MESOAMERICAN BURROWING TOADS: Rhinophrynidae

PHYSICAL CHARACTERISTICS

With its round and flat body and tiny, pointy-snouted head, the Mesoamerican burrowing toad is an odd-looking creature. It is so unusual, in fact, that some people might not even guess it is a type of frog until they see its hind legs and webbed feet sticking barely out from the saggy skin of its back. The skin droops along the sides of the body and up by the front legs, too. Its legs are short and pudgy, but very strong. The back legs are longer than the front legs, but the front legs are quite thick, as if the frog were a bodybuilder. The edges of the feet also have large, shovel-like bumps that help the frog dig. The toes of its front feet are much shorter than its very long back toes. It typically sits with its hind legs bent up against the body, so they almost disappear. In all, it looks like a dark, round blob with four feet poking out the sides. The head is little more than a cone-shaped lump on the front of the frog. It is small and has no obvious neck supporting it. The reason that the head looks so small is that the frog's shoulders are much closer to the head than they are in other types of frogs. The shoulder blades even wrap around the back of the head, which makes the frog look as if it is always shrugging up its shoulders.

The frog has two dark, small, round, beady eyes, two even smaller nostrils sitting just in front of the eyes on the top of the short snout, and thick lips. Most of the skin on this frog is smooth, but the skin on the snout is tough because it is covered with hard bumps, called spicules (SPIK-yuhlz), that can only be seen with a microscope. The spicules are rounded on the bottom of the snout, but pointy on top. In many other frog species, two eardrums are easily seen on the sides of the head, but the tiny head peeking out from the shoulders in the Mesoamerican burrowing toad leaves no room for any eardrums to show.

Its tongue is different, too. The Mesoamerican burrowing toad does not have a long tongue to flip far out of its mouth as many other frogs do. Instead, it has a sticky, triangular-shaped tongue that it can stick straight out just a little way. Its mouth is toothless.

The frog also has an unusual pattern on its thick back skin that helps to tell it apart from other species. Its back is dark grayish brown, sometimes nearly black, with a single, thin, stripe of yellow, orange, or reddish orange running down the middle. This stripe may be broken here and there, but it is still obvious. It also has yellow and/or orange spots or blotches on either side of the stripe that continue down its sides. Its legs may have a few spots, but the limbs are mostly just grayish brown, and the webbing is typically a lighter gray or bluish color. Its head is also a little lighter in color than the back and may be light gray to light brown. Its underside is dark brown, gray, or bluish gray and has none of the speckles, spots, or stripes of the back.

The name "toad" in this species can be confusing, because it is not actually a toad at all. The only true toads are in the family Bufonidae. They have warty skin and short legs. The Mesoamerican burrowing toad has smooth skin and quite large hind legs. Even though its common name includes the word "toad," scientists consider it to be a frog and not a true toad. This is a small- to medium-sized frog, and the adults grow to 1.8 to 2.6 inches (4.5 to 6.5 centimeters) long from the tip of the snout to the end of the rump.

A HOLE IN ONE

Tadpoles breathe through their gills. Water rushes in; blood in the gills removes the oxygen from the water, and the water flows out through a tiny hole on the tadpole's side. This tiny hole is called a spiracle (SPIH-reh-kul). Tadpoles of the Mesoamerican burrowing toad, clawed frogs, and Surinam toads have two spiracles instead of one, and they are located on the bottom of the tadpole instead. Partly because they have the extra spiracle and because of the location of the two holes, scientists think that these three groups of frogs are very closely related.

The tadpole of the Mesoamerican burrowing toad also looks a bit different from the average tadpole. It has the typical head and long tail, but the head is wider and flatter than other tadpoles, and it has unusual, tiny, fleshy "whiskers" poking from the front of the mouth. These bits of flesh are called barbels (BAR-bulls). Unlike most other frog species, these tadpoles do not have horny beaks on their mouths. Other tadpoles use these beaks to scrape algae (AL-jee), or tiny plantlike organisms that live in water, off rocks or plants for their meals. The Mexican burrowing toad tadpole eats instead by sifting out little bits of floating algae from the water. The only other tadpoles that look like the burrowing toad tadpoles are those of the Pipidae family, which include the clawed frogs and Surinam toads that live mostly in South America and Africa. Like the Mesoamerican burrowing toad, the clawed frogs and Surinam toads also have a very unusual look that includes an oddly flat head. Scientists believe that these two families of frogs are very closely related because the tadpoles are so much alike and because of some details in their skeletons. Paleontologists (PAY-li-un-TA-luh-jists), or scientists who study fossils, at the Carnegie Museum of Natural History are also now studying a new fossil discovered at Dinosaur National Monument, which is located in Colorado and Utah. This fossil may be a newly discovered relative of the two families.

GEOGRAPHIC RANGE

This family has only one living species, which may be called a Mesoamerican burrowing toad, Mexican burrowing toad, or simply, a burrowing toad. It is found in North and Central America. In North America, it lives in the southernmost part of Texas and in Mexico. It also is found in the Central American countries of Honduras, Guatemala, Belize, El Salvador, Nicaragua, and Costa Rica. At one time, other burrowing toads lived on Earth as far north as Canada. Two now-extinct species lived in what is now Wyoming about 40 to 50 million years ago, and another lived in Saskatchewan, which is in western Canada, about 32 million years ago. Scientists learned about these three species from fossils collected from the two places.

