Originally published in 1966 as an aid for the BSCS Green Version inquiry on amphibian life cycles and conducted by Kenneth W. Perkins, this article has been updated with general amphibian and life cycle background information.
Amphibians are some of the most fascinating vertebrates on Earth because of their unique life cycles. Unlike mammals or birds, amphibians undergo fantastic changes as they grow, often living in both water and on land at different stages of their lives. Frogs, toads, salamanders, newts, and caecilians are all amphibians belonging to the class Amphibia, with frogs and toads specifically being anurans in the order Anura. The word amphibian comes from Greek and means “double life,” which exactly describes how these animals depend on both aquatic and terrestrial environments. Understanding the life cycle of amphibians, such as the life cycle of a frog, helps students learn not only about herpetology and biology, but also about ecosystems, adaptation, and environmental health, all of which correspond to core ideas in Next Generation Science Standards.
All amphibians are cold-blooded, or ectothermic, meaning their body temperature changes with their surroundings. Most amphibians have smooth, moist skin, rather than fur or scales. This skin is very important because many amphibians absorb oxygen and water directly through it. Because their skin dries out easily, they usually live near water or in damp environments.
Another of the important characteristics of amphibians is that they usually begin life in water and later move onto land. This transition happens through a process called metamorphosis, a major physical transformation that changes an animal’s body shape, organs, and way of living.
Although there are some variations among species, most amphibians follow a similar four-stage life cycle: egg, larva, metamorphosis, and adult.
The amphibian life cycle begins with eggs. Most amphibians lay their eggs in freshwater or very moist environments because the eggs do not have hard shells like bird or reptile eggs. Instead, they are soft and jelly-like, containing embryos and a small amount of yolk, which allows oxygen and water to pass through.
Frogs and toads often lay hundreds or even thousands of eggs at once, commonly known as frogspawn, usually grouped together in clusters or long strings. Laying many eggs increases the chances that at least some will survive, since many eggs are eaten by predators such as fish, insects, or birds.
Once the eggs hatch, the young amphibians enter the larval stage, adopting a distinct larval form. In frogs and toads, the larva is called a tadpole. Tadpoles look very different from adult frogs. They usually have long tails, round bodies, and no legs at first. Tadpoles live completely in water and breathe through gills, much like fish.
During this stage, tadpoles mostly eat algae and plant material, making them herbivores. Their bodies are designed for swimming, not for life on land. Salamander larvae are similar—they also have external gills and live in water, although they may look more like tiny adults with tails.
Metamorphosis is the most dramatic stage in an amphibian’s life cycle, involving a complete shift in morphology. During this process, the larva slowly changes into an adult form of the amphibian. In frogs, this includes growing hind limbs or hind legs first, then front legs, as the animal transitions into a froglet. The tail gradually shrinks and is absorbed into the body as the back legs strengthen. At the same time, the gills disappear and lungs develop, allowing the animal to breathe oxygen.
The digestive system also changes. Frogs switch from eating plants as tadpoles to eating insects and other small animals as adults. These changes can take weeks or even months, depending on the species and environmental conditions, such as temperature and food availability.
Once metamorphosis is complete, the amphibian becomes an adult. Adult frogs and toads usually leave the water and live mainly on land, although they often stay near ponds, lakes, or wetlands. Adults breathe with lungs and through their skin, and most are carnivores, feeding on insects, worms, spiders, or small animals.
When adults are ready to reproduce, they return to water to mate and lay eggs, starting the life cycle all over again. Some amphibians repeat this journey many times throughout their lives.
Not all amphibians follow the exact same pattern. Salamanders often have less dramatic metamorphosis than frogs, and some species keep their gills and remain aquatic for their entire lives. Caecilians, belonging to the order Gymnophiona, are worm-like amphibians mostly found underground in tropical regions and have life cycles that scientists are still studying. Some amphibians give birth to live young instead of laying eggs.
