Metamorphosis From Tadpole to Frog

Learn about metamorphosis in amphibians and what factors can influence it.

What is Metamorphosis?

Metamorphosis refers to the transformation of a larva to its adult form. This process includes physiological, morphological, biochemical and even behavioural changes1. Many animals undergo metamorphosis, like the butterfly.

Monarch butterfly – by Kenneth Dwain Harrelson
by Kendra Melville

But perhaps the most well-known of metamorphic cycles is that of amphibians.

Metamorphosis in Frogs

Metamorphosis in frogs is one of the most dramatic examples seen the animal kingdom. For most frogs, metamorphosis begins after hatching and they develop from young tadpoles into adult frogs. Take a look at these animations to show how an egg develops into a tadpole, and how a tadpole develops into a frog.

provided by Keith Sillar, based on Nieuwkoop Faber 1956
by Kolafie

The common metamorphic cycle is also summarised below:

Frogs undergo massive physiological and morphological changes during this transformation to prepare them for adult life. All these changes require vast plasticity of their tissues and nervous systems1. [image compare?]

Some existing features are remodelled to optimise their usage for adult life1. Click on the buttons below to find out the different body systems change.

Some features are newly generated. New cells grow and develop in order to develop entirely new organs to support adult life. For example, legs and lungs are newly grown during metamorphosis and replace the tail and gills, which are absorbed back into the body. This process is highly controlled and each stage is carefully timed in sequence.

Watch the video below which summarises the metamorphosis of the frog.

Metamorphosis of Movement

The metamorphosis of the locomotor system is especially complex. A tadpole lives an aquatic lifestyle and so their muscles and neural networks are optimised for tail swimming. Within its spinal cord, a tadpole has a network of neurons that specialise in creating rhythmic and alternating signals that tell the muscles in the trunk and tail to sway left and right, allowing it to swim through water.

by Miika Silfverberg

When the tadpole begins to metamorphose, it must prepare for an amphibious lifestyle. As the tadpole begins to develop legs, a network optimised for limb movement must form. The network becomes more complex as it links to extensor and flexor muscles in the legs, which activate in an alternating and coordinated fashion. This allows the frog to move and kick their hind legs synchronously to swim and jump through both aquatic and terrestrial environments.

by Steve Hodgson

You can read about the metamorphosis of the locomotor system in more detail on a dedicated page, coming soon!

How is Metamorphosis Controlled?

Metamorphosis is controlled by thyroid hormones, produced by the thyroid gland which is present in all vertebrates, including humans. In humans, thyroid hormones help regulate our metabolism and contribute to healthy growth and development.

In frogs, thyroid hormones activate gene expression in cells that allow them to undergo development processes to complete metamorphosis7.

Increasing concentrations of hormones circulate through the body in the blood as metamorphosis progresses. This is carefully controlled in the body to ensure that all adult features are fully developed and functioning before the larval features are degraded1. For example, the growing tadpole must make sure their lungs are fully developed for breathing air before they can begin resorbing their gills, or else they couldn’t breathe!

Young tadpole (Rana temporaria) with gills visible – by Lia Gilmour
Adult Rana sylvatica – by Oh Weh

Without sufficient thyroid hormones or if something is interfering with the body’s response to the hormones, then metamorphosis does not occur. You can read about how thyroid hormones controls metamorphosis in more detail on another page, coming soon!

Different Strategies in Amphibian Metamorphosis

While frogs such as the Wood Frog, Rana sylvatica and the American bullfrog, Lithobates catesbeianus (previously Rana catesbeiana) exhibit the common type of metamorphosis, not all frogs develop this way.

The Puerto Rican coqui tree frog, Eleutherodactylus coqui undergoes direct development, where the egg directly develops into an adult organism, with no intermediary free-living larval stage2. Unlike the common frog, metamorphosis occurs entirely inside the egg and the coqui tree frog hatches as fully formed tiny froglets, often with small tails.

