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Tadpoles and using them to understand brains

Why so many eggs?

Learn why there are so many tadpoles in your pond...



During spring, male and female frogs arrive at ponds to mate. In some places you will see a pond full of frogs, like in the picture below!

Frogs_in_our_pond_Trevor Rickard

If you are there at the right time you will hear the males trying to attract females with their croaking calls.

Listen to the croaking sounds of males luring in females:

This photo shows how frogs mate. Notice the female is a different colour to the male.


Frog spawn

Once courtship has taken place, the female  lays huge numbers of eggs (up to 4000) and the male fertilises them as they are laid.

European_Common_Frog_Spawning_(Rana_temporaria)_Thomas Brown

These eggs are called frog spawn and are laid in clumps, with a protective jelly layer surrounding the fragile developing tadpole. The jelly also helps the frogs pawn to float on the surface of the water.

Look closely at this picture and you’ll notice its completely full of frog spawn!


Once the eggs are laid and fertilised, both the parents leave the pond. The tadpoles develop without help from their parents. Laying lots of eggs is therefore a good idea when there is no help to survive from the adult frogs. Other animals like birds have only a few babies at a time and spend time and energy looking after them.

Who eats tadpoles?

It may seem strange to put so much energy into laying thousands of eggs. But frogs have good reason for doing this- pretty much everything wants to eat a tadpole! Just like this kingfisher…

Kingfisher_eating_a_tadpole_Pierre Dalous_creativecommons

Many animals will also eat frogspawn or indeed baby froglets. So at each stage of a frogs life danger is around every corner. Insect nymphs (like the damselfly nymph below), newts and fish will all gobble up a tadpole if they get the chance!


Animals that eat tadpoles and frogs are predators. The tadpoles or frogs are therefore called the prey.

Predators of tadpoles include:

  • birds
  • newts
  • dragonfly larvae
  • fish
  • great diving beetle and its larvae

Click here to find out more about tadpole predators. 

Ducks have even been reported to gobble up frogspawn! Foxes and badgers will also eat little froglets as they leave the pond.

Check out this massive diving beetle larvae munching on a tadpole!

Disticus larvae

Lots of eggs

If everyone wants to eat your babies and you are not there to protect them, its a good idea to lay lots of eggs! Many tadpoles, means it is more likely to have some will survive.  

It is thought that only 1 in 50 eggs makes it to become a froglet without being eaten. That’s not good odds!

At each stage in the picture below, many tadpoles are eaten. So  not many surviving froglets leave the pond.


Care from parents

When a mother or father looks after its offspring for some or most of its life, this is called parental care. The parents are caring for the offspring or babies. This means feeding them, sheltering them and protecting them from harm or predators, like this blue tit with its chicks.


Some animals do not look after their offspring.

Plants cannot physically look after their seeds, so produce lots. They do not care for or protect these offspring. Less of these offspring will therefore survive. Producing lots of eggs means that at least some of these offspring will survive, like this oak tree with lots of acorns, or a butterfly laying lots of eggs like these on a leaf.


Other animals give a lot of care and protection.

For example humans, some other mammals and some birds  have only a few offspring. This means parents can focus their effort into making sure those offspring survive.



Media credits under creative commons licence: Frogs in pond-Trevor Rickard; Frogs croaking- Benboncan@Freesound.org; Frogs mating-Piet Spaans; Frogs mating and spawn- Thomas Brown; Pond full of spawn-Piet Spaans; Kingfisher- Pierre Dalous; Diving beetle larvae eating tadpole-Gilles-San-Martin; fox cubs- Su May; duck with ducklings- Kim Taylor; Acorns- Everly Simack; butterfly eggs-Волков Владислав Петрович; blue tit with chick- David Howes.

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