The Mexican burrowing toad (Rhinophrynus dorsalis) is the single living representative of the family Rhinophrynidae. It is a unique species in its taxonomy and morphology, with special adaptations to assist them in digging burrows where they spend most of their time. These adaptations include a small pointed snout and face, keratinized structures and a lack of webbing on front limbs, and specialized tongue morphology to assist in feeding on ants and termites underground. The body is nearly equal in width and length. It is a dark brown to black color with a red-orange stripe on its back along with splotches of color on its body. The generic name Rhinophrynus means 'nose-toad', from rhino- (ῥῑνο-), the combining form of the Ancient Greek rhis (ῥίς, 'nose') and phrunē (φρύνη, 'toad'). The Mexican burrowing toad diverged from other amphibians over 190 million years ago and has been evolving independently for a longer period of time than the evolutionary differences between mammals like humans, fruit bats, polar bears and killer whales. Its closest sister group is Pipidae, or the aquatic clawed frogs.
Description
The Mexican burrowing toad has a unique appearance that makes it easy to distinguish from other organisms. This species' body is flat with a width and length that are almost equal. It is covered in loosely fitting wrinkled skin which becomes taut and shiny when the frog's body swells up during its mating call. Its head is small and triangular, and projects out of its body in a small point with very small eyes. They have no neck and no visible ear holes or tympanum. Its legs are short and muscular, and are structured for burrowing as indicated by its name. Its feet also have adaptations for burrowing, mainly nail-like keratinized structures at the end of each digit. Its front feet lack webbing between the digits to free them up for burrowing, and its back feet are short and extensively webbed. The toad's coloration ranges from a dark brown to black. A bright red-orange stripe runs on its back from head to tail, and the body is covered with other red-orange splotches in varying patterns. Its underside is gray to dark brown and does not have the red splotches like the rest of the body. The toad is sexually dimorphic, with females being larger than males. Adults of the Mexican burrowing toad grow to be between 75 and 88 mm or about 3.0 to 3.3 inches (snout-vent length).
Feeding specializations The toad's snout is covered in an armor of small keratinous spines, and its lips are sealed by secretions from glands under the mandible. Its lips have a double closure along their maxillary arch, which are enhanced by the glands under the jaw. Morphological studies reveal that the frog has a type of tongue protrusion that is distinct from that of other frogs. Many other frog species project their tongues by a lingual flip, a behavior where the tongue is strongly flipped through the lips and out of the mouth. In this species however, the tongue stiffens and protrudes out of the mouth by moving the jaw backwards. This mechanism is specialized for capturing small insect prey in burrows.
Distribution The Mexican burrowing toad is found in tropical and subtropical dry broadleaf forest, savannas, and thorn scrub (e.g. Tamaulipan mezquital) in the lowlands of Central America, Mexico, and extreme south Texas, USA. Due to its wide range, the species is categorized as least concern by the IUCN, but some local and regional populations are protected and listed as threatened by various governments within its distribution. Rhinophrynus dorsalis occurs in the Lower Rio Grande Valley of south Texas, USA, ranging southward through the costal lowlands of the Gulf of Mexico and Caribbean Sea in eastern Mexico including much of the Yucatán Peninsula, into northern Guatemala, Belize, extreme northwest Honduras, and an isolated record from northeast Nicaragua. Another geographically isolated population occurs in the lowlands of the Pacific coast, from extreme southern Michoacán, Mexico, southward into coastal areas of Guatemala, El Salvador, Nicaragua, Honduras, and northwest Costa Rica.
Evolutionary history The oldest fossil of the genus is Rhinophrynus canadensis known from the late Eocene of Saskatchewan, Canada. Other fossils are known from the Oligocene of Florida.
Habitat Its natural habitats include forest, savanna, shrubland, grassland, and inland wetlands. It primarily inhabits lowland areas of tropical dry and moist forests. It is generally associated with areas which are seasonally flooded because it relies on temporary ponds for breeding. It usually remains underground in the dry season following the breeding period. Its eggs and larvae develop in temporary pools formed by heavy rains, and the adults remain in fairly small areas.
Behavior This species is nocturnal. These frogs make burrows to survive the dry season without suffering from lack of water. They use their strong short limbs and nails to dig into the soil and create burrows. The frog can survive long periods of drought inside this burrow. When the frog is making its vocalization or when it is alarmed its body becomes inflated and resembles a balloon, with its already short head and limbs almost disappearing. This mechanism is not deeply studied, and may require more research to determine its physiology.
Diet The Mexican burrowing toad primarily subsists on ants and termites that they forage underground. Their features are specialized for underground foraging, especially the way the tongue is used by shifting it forward rather than the lingual flip seen in other frogs. This mode is unique among anurans, and is highly specialized for capturing small insects in burrows.
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