SPADEFOOT TOADS: Pelobatidae

PHYSICAL CHARACTERISTICS

The spadefoot toads are named for the small scoops, or spades, on the bottoms of their hind feet. They use their spades, which are made of the same material as fingernails, to move away the dirt as they burrow into the soil. The spadefoot toad is actually not a true toad. All true toads are grouped into the family called Bufonidae. Spadefoot toads do look a bit like true toads, because they have round, plump bodies, but they do not have very warty skin. Their skin is actually quite smooth and moist, although tiny lumps are sometimes noticeable on their backs. These small lumps may be tipped in red. In addition, spadefoot toads also have teeth on the upper jaw. True toads do not.

Many of the species in this family have brown to gray backs, sometimes with faint stripes or spots, and light-colored bellies. Their eyes have vertical, catlike pupils. Some, like the Great Basin spadefoot, have a large lump between the eyes. Depending on the species, adults may grow to 2 to 3.2 inches (5.1 to 8.1 centimeters) from the tip of the snout to the end of the rump.

Their eggs are tiny and typically dark brown. The tadpoles are usually tan to brown in color. Some have orange speckles and see-through tails.

GEOGRAPHIC RANGE

Four species live from Europe and western Asia to northwestern Africa. The remaining seven are North American species, found from southern Mexico through the United States and to southern Canada.

HABITAT

Spadefoot toads are burrowing frogs that live in areas with loose, often sandy soil and usually dry weather. Some, like the Plains spadefoot toad, can live in almost desert-like conditions. They come above ground, usually at night following a heavy rain or when the air is humid, to find food. Spring rains also bring the frogs onto land for mating. Those that live in the driest of places, however, may stay underground for all but two weeks of the year.

DIET

Unlike most other frog species, the tadpoles of spadefoot toads are not just vegetarians. They will suck in water and sift out bits of plants, as some other species of tadpoles do, but they will search the water to catch and eat insects and other invertebrates (in-VER-teh-brehts), which are animals without backbones. Once they become frogs, they switch to an all-meat diet and eat snails, spiders, earthworms, and various insects, including beetles, grasshoppers, and caterpillars.

BEHAVIOR AND REPRODUCTION

The spadefoot toads are burrowing frogs that spend their days and many of their nights underground where the ground is moist. They use the spades on their feet to dig rump-first into the ground. They shovel with one foot at a time and wiggle their bodies backwards into the burrow. During the rainy season, their burrows are only a couple of inches (5 centimeters) deep, but during long dry times, they may burrow down 3 feet (1 meter) or more. On rainy nights, or nights that are humid, they will come above ground to look for food. They need the moisture in the air because they can die if their skin dries out. When the soil becomes dry even deep below the surface, these frogs may snuggle inside layers of their own dead skin to keep themselves at least slightly moist and away from the dry soil, which might soak up what little moisture they have. They can survive inside these cocoons of dead skin for many weeks.

Because they stay underground much of the year, the spadefoot toads can avoid many of their predators. Even when they are above ground, the browns and grays of their skin can help to hide them from hungry eyes, especially if they stay perfectly still. If a predator does spot them, the frogs can defend themselves by sucking air into their lungs and blowing up their bodies to make them look bigger than they are. Their larger bodies might be enough to frighten away certain predators. Some species, like the eastern spadefoot toad, have skin that gives off bad-tasting and often smelly ooze that might discourage a predator. Despite all of these defense tactics, these frogs sometimes become lunch for their predators, including birds, such as owls and crows; mammals, like coyotes; and snakes.

Dangers aside, the spadefoot toads leave the protection of their underground burrows to mate on land. The sound of rain drumming on the ground overhead brings out hundreds of male spadefoot toads, which hop to puddles or shallow ponds and begin calling while floating in the water. Depending on the species, the calls may sound like crows cawing, sheep baaing, or a finger squeaking against a balloon. The calls can become quite loud, and people have reported hearing them from a mile (1.6 kilometers) away. In the water pools, the males set up territories and keep their distance from one another. The males call most at night, but may also call sometimes during the day. The females arrive and select mates. To mate, a male climbs onto a female's back and clings to her with his forelegs wrapped just in front of her hind legs. The eggs, which can number several hundred to more than a thousand per female, stream from her body and stick in clumps to underwater plants, stones, and other items. The eggs usually hatch into tadpoles within a week. If the weather is especially warm, they may hatch in just one to three days.

NO RELATIVE OF MINE

For many years, scientists had thought the Asian toadfrogs and spadefoot toads were so similar that they should both be placed in the same group, called a family. Closer studies revealed that the two were much more different than originally thought, and in 1985 scientists separated the toadfrogs into their own family.

Dry weather is always a threat to a tadpole, which must turn into a froglet before its watering hole evaporates. Some species, like the Plains spadefoot toad, can change into a froglet in just a few days. Usually, however, tadpoles need about one month to become froglets. Sometimes, females lay so many eggs in a small pool of water that the growing tadpoles run out of space and food. At these times, some of the tadpoles begin to eat each other, although they can recognize their littermates and do not gulp them down, too.

SPADEFOOT TOADS AND PEOPLE

People may hear these frogs occasionally during their mating season, but they rarely see the animals in the wild. Spadefoot toads are not especially popular in the pet trade, although some people do keep them in their homes. Nonetheless, at least one species has received attention. In 2003, the state government of New Mexico named the New Mexico Spadefoot Toad, a red and brown speckled species, as its official state amphibian.

