SHOVEL-NOSED FROGS: Hemisotidae

PHYSICAL CHARACTERISTICS

The species of shovel-nosed frogs have wide, rounded, and rather flattened bodies with thick, strong front and rear legs. The very short front toes have no webs between them. The hind toes, which are much longer than the front toes, have a little webbing in some species, but no webbing in others. The spotted snout-burrower, for instance, has no webbing between its back toes. The head of all species is short and blends back into the body without a noticeable neck. The frogs have rather small eyes with vertical pupils and a pointed snout with a hard, sharp end. This makes the snout look a bit like the edge of a shovel blade. The tongue in these frogs has a notch in the tip. They also have a groove or fold that runs across the top of the head from behind one eye to behind the other.

A close look at the heels of the rear feet reveals a large, flat bump, or tubercle (TOO-ber-kul). The tubercle, which is hard and rough like the callous a person might get on his or her hand, is located on the inside of each heel.

Beneath the skin, the shovel-nosed frogs have a thick skeleton, which gives them a very solid body. The bones in much of the frogs' front and back feet, not including the toe bones, are fused together for added strength. The shovel-nosed frogs do not, however, have a breastbone, also known as the sternum.

Many frogs in this family are brown or purple with yellow markings. The spotted snout-burrower, for example, is dark purple or brown with small yellow spots. The marbled snoutburrower is a bit different. It may have a brown or a dark green head and back. The back is covered with numerous dark brown to black blotches. Its head often has noticeable dark stripes extending from the snout past the eye and to the back of the head. Its sides and legs have yellow- to cream-colored speckles.

Many of the shovel-nosed frogs are quite small and only reach 1 inch (2.5 centimeters) long from snout to rump. The spotted snout-burrower is an exception. This species, the biggest member of the family, grows to 3 inches (8 centimeters) long.

GEOGRAPHIC RANGE

Shovel-nosed frogs live in central and southern Africa, a region known as Sub-Saharan Africa. In particular, the frogs make their homes in a stretch of land from the west coast of central Africa across to Ethiopia in the east, down the east coast, and back across to the west side of the continent around Angola. Some, like the marbled snout-burrower and Guinea snout-burrower, live over a very large area, including many countries in Africa. Others live in very small areas. In 2002, a new species was found in western Zambia, which is in southern Africa, but nowhere else. The Ethiopian snout-burrower, which is also known as the Lake Zwai snout-burrower, is only known to live in parts of Ethiopia.

HABITAT

Shovel-nosed frogs live underground in dry, grassy areas. During the wet season, however, the rains fill the land with small, deep pools of water, and the frogs come out to feed and to mate. They lay their eggs underground, but the tadpoles move into these pools of water or in other small ponds that remain filled with water all year and develop there. Shovel-nosed frogs may live in lowland areas or in places as high as 5,900 feet (1,800 meters) above sea level.

DIET

These frogs search for food during the rainy season. At night, they may look about on land for insects. They are also good diggers and put this talent to use when finding a meal. They tunnel along just a few inches below the surface and seek out termites and earthworms there.

BEHAVIOR AND REPRODUCTION

The strong body, muscular legs, heavy skeleton, and shovel-like snout together help these frogs to be excellent diggers. They all dig head first into the muddy banks near water pools or small ponds, moving the head up and down to take advantage of the "shovel nose" to push away soil. Even the tubercles on the heels of their hind feet give them an added push when they are forcing their front ends into the ground. Most other burrowing frogs use their rear legs as the main digging limbs and dig themselves backward into the soil. During the dry season of the year, the frogs stay in their burrows and rest. This resting period during dry weather is called estivation (es-tih-VAY-shun). When the rains come, the frogs come out to look for food or to mate.

When the mating season starts, the males call to attract females. They call from the ground in a hiding spot under plants next to a pool of water or small pond. The call is a long buzzing sound. The call of a male marbled snout-burrower, for instance, is a buzz that lasts about two seconds. When a female pairs up with a male, he crawls onto her back and holds on as she starts digging head first into the ground to make a burrow under a log or stone. With the male still on her back, she continues tunneling. When the pair are completely buried, she lays a clump of one hundred fifty to two hundred eggs, although some females only lay about three dozen eggs. Each egg is about 0.1 inches (2 to 2.5 millimeters) in diameter and has a capsule around it.

A PINCH OF THIS, A DASH OF THAT

Depending on which features a scientist considers, the shovel-nosed frogs may look a bit like frogs in other families. As a result, researchers have had a hard time figuring out where the shovel-nosed frogs belong. Some think they should really be listed as part of another family. These families include the true frogs in the family Ranidae, which share several characteristics with the shovel-nosed frogs including a notched tongue tip; the rain frogs, which are members of the family Microhylidae and are burrowers like the shovel-nosed frogs are; and the African treefrogs of the family Hyperoliidae, which have vertical pupils in their eyes like those in the shovel-nosed frogs. Other scientists, however, consider the shovel-nosed frogs to be unusual enough to be listed in their own family, as they are in this volume.

