Spadefoot Toads (Pelobatidae)

(vertebrate zoology) A family of frogs in the suborder Anomocoela, including the spadefoot toads.

(Pelobatidae)

Class: Amphibia

Order: Anura

Family: Pelobatidae

Thumbnail description
Rotund, moderate-size frogs with vertical pupils and a keratinous tubercle on the hind foot

Size
2.0–3.2 in (51–81 mm)

Number of genera, species
3 genera; 11 species

Habitat
Spadefoot toads usually live in arid to semiarid areas, such as fields and woodlands with sandy or loose soil

Conservation status
No species listed by IUCN

Distribution
North America and Europe to central Asia and northern Africa

Evolution and systematics

The earliest fossil Pelobatidae are from the late Cretaceous of North America, and extend through the middle Eocene of Europe to the Pleistocene. Pelobatidae are characterized by having sacral vertebrae (the vertebrae attached to the hips) fused to their "tail" vertebrae (whereas these are jointed in many frogs); the presence of a metatarsal spade; and bony ornamentation on the bones covering the brain (the frontoparietals). Within the family, the Nearctic genera Spea and Scaphiopus are each other's closest relative; some researchers recognize only a single genus, Scaphiopus. For a long time, members of the family Megophryidae commonly were considered to be a subfamily of Pelobatidae. Research eventually showed, however, that megophryids are not the closest relative to pelobatids, and the megophryids were removed from Pelobatidae family in 1985. The extinct subfamily Eopelobatinae has also been considered a member of Pelobatidae by some authors, but this relationship is not well supported. No subfamilies are recognized.

Physical characteristics

Frogs in this family are often mistaken for toads (exemplified by the common name, "spadefoot toads"). They do not have the warty skin of true toads, however, and they have teeth in the upper jaw (which true toads lack). All pelobatids have teeth on the maxilla and premaxilla. Palatines, bones that support the upper part of the inside of the mouth, are absent from the skull, and there are two frontoparietal bones covering the brain. Some species (Pelobates and Scaphiopus) have exostosis, or additional bony deposits, on the dorsal surface of the skull.

In members of this family, the facial nerve exits through the anterior acoustic foramen in the auditory capsule. The pupil is vertically elliptical. There are eight vertebrae before the pelvis, and ribs are absent. The pectoral girdle is arciferal with a distinct sternum and omosternum. The small leg bones, the fibulare and tibiale, are fused only at their ends.

Larvae are aquatic with complete larval mouths (beaks).

Distribution

The family has a discontinuous distribution. The genus Pelobates occurs throughout most of western Eurasia and in the northwestern tip of Africa. The genera Scaphiopus and Spea occur throughout temperate North America, north to southern Canada, and south to southern Mexico.

Habitat

Spadefoot toads normally are found in arid to semiarid areas, such as fields, farmlands, dunes, and woodlands. They prefer rocky or sandy areas or regions where the soil is loose. Spadefoot toads typically inhabit low-lying areas that retain water after heavy rains. In eastern North America, one species occurs in cool, moist areas.

Behavior

All spadefoot toads are adapted to digging (fossorial). They are primarily nocturnal, but males call both day and night. Usually, males call while floating near the surface of shallow waters. They are very secretive and spend most of their time hiding in burrows. Those that live in strictly desert areas are active on the surface for only about two weeks during the year. As an adaptation to living in dry places, all spadefoot toads burrow down far enough so that the moisture content in the soil is the same as in their skin. Some may form a cocoon of dead skin to help protect against desiccation. These behaviors ensure that they can live for long periods without losing much water to the environment. During the rainy season, they burrow only about 2 in (5 cm) below the surface, but during droughts they can be found more than 3 ft (1 m) underground. They burrow feet first, like most other digging frogs, but spadefoot toads have thick, shovel-like, keratinous spades on their feet to help them move dirt quickly. They alternate from left to right, pushing dirt forward, while rocking their bodies backward into the hole they are excavating.

Like most frogs, spadefoot toads rely on several antipredator mechanisms to ward off would-be attackers. If they detect motion, they stay completely still and depend on their camouflaged skin to blend in with the environment. If threatened, they inflate their lungs to make themselves appear bigger. Some toads also produce distasteful skin secretions, which often are accompanied by a strong odor (some smell like garlic, and others like peanut butter).

Feeding ecology and diet

During the night, when there is enough moisture in the air to keep them from becoming desiccated, spadefoot toads come out of their burrows to hunt for food. The adult diet generally consists of invertebrates, including beetles, snails, spiders, and caterpillars. The diets of spadefoot tadpoles are much more remarkable. Most anuran larvae eat vegetative matter, but spadefoot larvae include some of the few species that eat aquatic insects and small crustaceans as well. They also eat plant material, filtering particles from the water column. Spadefoot tadpoles sometimes group together in huge schools, which may help stir up settled plant material from the bottom of the pond. Schooling also may help protect against predation by insect larvae.

Because spadefoots breed in relatively shallow, temporary waters, they are under constant stress from drying waters, increasing temperatures, reduced food densities, and crowding. If the density of tadpoles reaches a certain point, some of the larvae of certain species eat their fellow tadpoles. The cannibal morphs develop larger heads, sharp beaks, strong jaw muscles, and shortened intestines. Research indicates that cannibalism is adaptive in these species, because it allows for increased caloric intake, which, in turn, accelerates the rate of growth of the tadpoles. This ensures that the larvae reach the necessary size to metamorphose before the pond dries. Research also indicates that the cannibals use chemical cues to recognize related tadpoles and avoid eating their close relatives.

Reproductive biology

Because spadefoot toads live where rains and available water are unpredictable, they do not have a breeding season. Instead, males wait in their burrows for optimal weather conditions (triggered by the low-frequency vibrations of rainfall) and then race to any available waters and let out a relatively loud call for their size (much like a deep "bleet" from a sheep). This call can be heard for about a mile (1.6 km), and other males will join in and set up adjoining territories in the water. Females then emerge from their burrows and join the males in their territories.

Spadefoot toads generally breed in shallow, temporary pools, such as cow ponds and drainage ditches. Because these waters may last for only a few weeks, much of the population mates on the first night of heavy rains. Males clasp females in front of the hind legs during amplexus (mating). Hundreds of thousands of small, dark eggs are laid in gelatinous clumps or bands attached to aquatic vegetation. Because the waters in which the eggs are laid may have begun to evaporate, to survive to adulthood these eggs must develop into toadlets that can leave the water in a matter of days. Thus the developmental cycle of most spadefoot toads is fast. Hatching occurs within 24–72 hours in hot weather or up to one week in cooler temperatures. Generally, spadefoot tadpoles metamorphose in about four weeks. Tadpoles range from tan to dark brown; some are finely dotted with orange pigmentation and have transparent tail fins (with dark rims) that typically maintain transparency throughout the larval period.

Conservation status

No species are listed by the IUCN. In some parts of their range, several species are not considered threatened or endangered. However, population numbers and geographic ranges of several species are showing signs of rapid decline throughout their ranges. Several species (e.g., Scaphiopus holbrookii, Spea intermontana) are of special concern in parts of their range, because they are found only rarely. Other species, such as Pelobates fuscus, are listed as protected, threatened, or endangered by some agencies.

Significance to humans

Spadefoot toads are of no special significance to humans, though a few species are found in the pet trade.

Species accounts

Resources

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

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

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.

Pfennig, David W. "Polyphenism in Spadefoot Toad Tadpoles as a Locally Adjusted Evolutionarily Stable Strategy." Evolution 46 (1992): 1408–1420.

[Article by: Anne M. Maglia, PhD]