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.

GEOGRAPHIC RANGE

Frogs live in North, Central, and South America, in Europe and Asia, in Africa, and in Australia. They do not live in extremely cold areas, such as the Arctic, or on many of the islands out in the ocean. The largest number of frog species is in hot and humid tropical areas, but some make their homes in places that have all four seasons, including a cold winter. Frogs usually stay out of very dry areas, but the water-holding frog and a few others are able to survive in dry grasslands and even deserts. The majority of frogs live in valleys, lowlands, or only partway up the sides of mountains. Some, however, survive quite well high above the ground. The Pakistani toad is perhaps the highest-living frog. It makes its mountain home at 16,971 feet (5,238 meters) above sea level in the Himalayas.

HABITAT

The majority of frogs start their lives in the water as eggs, then hatch into tadpoles, which remain in the water until they turn into froglets. At that point, frogs of some species may leave the water and make their homes on land, while others may stay in the water. Some species are able to survive without ever having to even dip their feet in a puddle. Most of these frogs spend hours everyday underground or in some other moist place.

A number of frog species that live in dry areas, such as grasslands or deserts, stay underground and enter a state of deep sleep, called estivation (es-tih-VAY-shun) for much of the year. There, they wait for the rainy season and then climb back up to the ground to eat and to mate. Other frogs that live in colder places that have a frigid winter find shelter, sometimes also underground, and also enter a state of deep sleep, called hibernation (high-bur-NAY-shun). They remain in hibernation until warmer weather arrives in the spring.

DIET

Most frogs eat mainly plants when they are tadpoles and switch to a diet of mainly insects once they turn into froglets. Some tadpoles also eat little bits of dead animal matter that float down to the bottom of the water, and the tadpoles of a few species will even eat an insect or other invertebrate (in-VER-teh-breht), which is an animal without a backbone. Not all adult frogs will only eat insects. Many of the larger species will gulp down anything they can catch and swallow. Bullfrogs, which are common throughout much of North America, are one type of frog that will almost eat anything that comes within reach, including ducklings and other bullfrogs.

BEHAVIOR AND REPRODUCTION

Like other amphibians, frogs can breathe through their skin, but they can only do so if the skin is moist. Most frogs are active at night, which is when the air is more humid. Humid air helps them keep their skin moist. During the daytime, these frogs sit still in moist places, like under a rotting log, in a muddy place, underground, or in the crack of a rock. Even when frogs are active at night, they spend a good part of the time sitting still. This is how many species hunt. They remain in one place and wait for an insect or other prey animal to wander past, either grasp it with their mouths or flick out their tongues to snatch it, and swallow it whole. Most frogs have sticky tongues that attach in the front of the mouth and flip outward. Some frogs, including the poison frogs, take a more energetic approach to hunting, and hop about looking for their next meals.

Frogs often mate based on the weather. Those that live in warm, humid places may mate any time of year but usually only do so during or after a rainstorm. Frogs that make their homes in colder climates commonly wait until the temperatures warm and the spring rains have come. For species in especially dry areas, the rainy season is the time for mating. The males of almost all frog species call during the mating season. They make the calls by sucking in and letting out air from the vocal sac, which is a piece of balloon-like skin in the throat area. Most frogs, like the spring peeper, have one vocal sac, but some species, including the wood frog, have two. The males of each species have their own calls. The calls not only attract females but sometimes tell other males to stay away and find their own mating places. In a few species, calls may not be enough, and two males may fight. Most fights are little more than wrestling matches, but in some species, like the gladiator frogs, males have sharp spines and often injure one another. In many frog species, the males call together in a group. This type of group calling is called a chorus (KOR-us). In some species, the males all call and mate over a very short time, often within a few days. Frogs that breed over such a short time are called explosive breeders.

To mate in most species, the male scrambles onto the back of a female in a piggyback position called amplexus (am-PLEK-sus) and hangs onto her. As she lays her eggs, he releases a fluid. The fluid contains microscopic cells called sperm that mix with the eggs. This mixing is called fertilization (FUR-tih-lih-ZAY-shun). Once fertilization happens, the eggs begin to develop. The tailed frogs do things a bit differently. The males have "tails," which are actually little bits of flesh they use to add their fluid to the eggs while the eggs are still inside the female's body.

FROGS IN DANGER

In the 1990s, scientists noticed that the number of frogs around the world was dropping. Some species were nearly gone, and others were already extinct. They began trying to figure out why and now believe that many things may be to blame, including air and water pollution, habitat destruction, and infection with a fungus, called chytrid (KIT-rid) fungus. They also believe that introduced species are a danger to frogs. People often add fish to streams or ponds without thinking about what will happen to the frogs that use the water, too. In many cases, fish eat frog eggs, tadpoles, and sometimes adult frogs. Just a few fish in a pond may be enough to gobble up every frog egg and tadpole for the whole season. Since most adults only live and breed for a few years, the fishes can quickly wipe out an entire frog population.

