Frogs and Toads - Physical Characteristics
Like mammals, birds, bony fishes, reptiles, and other amphibians, frogs are vertebrates (VER-teh-brehts). A vertebrate is an animal with a spine, or backbone. Compared with all the other vertebrates, frogs are the only ones that have this combination of features:
- A wide head and large mouth
- Two big, bulging eyes
- A short body with only eight or nine bones in the spine
- Two extra bones in the ankle area that make their long legs even longer
- A long, rod-shaped bone, called a urostyle (YUR-oh-stile) in the hip area
- No tail
Most of the frogs are about 1.5 to 3.0 inches (3.5 to 7.5 centimeters) long from the tip of the snout to the end of the rump. Some are much smaller. The smallest species are the Brazilian two-toed toadlet and the Cuban Iberian rain frog, which only grow to about 0.4 inches (1 centimeter) long. These compare with the unusually large Goliath frog, which can grow to 12.6 inches (32 centimeters) long and weigh 7 pounds (3.25 kilograms).
Depending on the species, the skin on a frog may be smooth, somewhat bumpy, or covered with warts. Although many people think that all warty frogs can be called toads, only those in one family of frogs are true toads. The members of this family typically have chubby bodies, rather short hind legs, and many warts. What sets them apart from other frogs—even those that are also chubby, warty, and short-legged—is something called a Bidder's organ. A Bidder's organ is a female body part that is found inside a male toad. This tiny organ does not appear to do anything in a healthy male toad, but it does help scientists tell a true toad from all other kinds of frogs.
A great number of frogs are green, brown, gray, and other colors that look much like the background in the place they live. They also have spots, stripes, and other patterns that help them blend into their surroundings. Many of the poison frogs, among others, are not camouflaged. They have bright colors that make them very noticeable.
Most species of frogs lay eggs that hatch into tadpoles. Tadpoles are sometimes described as a sack of guts with a mouth at one end and a tail at the other. Often, the mouth on a tadpole is hard like the beak of a bird and is able to scrape bits of plants off the sides of underwater rocks. Some tadpoles instead have a fleshy mouth. These tadpoles suck in water and strain little bits of food out of it. Including their tails, tadpoles are often as long as or longer than the adult frogs. As the tadpoles change into young frogs, however, the tail slowly becomes shorter and shorter until it is gone.