HABITAT

The Mesoamerican burrowing toad mainly lives in underground burrows in lowland areas, which are nearer the Gulf of Mexico and Pacific Ocean and away from the hilly, mountainous regions farther inland. Some live in grassland areas, but others do well in forests that become very dry for part of the year. They can exist in tropical areas or in somewhat cooler subtropical grasslands and forests.

DIET

Although scientists know of no one who has actually seen them eating, they think that the Mesoamerican burrowing toad eats ants and termites that it finds in its underground burrows. They learned this by capturing some of these frogs and looking at what was in their stomachs. This is a somewhat common way for scientists to learn about the diet of animals that live most of their lives out of human sight. Based on the frog's short tongue, scientists believe that this frog digs through the soil until it pops just the tip of its snout into a termite or ant tunnel. From there, it can sit still to wait for one of the insects to crawl by and then quickly stick out its short, gummy tongue to pick up the insects one at a time and gobble them down. This type of sit-and-wait hunting is called ambush (AM-bush) hunting. Tadpoles, which live in pools of water, feed by sucking in water and picking out little bits of algae. This is called filter feeding.

BEHAVIOR AND REPRODUCTION

The frog uses its strong legs to dig backwards into the ground. As its legs work, the frog twists while blowing up its body with air and then letting the air out. When it is deep enough into the soil, the dirt falls in on top of the frog and eventually covers up the tunneling frog completely. It stays underground most of the time, coming up to the surface during the rainy season, which is when it mates. It may leave its burrows and move above ground at night to look for food, but its large body and short legs make it rather clumsy on land.

The breeding season begins when the heavy rains come. Sometimes, the males make a few calls from their burrows, but they do most of their calling once they find little pools of water. These can be puddles in a farm field, water-filled ditches along a road, or any other small watering hole that forms when the rains come. Males do not breed in ponds that have water in them all year long. From its pool of water, the male begins calling. In many species of frogs, one or two vocal sacs inflate when they call, and each sac looks like a balloon that blows up around its chin. The burrowing toad's sac stays on the inside of its body, so when it fills up with air, the frog's entire body blows up. Every time the male makes his loud mooing call, the air rushes out of his body and pushes the frog backward in the water. Since the male may make its short "ooo" calls 15 to 20 times a minute, the frog continues to scoot around the water backward.

A female comes to a breeding pool, which may contain many males, and picks out a mate by bumping her snout into his chest and throat. He responds by climbing onto her back and grabs onto her with his forelegs just in front of her back legs. She lays a few eggs at a time, sometimes only one, and may mate with different males on several different days, especially if the area gets more than one drenching rain. By the end of the breeding season, each female may have produced thousands of eggs. After she lays the eggs, they sink to the bottom of the water, where they hatch into tadpoles a few days later. The tadpoles group closely together, forming living balls of tadpoles. The larger tadpole balls can reach 3.3 feet (1 meter) in diameter. The tadpoles change into froglets in one to three months. If the water is warmer, they make the change, called metamorphosis (MEH-tuh-MORE-feh-sis), sooner. In cooler water, metamorphosis occurs later.

MESOAMERICAN BURROWING TOADS AND PEOPLE

If enough frogs are calling at once, the noise sounds something like running machinery, and people sometimes hear this so-called chorus (KOR-us) of breeding calls from up to a mile away. Outside of the breeding season, however, people rarely see this frog. It is not popular in the pet trade. It did, however, draw at least a little attention in 2004 when a part-time stunt-man rode his unicycle in Death Valley, California, to raise awareness of the Mesoamerican burrowing toad.

CONSERVATION STATUS

This species is not considered to be at risk. It is quite common in parts of Mexico and Central America, but it is rare in the United States and only lives in a few scattered areas in Texas. For this reason, Texas Parks and Wildlife lists it as threatened.

FOR MORE INFORMATION

Books:

Lee, Julian C. The Amphibians and Reptiles of the Yucatán Peninsula. Ithaca, NY: Comstock Publishing Associates, Cornell University Press, 1996.

Meyer, John R., and Carol F. Foster. A Guide to the Frogs and Toads of Belize. Malabar, FL: Krieger, 1996.

Web sites:

Beaudry, B. 1999. "Rhinophrynus dorsalis." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/information/Rhinophrynus_dorsalis.html (accessed on February 10, 2005).

Cannatella, David. "Rhinophrynidae." Texas Memorial Museum, University of Texas.http://www.zo.utexas.edu/research/salientia/rhinophrynidae/rhinophrynidae.html (accessed on February 10, 2005).

"Family Rhinophrynidae (Burrowing Toad)." Animal Diversity Web.http://animaldiversity.ummz.umich.edu/site/accounts/information/Rhinophrynus_dorsalis.html (accessed on February 10, 2005).

"Mexican Burrowing Toad." Amphibian Conservation Alliance. http://www.frogs.org/amphibianet/species.asp?Genus=Rhinophrynus&Species=dorsalis (accessed on February 10, 2005).

"Mexican Burrowing Toad." eNature, National Wildlife Federation. http://www.enature.com/fieldguide/showSpeciesFT.asp?fotogID=1098&curPageNum=4&recnum=AR0712 (accessed on February 10, 2005).