Mesoamerican burrowing toads

(Rhinophrynidae)

Class Amphibia

Order Anura

Family Rhinophrynidae

Thumbnail description
Moderate-sized burrowing frog with rotund body; triangular head with truncate snout and tiny eyes; exceptionally short, powerful limbs; and loose, pustulose skin

Size
1.8–2.6 in (45–65 mm)

Number of genera, species
1 genus; 1 species

Habitat
Seasonally dry forests and savannas in lowland tropics and subtropics

Conservation status
Not threatened

Distribution
Extreme southern Texas and lowlands of Mexico and Central America to Guatemala on the Atlantic slope and from Guerrero, Mexico, to Costa Rica on the Pacific slope

Evolution and systematics

Rhinophrynus dorsalis (the burrowing toad, Mexican burrowing toad, or Mesoamerican burrowing toad) is the only living representative of the anuran family Rhinophrynidae. Despite its common name, the Mesoamerican burrowing toad is not a toad. It is a curious, almost absurd-looking frog that is related most closely to another bizarre group of frogs—the pipids, of which the flat-headed Pipa pipa (Surinam toad) and Xenopus laevis (African clawed frog) are the most familiar representatives in laboratories and the pet trade. No subfamilies are recognized.

Externally, the Mesoamerican burrowing toad resembles several other burrowing frogs (e.g., the microhylid Breviceps, the hemisotid Hemisus, and the myobatrachid Myobatrachus), but several skeletal features of the adults and characteristics of the larvae indicate that the Mesoamerican burrowing toad is allied with pipid anurans (e.g., the living Xenopus, Silurana, Pseudhymenochirus, Hymenochirus, and Pipa and many fossil taxa). The tadpoles of both pipids and the Mesoamerican burrowing toad have broad, flat heads with wide, slitlike mouths that lack keratinous mouthparts and bear marginal barbels; there is a pair of spiracles (instead of only one) located on the underside of the tadpole body rather than on its side, as in most other anurans.

As a group, pipoid frogs (i.e., Rhinophrynidae and Pipidae) have a rather extraordinary fossil record, in terms of both numbers of fossil representatives and their ages. The Mesoamerican burrowing toad is no exception. It is known from the Upper Pleistocene of Mexico in deposits less than one million years old. A related, extinct species, Rhinophrynus canadensis, was described from the Lower Oligocene (ca. 32 million years ago) of Saskatchewan, Canada. Older fossils (ca. 40–50 million years old) include Eorhinophrynus septentrionalis from the Middle Eocene and the slightly younger Chelomophrynus bayi—both from Wyoming in the United States.

The taxonomy of this species is Rhinophrynus dorsalis Duméril and Bibron, 1841, Vera Crúz, Mexico.

Physical characteristics

Typical of burrowing frogs, the Mesoamerican burrowing toad has a short head with tiny eyes and a globular body with

loose skin that obscures the short, stout limbs, leaving only the immense hands and feet visible when the frog is at rest. The pectoral girdle and forelimbs are located far forward so that the shoulder blades actually wrap around the back end of the skull. Consequently, the head of the Mesoamerican burrowing toad seems to be even shorter than it actually is, and there is no indication of a neck and no room for a tympanum (external ear). The snout of the Mesoamerican burrowing toad is unique. The nostrils are located much closer to the eyes than to the end of the long, narrow snout, which is truncate at the end. The skin covering the snout firmly adheres to the skull beneath and bears cushionlike pads. Each epidermal skin cell in the snout region has a minute keratin spicule, which is not visible to the naked eye; the spicules are pointed on the top of the snout but rounded on the bottom. The lips are thick, and the lower lip is glandular.

The Mesoamerican burrowing toad lacks teeth and has an unusual triangular tongue. Unlike other anurans, in which the tongue is rolled over the edge of the lower jaw or flipped out of the mouth, in the Mesoamerican burrowing toad the tongue protrudes forward through the buccal groove and out the end of the snout for a short distance. Because this frog feeds underground on termites and ants, it is thought that the tongue is a special adaptation for feeding in confined quarters. Thus, having located and broken through to a subterranean ant or termite tunnel, the frog can place the tip of its snout against the hole and simply extend its sticky tongue each time it detects a passing insect and then retract its tongue and the prey into its mouth.