Depending on the species, a frog may lay less than a dozen eggs at a time or more than a thousand. The typical female frog lays her eggs in the water, often in underwater plants, and she and the male leave the eggs alone to develop on their own. In a few species, one of the parents stays behind to watch over the eggs and sometimes stays to cares for the tadpoles, too. The typical frog egg develops in the water into a tadpole. In some species, the egg develops instead in a moist spot, and in a few species that moist spot is inside a pouch or on the back of one of the parents. A number of the frogs that have their young on land lay eggs that skip the tadpole stage and hatch right into baby frogs. In most frogs, however, the eggs hatch into tadpoles that continue growing in the water. Most tadpoles begin to change into froglets within a month or two, but some remain tadpoles for a year or more. The change from a tadpole to a froglet is called metamorphosis (meh-tuh-MOR-foh-sis). In this amazing process, the tadpole's tail becomes shorter and shorter, tiny legs sprout, and the tadpole begins to take on the shape and color of the adults. Soon a tiny froglet, often still with a little bit of the tail left, takes its first hops.

FROGS AND PEOPLE

Many people greatly enjoy the sound of frogs calling on a spring or summer night. In some places, people even gather together to listen to frog choruses. Some people eat frogs, especially frog legs, and occasionally tadpoles. Frogs are also popular as pets. Perhaps more importantly, some frogs have chemicals in their skin that are helping to treat human medical conditions. In addition, scientists are watching frog populations very closely, because frogs can help them tell whether the environment is healthy. A population that suddenly disappears from a pond, for example, may be a warning sign that the water is polluted.

CONSERVATION STATUS

The World Conservation Union (IUCN) lists thirty-two species that are Extinct, which means that they are no longer in existence; one species that is Extinct in the Wild, which means that it is no longer alive except in captivity or through the aid of humans; 367 species that are Critically Endangered and facing an extremely high risk of extinction in the wild; 623 species that are Endangered and facing a very high risk of extinction in the wild; 544 that are Vulnerable and facing a high risk of extinction in the wild; 302 that are Near Threatened and at risk of becoming threatened with extinction in the future; and 1,165 that are Data Deficient, which means that scientists do not have enough information to make a judgment about the threat of extinction. ∎

FOR MORE INFORMATION

Books:

Behler, John. Simon and Schuster's Guide to Reptiles and Amphibians of the World. New York: Simon and Schuster, Inc., 1989, 1997.

Clarke, Barry. Amphibian. New York: Dorling Kindersley, 1993.

Florian, Douglas. Discovering Frogs. New York: Charles Scribner's Sons, 1986.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians (Smithsonian Handbooks). New York: Facts On File, 1991.

Harding, J. H. Amphibians and Reptiles of the Great Lakes Region. Ann Arbor: The University of Michigan Press Institution Press, 1997.

Lamar, William. The World's Most Spectacular Reptiles and Amphibians. Tampa, FL: World Publications, 1997.

Maruska, Edward. Amphibians: Creatures of the Land and Water. New York: Franklin Watts, 1994.

Miller, Sara Swan. Frogs and Toads: The Leggy Leapers. New York: Franklin Watts, 2000.

O'Shea, Mark, and Tim Halliday. Smithsonian Handbooks: Reptiles and Amphibians (Smithsonian Handbooks). New York: Dorling Kindersley Publishing, 2002.

Periodicals:

Hogan, Dan, and Michele Hogan. "Freaky Frogs: Worldwide Something Weird Is Happening to Frogs." National Geographic Explorer (March–April 2004: 10).

Masibay, Kim Y. "Rainforest Frogs: Vanishing Act?" Science World (March 11, 2002): 12.

Sunquist, Fiona. "The Weird World of Frogs." National Geographic World (March 2002): 14.

Walters, Mark Jerome. "Spotting the Smallest Frog: As hopes fade for one species, a tiny frog comes into view." Animals (May–June 1997): 8.

Web sites:

"Anura Species Database." LivingUnderworld.org.http://www.livingunderworld.org/anura/families/ (accessed on May 15, 2005).

Morell, Virginia. "The Fragile World of Frogs." National Geographic.http://www.nationalgeographic.com/ngm/0105/feature6/index.html (accessed on February 12, 2005).

"North American Reporting Center for Amphibian Malformations." National Biological Information Infrastructure.http://frogweb.nbii.gov/narcam/index.html (accessed on May 15, 2005).

Stoddard, Tim. "Island hoppers: Sri Lankan tree frogs end game of hide-and-seek." BU Bridge.http://www.bu.edu/bridge/archive/2002/10-18/frogs.htm (accessed on February 12, 2005).

Trivedi, Bijal P. "Frog Fathers Provide Transport, Piggyback Style." National Geographic Today.http://news.nationalgeographic.com/news/2002/08/0807_020807_TVfrogs.html (accessed on February 12, 2005).