrane or axon membrane?!



Some exam boards prefer you to mention simply the “axon”, others just the “membrane”, or the “membrane of the axon”. 


When we are talking about a difference in charge over an area, we always refer to what area that is- for example a potential difference over the axon membrane. 

Some exam boards will prefer you say that the “membrane” or “axon membrane is depolarised”…but others will be happy with just the “axon is depolarised”

Check what is preferred by your exam board and incorporate into your notes links below:





Exam Board Links

pencil22small_foka.tkClick on the links below to access the specifications for listed exam boards:







Toad spawn

Toad eggs are the same size as Frog eggs but are laid in a string, often among weeds in the pond. The string can be more than 1 meter long and contain a double row of eggs. Here is a photo of a small piece from a pond in Hampshire.

A small piece of toad spawn from a pond in Hampshire.

American Bullfrog

Below is an American Bullfrog. The Latin name of this frog is Lithobates catesbeianus. It is sometimes also called Rana catesbeianus. 

American_Bullfrog_(Rana_catesbeiana)_-_Algonquin_Provincial_Park_Ontario_By Ryan Hodnett Own work

Click here for more information on the American Bullfrog from the ARKive website.

Media credits: American Bullfrog-Ryan Hodnett