Despite its stocky form, this species is an accomplished burrower. The body is highly flexible, and the stout hind limbs are equipped with large feet with short, thick digits and a pair of digging "spades." While pivoting its body in a circle around its forelimbs, the frog shifts soil away from itself by digging with its hind feet and inflating and deflating its body; it soon disappears, rear end first, into soil, which then fills in over the head as the frog disappears down the shaft it is excavating. Typically, the Mesoamerican burrowing toad is dark brown or nearly black dorsally, whereas the venter varies from dark brown to gray and usually has no pattern. There is a bright stripe on the middle of the back of the frog from its head to the vent; the vertebral stripe is flanked by scattered blotches or spots that vary from bright yellow to yellow-orange or reddish orange.

Distribution

Although extinct rhinophrynids occurred in North America, the Mesoamerican burrowing toad is restricted to the southern tip of Texas in the United States and the lowlands of southern Mexico and Central America (Honduras, Guatemala, Belize, El Salvador, Nicaragua, and Costa Rica).

Habitat

The Mesoamerican burrowing toad is found in savanna habitats and seasonally dry forests.

Behavior

Because this species is fossorial (adapted to digging), it is seen above ground only when it emerges to breed during the rainy season. At this time, adults usually are found in flooded pastures, roadside ditches, pools in savannas, and other ephemeral bodies of water. The frogs spend the dry season underground. Thus, virtually nothing is known about their nonbreeding activity patterns and their interactions with one another and other species.

Feeding ecology and diet

No one has reported observing the Mesoamerican burrowing toad feed. Presumably they do so underground and specialize in termites and ants that use subterranean burrows, because these insects have been recovered from stomach contents of the frogs.

Reproductive biology

Adult members of the species emerge from their subterranean burrows at the beginning of the rainy season to breed. Males call from temporary bodies of water at the water surface. When they call, their internal vocal sacs become enormously distended; with each abrupt inflation of the vocal sacs, the frog is rotated and pushed backward in the water. The loud call has been described as an "uooooooooo" that lasts about 1.4 seconds and is repeated 15–20 times a minute. Choruses of these frogs can be heard over great distances. A female Mesoamerican burrowing toad initiates contact with a breeding male by nudging him in the throat or the chest with her snout. The male then grasps the female from above in the inguinal region and fertilizes the single egg or small groups of eggs that she deposits in the water. Each female produces several thousand eggs. Because she expels only a few at a time, it is possible that each female mates with many males during the breeding season. The fertilized eggs sink to the bottom of the temporary pond and hatch into tadpoles in a few days. The developing tadpoles filter-feed on algae and congregate into swimming groups. These groups may be composed of as few as 50 individuals swimming in a coordinated "ball" about 3.9 in (10 cm) in diameter to several thousand tadpoles in a congregation more than 3.3 ft (1 m) in diameter.

Conservation status

The Mesoamerican burrowing toad is not threatened.

Significance to humans

None known.

Resources

Books

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

Periodicals

Henrici, Amy C. "Chelomophrynus bayi (Amphibia, Anura, Rhinophrynidae), a New Genus and Species from the Middle Eocene of Wyoming: Ontogeny and Relationships." Annals of the Carnegie Museum 60 (1991): 97–144.

Trueb, Linda, and David Cannatella. "The Cranial Osteology and Hyolaryngeal Apparatus of Rhinophrynus dorsalis (Anura: Rhinophrynidae) with Comparisons to Recent Pipid Frogs." Journal of Morphology 171 (1982): 11–40.

Trueb, Linda, and Carl Gans. "Feeding Specializations of the Mexican Burrowing Toad, Rhinophrynus dorsalis (Anura: Rhinophrynidae)." Journal of Zoology, London 199 (1983): 189–208.

Linda Trueb, PhD