Anura
Frogs and toads are tailless amphibians. Today, about 3,500 species of frogs have evolved from their ancestors which appeared 180 million years ago. Frogs can be found on most islands and on every continent except Antarctica. Their habitats are diverse, from desert and savannah to mountains and tropical rain forests. The greatest concentration of anurians is found in the tropics.
The term “frog” refers to the tailless amphibian that is smooth skinned, semi-aquatic, and web-footed, with long and powerful legs used for jumping. The term “toad” refers to the species that is terrestrial and lives in dark, damp places away from water. They have stout bodies with skin often covered with wart like bumps that secrete poisonous fluids. The hind legs are not as long nor as powerful as the frogs, and they move in a series of short hops. The term frog may be applied to either frogs or toads.
Frogs range in size from ½ inch to 12 inches in length. They differ in color from the familiar green or brown to some extremely colorful species found primarily in the tropics. Most frogs have the same body type. The neckless body is short and compact with a round head and bulging eyes. The front legs have four short digits which help the animal sit and aid in breaking the fall when it leaps. The hind legs, with five digits, are the strongest and most developed to aid in locomotion. Most aquatic species have webbed hind feet. Tree dwellers may have areas of sticky webbing between digits on all 4 feet that enable them to cling to a surface. Most frogs have a sticky tongue that can quickly dart out to capture small prey. Adults eat insects, earthworms, minnows, spiders and often smaller frogs.
The internal organs of frogs often differ from other animals. To begin with, they have a three chambered heart. Adult frogs, in addition to breathing with their lungs, can also breathe through their moist skin. The outer layer of skin is shed often, and is usually consumed. Most frogs have fairly good eyesight, and their bulging eyes allow them to see in all directions. Located just behind the eyes are rather large discs that are the eardrums. Frogs have a larynx and in many species, the males have an expandable vocal sac. The male has the strongest voice and make “advertisement calls” used to attract females and sometimes threaten other males in the area.
Frogs are ectothermic, but can produce some internal body hear by metabolism. Body temperatures range from 37 to 97 degrees, but in a state of torpor, most can survive for long periods at temperatures between 32 and 48 degrees. During torpor, frogs might burrow underground, or bury themselves in mud at the bottom of a pond or stream. Basking, which aids in regulating body temperature, also causes water loss and is restricted to frogs living near water. Frogs not only breathe through their skin, but can absorb water through their skin when submerged. When on land, frogs have no physiological control over water evaporation.
The majority of frogs breed in water, and their breeding habits seem closely controlled by the weather. The male generally enters the water and emits advertisement calls to females. During the act of mating, the males fertilize the eggs as they leave the female's body. Tadpoles hatch from the eggs within 2 to 25 days depending on species and temperature of the water. Some species lay thousands of eggs at one time, but many of these eggs are eaten by ducks, fish or insects. Nests vary greatly based on environment and include using rainwater collected in leaves, attaching to leaves hanging above water and floating foam nests. Select species are laying fewer, but larger eggs on land and tending to them until they hatch. Often, these young complete their development with one parent in special brood pouches. Frogs that breed on land often bypass the tadpole stage and hatch as fully formed miniature frogs. Most tadpoles eat plants or scavenge decaying animal matter, but some are true carnivores. The metamorphosis of tadpoles to frogs can be completed within a week or take up to 3 years. Clutch size varies from 2 to 25,000 eggs. The fewest number represents eggs laid on land, while the largest number are laid in water.