CONSERVATION STATUS

The World Conservation Union (IUCN) considers one species to be Endangered, which means that it faces a very high risk of extinction in the wild. The U.S. Fish and Wildlife Services does not list any species as being at risk. The IUCN Endangered species is Varaldi's spadefoot toad, which lives in a few small areas in Morocco in northwestern Africa. Like many other spadefoot toads, it lives mainly underground in sandy soils, but comes on land to mate in puddles and pools of water. As humans farm in areas where the frog lives, the soil is beginning to pack down, which makes the frog's digging more difficult, and new pollutants are entering the watering holes. Both are likely hurting the frogs. In addition, some frogs are able to breed in larger ponds that are filled with water all year, but these ponds often are also home to fishes that eat the frogs.

Although no other species is considered to be at risk, some populations, including Couch's spadefoot toad, appear to be disappearing due to pollution and/or habitat destruction.

PLAINS SPADEFOOT TOAD (Spea bombifrons): SPECIES ACCOUNT

Physical characteristics: Plump and round-bodied, the Plains spadefoot toad has wrinkled skin. Its head has a short, rounded, and slightly upturned snout and two very large eyes with catlike, vertical pupils. Between its eyes and running down to the top of its snout is a blisterlike hump, also known as a boss. Its short forelegs end in feet with small toes that have no webbing between. It has webbed feet on its large back legs, and the bottom of each foot has a single, small, black scoop, or spade, below the toes. Its back is light tan, milk chocolate-colored, greenish brown, or gray, sometimes with four light-colored stripes running from head to rump, or with numerous faint, darker brown spots. The toads occasionally have red or yellow sandlike bumps on their backs, which may sit in small dark spots. The underside is white, although the males sometimes have a noticeable blue or gray tint on the sides of the throat. Adults grow to about 1.5 to 2.5 inches (3.8 to 6.4 centimeters) from the tip of the snout to the end of the rump.

Geographic range: The Plains spadefoot toad is a North American species and lives from northern Mexico into the southern tip of Texas and in a wide area from northern Mexico through many central U.S. states and into southern Canada.

Habitat: It makes its home in the dry prairies and farm fields that are common in central North America.

Diet: Night hunters, adults eat insects and other invertebrates.

Behavior and reproduction: The Plains spadefoot toad is a fossorial (faw-SOR-ee-ul) animal, which means that it lives most of its life underground. The small spades on its feet help it dig hind-end-first into the loose, often sandy soil of its habitat. It leaves its burrows at night after a rain or when the air is humid to look for food. It also comes out of its underground burrow to mate. When the spring rains drench the land, hundreds of these frogs will all hop from their burrows at once to mate. Because so many frogs mate together over a short time, scientists call them explosive breeders. The males find small puddles and shallow ponds and begin making their squeaky calls to attract females. While mating, each female lays hundreds of eggs, which stick to underwater plants, rocks, and other objects. Within two days, the eggs hatch into tadpoles, and these change into froglets in as little as two weeks. This quick egg-to-tadpole-to-froglet growth is important, because they live in a habitat where puddles and ponds can dry up in a very short time. A tadpole cannot survive without water. Males and females may mate again later in the year if another heavy rain soaks the ground.

Plains spadefoot toads and people: People only notice this toad when a group of males is calling together.

Conservation status: This species is not considered to be threatened. In some areas, however, its habitat is disappearing at an alarming rate. A few states and provinces have now begun taking measures to protect it. ∎

FOR MORE INFORMATION

Books:

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians. New York: Facts on File, 1991.

Stebbins, Robert C. A Field Guide to Western Reptiles and Amphibians. Boston: Houghton Mifflin, 1985.

Tyning, Thomas. A Guide to Amphibians and Reptiles. Boston: Little, Brown and Company, 1990.

Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. 2nd ed. San Diego: Academic Press, 2001.

Periodicals:

Fellman, Bruce. "To eat or not to eat." National Wildlife. February-March 1995 (volume 33): page 42.

Freeman, Melanie. "The Spadefoot Toad." Boy's Quest. February-March 2002 (volume 7): page 2.

Web sites:

"Eastern Spadefoot Toad." Connecticut Department of Environmental Protection.http://dep.state.ct.us/burnatr/wildlife/factshts/esptoad.htm (accessed on February 14, 2005).

"Eastern Spadefoot Toad." Savannah River Ecology Laboratory, University of Georgia.http://www.uga.edu/srelherp/anurans/scahol.htm (accessed on February 14, 2005).

"Frogs and Toads Found in Texas." Texas Parks and Wildlife. http://www.tpwd.state.tx.us/nature/education/tracker/amphibians/species/ (accessed on February 14, 2005).

"Great Basin Spadefoot Toad." B.C. Frogwatch Program, Ministry of Water, Land, and Air Protection, Government of British Columbia.http://wlapwww.gov.bc.ca/wld/frogwatch/whoswho/factshts/spadeft.htm (accessed on February 14, 2005).

"Official State Amphibian: New Mexico Spadefoot Toad." Netstate.com.http://www.netstate.com/states/symb/amphibians/nm_spadefoot_toad.htm (accessed on February 14, 2005).

"Plains Spadefoot, Spea bombifrons." Northern Prairie Wildlife Research Center, U.S. Geological Survey.http://www.npwrc.usgs.gov/narcam/idguide/speab.htm (accessed on February 14, 2005).

"Plains Spadefoot Toad." Lee Richardson Zoo. http://www.garden-city.org/zoo/animalinfo/plains_spadefoot.htm (accessed on February 14, 2005).