Frogs and toads are often distinguished by appearance—frogs generally have smooth, moist skin, while toads have drier, warty skin. Despite these differences and varying habitat preferences, these two groups of amphibians share a common evolutionary blueprint. This shared biological design dictates their life cycle, physiology, and overall body structure, reflected in their scientific classification and adult traits.
Shared Taxonomic Order
Both frogs and toads belong to the biological order Anura, a Greek term meaning “tail-less.” This classification establishes a fundamental shared characteristic: the adult form of all species within this order lacks a tail. Anura is one of the three living orders within the class Amphibia, placing them alongside salamanders and caecilians.
The Anura order accounts for approximately 88% of all existing amphibian species. This taxonomic grouping confirms that the differences commonly used to distinguish frogs and toads are informal and superficial, often relating to skin texture and leg length.
Universal Life Cycle
The universal life cycle involves a dramatic transformation known as metamorphosis. This process begins when the adult female lays anamniotic eggs, often called frogspawn, in water or a moist environment. These eggs lack a protective shell and are surrounded by a jelly-like substance for moisture retention.
The eggs hatch into a larval stage known as a tadpole, which is adapted to aquatic existence. The tadpole is morphologically distinct from the adult, possessing a long tail for swimming and gills for underwater respiration. Tadpoles have a primarily herbivorous diet of algae and plant matter. The length of the tadpole stage varies widely, from a few weeks to multiple years.
The transition to adulthood involves rapid hormonal restructuring regulated by thyroxin. During metamorphosis, hind legs develop first, followed by forelegs. The gills are replaced by lungs, and the tail is gradually absorbed. This complex process transforms the aquatic, gill-breathing larva into a terrestrial or semi-aquatic, lung-breathing adult. The necessity of water or a consistently moist environment for the larval stage is a requirement for all frogs and toads.
Common Adult Physiological Traits
The adult body form of all anurans is a compact, short-bodied structure with specialized physiological adaptations. Their skeletal design is optimized for locomotion, featuring a short vertebral column with fewer than ten vertebrae and a fused tailbone, known as the urostyle, which adds rigidity to the pelvis. The bones of the lower forelimbs and lower hind limbs are fused into single, strong bones, which helps absorb the shock of landing after a jump or hop.
Adult frogs and toads employ a unique bimodal respiratory system, using both lungs and their highly permeable skin. The lungs are simple, sac-like organs, and air is forced into them using buccal pumping, as they lack a diaphragm. A significant portion of their gas exchange, particularly the release of carbon dioxide, occurs through the skin in a process called cutaneous respiration. This reliance on moist, glandular skin for oxygen and moisture absorption makes all anurans susceptible to dehydration.
Both frogs and toads are ectotherms, meaning they are unable to generate sufficient internal heat to regulate their body temperature and must rely on external sources. They manage their thermal state behaviorally, such as basking in the sun to warm up or retreating to water or burrowing to cool down. Their metabolic rate is consequently low compared to warm-blooded animals.
The adult diet is universally carnivorous, consisting primarily of insects, spiders, and other small invertebrates. They are opportunistic predators that typically capture prey using a specialized, sticky tongue attached to the front of the mouth. Larger species may consume small vertebrates, but their feeding mechanism is largely uniform, involving seizing live prey and swallowing it whole.