The expression that frogs have “two lives” reflects their biological classification as amphibians. The word “amphibian” originates from the ancient Greek words amphi (“of both kinds”) and bios (“life”), literally translating to “double life.” This duality is a fundamental survival strategy, allowing the animal to exploit both aquatic and terrestrial environments across its lifespan. The change from a water-bound larval stage to a land-dwelling adult defines this entire class of vertebrates.
The First Life Aquatic Development
The initial phase of the frog’s existence is spent entirely in water as a larva, commonly called a tadpole. These newly hatched creatures are built for an aquatic environment, possessing external or internal gills that efficiently filter dissolved oxygen from the surrounding water. Their primary means of movement is a long, laterally compressed tail, which acts as a propeller for swimming.
Tadpoles are typically herbivorous, grazing on algae and other vegetation using specialized, rasping mouthparts. This diet and their dependence on gills for respiration mean they are completely tied to their watery habitat for survival and nourishment.
The Transition Metamorphosis
The shift from the aquatic tadpole to the terrestrial frog is one of the most remarkable transformations in the animal kingdom, known as metamorphosis. This complex process is orchestrated primarily by an increase in the thyroid hormone, thyroxine, which acts as the master switch for the morphological changes. Thyroxine regulates the activation and repression of specific genes in different tissues, ensuring the orderly development of adult features while simultaneously dissolving larval structures.
Physical Changes
One of the most visible changes is the reabsorption of the tail, a process driven by programmed cell death, or apoptosis. Simultaneously, the four limbs develop from small buds, providing the future adult with a means of terrestrial locomotion. Internally, the respiratory system undergoes a radical change as the gills degenerate and lungs begin to develop for air breathing. The digestive system also shortens and restructures to accommodate the adult’s carnivorous diet.
The Second Life Terrestrial Adulthood
The fully transformed adult frog is equipped with a new set of physical characteristics that allow it to thrive outside of water. The newly developed, powerful hind limbs enable the frog to move by jumping and walking on land, a significant departure from the tadpole’s swimming propulsion. With the completion of metamorphosis, the lungs become the main respiratory organs for gas exchange, allowing the frog to utilize atmospheric oxygen.
Adult frogs are insectivores, relying on their sticky, projectile tongues to capture small invertebrates on land. They also possess a highly permeable, moist skin that facilitates a secondary form of gas exchange called cutaneous respiration. This adaptation allows them to absorb oxygen directly through the skin, especially when submerged. Despite their terrestrial lifestyle, adult frogs must return to a moist environment to prevent dehydration and to lay their eggs, thus restarting the cycle of their “double life.”