Metamorphosis in the frog

The life cycle of the frog involves an astonishingtransformation known as metamorphosis as it grows from an egg to an adult amphibian. Our beautiful native frog is now rarer and rarer. Thirty years ago every child living near a pond would scoop up frogspawn and setting the jar on a light windowsill watch the amazing process, eventually releasing the tiny frogs into long grass.

2023.03.Frog in frogspawn Salimfadhley crop Frog in frogsawn.  Photo: Thanks to Salim Fadhley, Creative Commons

The appearance of frogspawn was a classic sign of the coming of Spring.  In my biology lab at school tadpoles would take up a place in the beautiful glass aquarium and we would release them transformed into frogs by the great pond at the back of the building. Some rural Norfolk primary schools had a little pool in the classroom, scooped into a central table where the whole class watched the tadpole alchemy. The Irish poet Seamus Heaney writes:

“I would fill jampotfuls of the jellied

Specks to range on windowsills at home,

On shelves at school, and wait and watch until

The fattening dots burst into nimble-

Swimming tadpoles”.

Metamorphosis is an ancient biological process present in many creatures: amphibians such as newts, toads and frogs, fish, prawns, lobsters, jellyfish and many insects. The classic insect shape-shifter is the caterpillar, hatched from an egg. It eats voraciously, pupates and emerges as a beautiful butterfly.  Metamorphosis is a word taken from the Greek and means to change or transform in shape. Many cultures have built myths and stories around this ability to transform, sometimes by witchcraft or magic or the power of the gods.  In his poem 'Metamorphoses' the Roman poet Ovid, tells the story of Apollo, god of the sun and his relentless pursuit of the nymph Daphne. She, in desperation to remain alone in wild places, begs her father, a river god, to intervene. He transforms her into a laurel tree. Her graceful limbs grow numb, her breasts are enclosed in bark, her beautiful hair becomes leaves, her arms turn into branches and her feet are rooted. Apollo can feel her beating heart through the bark and frustrated and outwitted weaves a laurel wreath for his head.

In frogs metamorphosis is dramatic although gradual and each step from larva to tiny adult requires major physiological re-organization.This biological process is triggered by thyroid hormones.  In Britain breeding in most frog species starts soon after they emerge from hibernation and is centred on ponds and lakes. The male frogs croak by inflating their vocal sacs. They grasp a receptive female in an embrace known as inamplexus, their hold strengthened by thick nuptial pads that develop on their thumbs.  After a day or more the female releases her eggs and the male frog releases sperm to fertilise them. This external form of fertilization can only happen in water.

For about two or three weeks the frogspawn seems to remain inert.  Then the astonishing frog metamorphosis begins, as nourished by their stored yolk, the tadpoles develop from the eggs, struggling free from the jelly of the spawn mass. They will now transform changing from one shape to another until they become perfect, tiny frogs. Unlike insect pupae metamorphosing tadpoles remain active trying to avoid predators and seeking a nurturing environment.  The tadpoles have a strong mobile tail, no legs and a set of gills through which to breathe, extracting oxygen from the water.  They swim in groups using their minute but efficient teeth to nibble at filaments of algae and other plant material.  The tadpole tail shrinks and lungs and hind legs form. Then forelegs appear under the gill sac. Their diet begins to include flies and snails as they adapt from an aquatic to an eventual terrestrial environment.  The tadpole head becomes more distinct, the mouth changes to the wider jaws of a frog with a long sticky tongue. Its tail disappears and finally the gills stop working and the lungs take over. About twelve weeks after hatching metamorphosis is complete and a tiny, perfect frog hops from the water.

The process of metamorphosis is an intricate biochemical dance, a balance of hormones, temperature, humidity and food.



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Seamus Heaney “Death of a Naturalist”, 1966, Faber and Faber, London.


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