It has been suggested that a combination of environmental pressures and development adaptations encouraged the evolution of direct development2.

by California Department of Fish & Wildlife

Another type of development in amphibians is known as paedomorphosis. This is where juvenile traits such as external gills or the tail are kept by the amphibian even after it develops into a sexually mature adult3. Several amphibians exhibit this type of development like salamanders or newts, but the most famous is in the Mexican axolotl, Ambystoma mexicanum.

Axolotl – by John Clare

Paedomorphosis in the axolotl is a result of neoteny, which describes an extension of the juvenile stage into adulthood4. Axolotls have faulty pituitary glands which do not release the hormones necessary to stimulate the production of thyroid hormones in the thyroid gland5-p.743. This fault decreases the levels of circulating thyroid hormones and so the axolotl does not undergo normal metamorphosis. This leads to the axolotl becoming sexually mature at a juvenile stage. It also retaining juvenile characteristics, such as external gills and a tail fin and lives a fully aquatic lifestyle throughout its life.

Curiously, an axolotl can undergo metamorphosis when it is injected with thyroid hormone in a lab. You can read here to learn more about hormonal control of metamorphosis and the axolotl.

When Metamorphosis Goes Wrong

Metamorphosis is a highly controlled process in the frog, and there are many factors that can influence it.

Varied Temperature

In warmer water, tadpoles metamorphose more quickly, causing adult frogs to be smaller.

In colder water, the spinal cord development in larvae is affected and tadpoles develop a more effective escape response. This is called cold-hardening and gives the tadpoles a swimming advantage in their environment after they hatch6.

Food Availability

Tadpoles not only need a sufficient amount of food to survive, but to undergo normal metamorphosis as well.

When there is lower food availability, tadpoles begin metamorphosis when they are smaller compared to tadpoles with higher food availability7. They also undergo a more rapid metamorphosis. This causes the adult frogs to be smaller.

Tadpole Population

Tadpole metamorphosis can be influenced by the population density within their ponds.

When tadpoles are in a pond with a high population density of other tadpoles, they undergo metamorphosis early and rapidly in order to adapt to the increased competition for food7. This produces smaller adult frogs. Tadpoles are also less likely to fully complete metamorphosis8.

In general when an adult frog is smaller due to rapid metamorphosis, it comes with life-threatening disadvantages. Smaller adult frogs have reduced mobility and have more trouble eating large prey compared to larger frogs. This leaves them more vulnerable to predation and less able to gain the amount of energy they need from food to survive9.

Pollution

Pollution has had an increasingly negative impact on frogs and their development. The widely-used plastic bisphenol A (BPA) is used in many modern plastic products and has leached into a large amount of water sources around the world due to landfill and pollution. Evidence shows that BPA disrupts metamorphosis in frogs by inhibiting normal thyroid hormone function and prevents gene expression in cells that are necessary for development10. BPA not only affects hormone regulation in frogs, but in humans and many other animals as well.

Plastic bottles release BPA into the environment as they degrade – by GreenTumble

There is also more emerging evidence into the negative effects of climate change on metamorphosis. Decreasing water levels in ponds and lakes are causing frogs to undergo faster metamorphosis and leaves them more vulnerable to predation9.

You can read more about the effects of climate change on frogs, coming soon!

Remember to attribute photographs, videos or work where appropriate! This is not needed unless used online, but if you’re unsure please refer to the creative commons licence rules. For academic references and media credits of the images above, please refer to the next page.

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The Digestive System

 

The digestive system is modified to account for the change of the herbivorous diet of the tadpole to the carnivorous diet of the frog.

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The Skin

 

The skin adapts for the change from a purely aquatic lifestyle to an amphibious lifestyle.

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The Reproductive System

 

The urogenital system develops to allow for reproduction in adulthood.

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The Skeleton

 

The skeletal structure develops to accommodate the change from tail swimming to using legs to move around. The skull also needs to be remodelled for a frog's change in vision.