Amphibians are important indicators of environmental health. Because they live both in water and on land and absorb substances through their skin, they are very sensitive to pollution, climate change, and habitat loss. A decline in amphibian populations often warns scientists that ecosystems are in trouble and conditions are changing.
Studying amphibian life cycles helps students understand how animals adapt to their environments and why protecting habitats such as wetlands is so important. Amphibians control insect populations, serve as food for other animals, and play a crucial role in balanced ecosystems.
BSCS Green, a popular high school biology text from the 1960s, focused on biology in the living world. Activities supported by the text included numerous extended inquiry opportunities for students to manipulate environmental variables and then assess outcomes. The student project below investigates amphibian metamorphosis. The following directions include instructions for establishing and maintaining a tadpole culture for students to investigate frog metamorphosis and variables that may impact tadpole development.
The BSCS exercise in which amphibian metamorphosis is demonstrated in the classroom requires the use of pre-metamorphic tadpoles. Rana pipiens tadpoles have been difficult to obtain in the proper stage at the time when the exercise is needed in the classroom. Eggs and very young tadpoles are readily available; however, maintaining them in the laboratory for the 10-12 weeks required for the animal to reach the premetamorphic stage is difficult with limited facilities.
During spring, the tadpoles of one or another of the species of spring peeper can be gathered. These tadpoles undergo very rapid development and are ready for metamorphosis within a few weeks after they hatch.
Treatment with iodine or thyroxin will speed up the changes of metamorphosis, just as it does in Rana pipiens. The one difference in the reaction is that in the spring peeper the changes occur more rapidly than in the larger green frog. For this reason, a number of controls should be carried.
One of the techniques successfully used on these tadpoles is as follows. 10 mg of thyroxin is dissolved in 5 cc of 1% sodium hydroxide. This is then diluted to 1 liter with distilled water to make the stock solution of 1/100,000 concentration. This will keep indefinitely if refrigerated. The tadpoles are treated by placing them in solutions having concentrations ranging from 1/100,000,000 to 1/1,000,000.
Feeding tadpoles in the laboratory is often a problem. Lettuce or spinach, softened by boiling, can be used but if either of these is left in the culture trays too long, the resultant bacterial growth may be fatal to the tadpoles. You can achieve good results by feeding them algae, diatoms, and other organisms adhering to pond plants. These food sources will not cloud the water and will provide a considerably varied diet for the tadpoles.
Another satisfactory living food source for the tadpoles is lab-cultured algae and diatoms. The culture dishes can be any glass bowl covered by a glass plate, and the inoculum should be debris from any rich pond. The culture medium consists of 1-2 drops of liquid rose fertilizer per liter of water. Cultures should be kept in a north-facing window.
Amphibian life cycles are a powerful example of change and adaptation in nature and can be used to support the NGSS Disciplinary Core Ideas of Growth and Development of Organisms ((LS1.B), Interdependent Relationships in Ecosystems (LS2.A), Ecosystem Dynamics, Functioning, and Resilience (LS2.C), and Adaptation (LS4.C). Additionally, frog metamorphosis provides a unique opportunity for students to design and carry out independent research. Once a thriving culture is established, students can manipulate biotic and abiotic factors to study variable impacts. Ecosystem dynamics like overpopulation, food resources, and predator-prey relationships can be investigated. Long-term investigations make great capstone projects, science competition projects, and teach students scientific research skills.
From jelly-like eggs to aquatic larvae and finally to young frogs and land-dwelling adults, amphibians transform in ways that few other animals do. These changes allow them to survive in different environments and fill important roles in ecosystems. Learning about amphibian life cycles and experiencing them in the classroom not only builds scientific knowledge but also encourages respect for the natural world and the delicate balance that keeps it thriving.
Originally published in 1966 as an aid for the BSCS Green Version inquiry on amphibian life cycles and conducted by Kenneth W. Perkins, this article has been updated with general amphibian and life cycle background information.
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