After mating, the male digs back out of the ground and leaves, but the female stays behind with her eggs. As the eggs develop, rains continue to fall, eventually filling the pools and ponds. Water overflows and soon rises to cover and soak into the underground chamber where the female is staying with her eggs. By this time, usually less than two weeks later, the eggs begin to hatch into tadpoles. The female may dig a tunnel out of the burrow. The tadpoles use the tunnel to swim in the flooding water, out of the chamber, and into the pools and ponds. If the tadpoles hatch before the nest chamber is flooded, or in a year when the rains are not hard enough to overflow the pool or pond and flood the nest, the tadpoles in some species scramble onto the female's back, and she carries them out of the nest and to the water.

SHOVEL-NOSED FROGS AND PEOPLE

People rarely see this frog, which remains underground much of the year.

CONSERVATION STATUS

Of the nine or ten species in this family, the World Conservation Union (IUCN) considers one to be Vulnerable and facing a high risk of extinction in the wild; and four to be Data Deficient, which means that too little information is available to make a judgment about the threat of extinction. The Vulnerable species is the spotted snout-burrower, which is found in South Africa and probably in Swaziland, although scientists have not yet discovered it there. Members of this species live in groups, or populations, in several small areas that are separated from one another. The future of the frogs is threatened by the clearing of trees and plants in the frog's habitat. People clear the land to make way for sugar cane farms and housing developments. Fortunately, some of the frogs live in several protected areas, including parks, where land cannot be cleared. People are also, however, introducing fishes to the pools of water and small ponds that the frogs use for breeding. Often, these fishes eat the frogs and/or their tadpoles.

The four species listed as Data Deficient include the Masiliwa snout-burrower, Perret's snout-burrower, De Witte's snoutburrower, and a species known only by its scientific name of Hemisus barotseensis. Scientists have not done thorough searches for the Masiliwa snout-burrower or for De Witte's snoutburrower for many years and know little about either species. Perret's snout-burrower, which is found in Congo and Gabon, is rarely seen. Since it usually stays underground, however, it may be more common than it appears. Scientists also know little about Hemisus barotseensis, which was just discovered in 2002.

MARBLED SNOUT-BURROWER (Hemisus sudanensis): SPECIES ACCOUNT

Physical characteristics: The marbled snout-burrower also goes by the common names of marbled shovel-nosed frog, mottled shovelnosed frog, pig-nosed frog, and mottled burrowing frog. The marbled snout-burrower is typically brown with darker brown markings on its back and head and often a light-colored stripe down the middle of the back. Its back toes have a little webbing, but the front toes have none. Its front legs are thick and strong. Females sometimes grow to as much as 2.2 inches (5.5 centimeters) long from snout to rump. Some scientists consider this frog not to be a separate species, but instead to be a subspecies of another species, known as Hemisus marmatorus. Sometimes species are split into one or more subspecies. This means that the frogs are still members of the same species, but are slightly different. Perhaps they live in separate places or have slightly unusual looks or behaviors.

Geographic range: It lives in much of central and southern Africa.

Habitat: This is a burrowing frog that spends most of the year underground in dry areas, often with few if any trees.

Diet: Marbled snout-burrowers eat a variety of insects above and below the ground.

Behavior and reproduction: Much of the time, marbled snoutburrowers search for and eat various insects that they either find along the ground or in the underground tunnels that they dig. They are especially fond of termites, particularly when the termites develop wings, which they do during part of their life cycle, and leave their termite hills. The frogs wait near the exits to the hills and grab the termites as they fly out.

The frogs mate and have their young next to pools of water or small ponds that remain filled with water all year. Males call and attract females. When a female approaches a male, he grabs hold of her, and she begins digging head first into the soft mud near but outside the pool or pond. When she has dug out a burrow—with the male still clinging to her—she lays her eggs inside the underground nest chamber. The male leaves, but the female stays with her eggs as they hatch into tadpoles underground. When rains come, the pool or pond overflows and soaks the burrow. The tadpoles then swim out. If too little rain falls and the burrow does not flood, scientists think that the tadpoles probably squirm onto the female's back and she carries them out of the burrow and into the nearby pool or pond. In that water body, the tadpoles develop into froglets.

Marbled snout-burrowers and people: People rarely see this mainly underground frog.

Conservation status: The World Conservation Union (IUCN) lists only nine species of shovel-nosed frogs and does not consider this species to be separate from Hemisus marmatorus. According to the IUCN, Hemisus marmatorus is not at particular risk. It lives over a large part of Africa, including protected areas, and is probably quite common. ∎

FOR MORE INFORMATION

Books:

Burnie, David, and Don E. Wilson. Animal. New York: DK Publishing Inc., 2001.

Channing, A. Amphibians of Central and Southern Africa. Ithaca, NY: Cornell University Press, 2001.

Showler, Dave. Frogs and Toads: A Golden Guide. New York: St. Martin's Press, 2004.

Web sites:

Cannatella, David. "Hemisus." Tree of Life Web Project. http://tolweb.org/tree?group=Hemisus&contgroup=Neobatrachia (accessed on February 21, 2005).

"Hemisus marmoratum." Amphibans (Mamoru Kawamura).http://www.rieo.net/amph/exfrog/aka/hemisus/marmo.htm (accessed on February 21, 2005).

"Hemisus marmoratum." California Academy of Sciences.http://www.calacademy.org/research/herpetology/frogs/list7.html (accessed on February 21, 2005).