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The Nervous System

 

A tadpole sees from eyes that are positioned on opposite sides of the head. During metamorphosis, the optical nerves develop to accommodate a frog's binocular vision, where the eyes are positioned at the front of the head.

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1. Mating and Laying Spawn

 

Male and female frogs go to ponds in the winter. They mate in the spring, and the female lays big clumps of eggs.

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2. Frog Spawn

 

Frogs eggs are called frogspawn. Each round black egg is about 1 mm wide and is surrounded by a blob of jelly. Other animals produce spawn as well, which you can look at here.

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3. Maturing Frog Spawn

 

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

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4. Hatchlings

 

Then the tadpoles hatch! They are about 5 mm long and they can’t swim (yet). They can bend their body from side to side using special muscles along their trunks and tails.

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5. Young Tadpoles

 

When their tail is big enough, they swim off into the pond to start to feed. At first they have gills (the pale protrusions from the head region in the left photo) so they can breath underwater like fish. Young tadpoles feed by grazing the surface of pond weeds and also eating tiny floating plants called algae.

Click here to play a tadpole feeding game called Taddypole!

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6. Maturing Tadpoles

 

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

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7. Mature Tadpoles

The next stage in development is to grow back legs. Tadpoles during this stage need to eat meat in order to get the proper nutrients to grow.

If you are looking after tadpoles, be careful as they can eat each other if you don't give them meat to eat! Click here to learn more about how to look after tadpoles as pets.

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8. Froglet

Finally, tadpoles grow front legs and their tail shrinks until it almost disappears. This is when they climb out of the pond and start living on dry land. Small frogs are commonly called froglets.

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9. Adult Frog

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

After four years, the new frogs will become adults and will be ready to mate and begin the cycle again.

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Copy - 9. Adult Frog

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

After four years, the new frogs will become adults and will be ready to mate and begin the cycle again.

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Desert Habitats

Desert habitats are the driest habitats in the world. Most people only think of very hot habitats as being deserts, but cold habitats can be deserts as well! Animals and plants that live in deserts have the ability to survive on very little water and animals can control their body temperatures so they stay at the right level.

 Some examples of plants and animals that live in deserts are cacti, the desert tortoise and the artic fox.

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Coral Reef Habitats

Coral reefs are found in warm tropical oceans all around the world. Coral reefs can be found in both shallow and deep water and take hundreds of thousands of years to grow! They provide food and shelter to many fish and other animals, making them habitats that are home to so many different types of life.

Some examples of plants and animals that live in coral reefs are the sea star, sea grass, the octopus and clown fish.

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Rainforest Habitats

Unlike deserts, rainforests are very damp habitats that are filled with lush plant and animal life. Rainforests have an average rainfall of 2,000 to 10,000 millimetres a year! That can be 10 times more rain that falls in the UK! Animals and plants that live in rainforests are used to the wet and humid environment and are able to compete with all the different animals and plants around them.

Some examples of plants and animals that live in rainforests are orchids, the poison dart frog and the hummingbird.

 

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Pollution of Habitats

Pollution is the contamination of habitats with harmful substances. These harmful substances can be anything from plastics, to fertilisers used in fields, to waste products from manufacturing factories.

As humans continue to reproduce and the global population grows, we continue to produce a larger and larger amount of waste and pollution. This affects all air, land and water-based habitats and threatens the health and survival of the plants and animals that live in them, including humans!

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Land Use

As the human population gets larger, we require more land to live on. More houses and schools and everything else we need to live must be built and more resources need to be found. As we expand our towns and cities, this takes away space and resources from other animals and plants.

In order to build more buildings for humans, land must be cleared, which greatly reduces its biodiversity. This means that the range of animals and plants in the land gets smaller, which can harm the food chains in the habitat.

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Deforestation

Deforestation is an example of harmful land use by humans. Trees are an important part of the carbon cycle and they help to clean the air and produce oxygen. With a growing human population, we have started clearing larger and larger areas of trees and forests to build on the land or to obtain wood.

This reduces the biodiversity the the habitats and sometimes destroys habitats completely.

 

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