Spea bombifrons
Plains Spadefoot

Plains spadefoot toads, Spea bombifrons, are native to the Nearctic region. Their North American range stretches from southern Alberta and Saskatchewan, Canada south to the panhandle and lower tip of Texas, and into northern Mexico. They range as far west as southeastern Arizona and east to Nebraska, western Missouri, and western Oklahoma. Their range seems to be expanding along the Missouri River floodplain and in southern Alberta and Saskatchewan. There are disjunct populations in southern Colorado, northeastern Mexico, southernmost Texas, and along the Arkansas River in Arkansas. (Alberta Government, 2002; Holman, 2003; LeClere, 2005; Sargeant, et al., 1993; Alberta Government, 2002; Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; Holman, 2003; Lauzon, 1999; LeClere, 2005; Sargeant, et al., 1993)

Plains spadefoot toads are found mainly in the arid grasslands of western North America. They are restricted to areas with loose soils for burrowing. They can also be found in savannas, loess hills, sandhills, and semi-desert and desert scrub. They are found in shallower summer burrows and deeper winter burrows throughout the year. Larvae require small, ephemeral ponds for development, such as cattle ponds, vernal pools, playa lakes, and flooded agricultural fields. Suitable ponds are difficult to find in habitats that plains spadefoot toads prefer. The soft, sandy soils they prefer for burrowing are also usually permeable to water. Because of this they also require proximity to areas where soils are less permeable and permit formation of temporary breeding pools. Juveniles burrow into soft mud along the ponds in which they developed or crawl into crevices in dried mud or under plant litter. (Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; Lauzon, 1999; LeClere, 2005; McCormick, 2006)

Plains spadefoot toads measure 38.1 to 63.5 mm (2 to 2.5 inches) in length, and weigh approximately 30 grams. Females tend to be slightly larger. They are brown or gray, sometimes with hints of green, and dark splotches or warts. On their dorsal and lateral surfaces are 4 vague stripes, with the middle two stripes sometimes forming an hourglass-like shape. Some have red or orange spots as well. Plains spadefoot toads are called "spadefoot" because of a welll-developed, sharp, spade-shaped and black tubercle on each of their rear feet. This is a bony extension of the metatarsal, covered with keratin that is used to burrow with the rear feet. There is a raised bone (or "boss") between their eyes, and their pupils are vertical, like those of cats. Adult, breeding males have a keratinous "nuptial pad" on their thumbs. (Alberta Government, 2002; Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; Holmes, 1954; LeClere, 2005; McCormick, 2006; Sargeant, et al., 1993)

Plains spadefoot toads are not true toads (Bufonidae). They are members of a small, frog family, Pelobatidae. (Farrar and Hey, 2006; McCormick, 2006)

Plains spadefoot toad eggs usually hatch within two days of being laid. At 30 degrees Celsius eggs hatch in 20 hours. Tadpole development (time to metamorphosis) usually lasts 13 to 20 days, depending on ambient temperatures. In northern parts of the range, time to metamorphosis may be up to 60 days, with average times in Alberta being 21 to 34 days. Tadpoles reach up to 7 cm in length and have a dark olive/yellow color with irridescent highlights. Plains spadefoot toad larvae develop primarily in small, temporary ponds and larvae are tolerant of widely fluctuating and high water temperatures. They metamorphose rapidly, before breeding ponds evaporate. As members of the family Pelobatidae, plains spadefoot toads have one of the fastest development rates among amphibians. (Alberta Government, 2002; Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; Sargeant, et al., 1993)

After heavy rains, males travel to breeding ponds and begin to call. Calls attract both males and females, with louder choruses attracting the most individuals. Calls are loud and harsh and can be heard up to 3 km away. In some areas of the southwest Spea bombifrons uses the same breeding ponds as other Spea species and hybridization has been documented in the laboratory. Mating calls act as reproductive isolating mechanisms in these circumstances. Two mating call types have been identified in plains spadefoot toads. Spea bombifrons has been observed calling at mating ponds at temperatures as low as 10.5°C. (Alberta Government, 2002; Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; Lauzon, 1999; Sargeant, et al., 1993)

Plains spadefoot toads mate during or after heavy rains (from 2.5 to 10.4 cm). Times of rainfall vary with latitude, but in the center of the range of S. bombifrons, this is from May to August. Plains spadefoot toads live primarily on land, traveling to breeding ponds only to mate. They lay their eggs in temporary ponds created by the rain. These ponds are generally up to 1 meter in depth. They have been observed breeding in natural ponds, oxbow areas of rivers, and sloughs as well as irrigation and roadside ditches, flooded areas of playgrounds and constructions sites, and in flooded agricultural fields. Females lay up to 2,000 eggs, in masses of 10 to 250. Eggs fall to the pond bottom or are attached to vegetation or other submerged objects. Sexual maturity is reached in the second year after hatching. (Alberta Government, 2002; Cannatella and Bockstanz, 1999; LeClere, 2005; McCormick, 2006; Sargeant, et al., 1993)

After the eggs are deposited in a temporary pond, there is no further parental care. (Alberta Government, 2002; Cannatella and Bockstanz, 1999; Farrar and Hey, 2006; McCormick, 2006; Sargeant, et al., 1993)

The maximum lifespan of plains spadefoot toads is estimated at 13 years. (Farrar and Hey, 2006)

Plains spadefoot toads are nocturnal, emerging during humid nights to forage. During the day, they remain in their burrows. Plains spadefoot toads emerge less frequently and burrow more deeply during especially dry times, such as droughts or during summer. Burrows can be as deep as 1 meter. Because they rely on burrowing to protect them from dry conditions, plains spadefoot toads are restricted to areas with loose soils. Burrows are usually dug close to a plant or other object that provides shade at the entrance. In winter, plains spadefoot toads will burrow more deeply to avoid freezing temperatures. When from 2.5 to 10.4 cm of rain falls, these toads emerge from their burrows and migrate to breeding ponds. Lighter rains and snow melt will also attract smaller numbers of plains spadefoot toads from their burrows to congregate at temporary ponds. (Cannatella and Bockstanz, 1999; Holmes, 1954; Lauzon, 1999; McCormick, 2006)

Because plains spadefoot toads can survive long periods of time without food and remain inactive underground, they require little individual space. The average home range for plains spadefoot toads in Alberta is 10 square meters. Closely related eastern spadefoot toads (Spea holbrooki) have home ranges of 10.1 square meters, with male ranges being slightly larger than female ranges. Individuals have been recorded traveling 1.6 km to breeding sites and toadlets have been recorded 2 km from their natal ponds within weeks of metamorphosis. (Alberta Government, 2002)

Plains spadefoot toads have a mating call that is approximately 0.5 - 1.0 second in length and sounds similar to that of a mallard. This call, and another low, rough call, both act as mating calls. A male's mating call can be heard as far as 3 km away. (McCormick, 2006)

Plains spadefoot toad tadpoles can develop into carnivorous "trophic morphs" or omnivorous "trophic morphs." Carnivorous tadpoles develop faster than omnivorous tadpoles and eat other spadefoot toad tadpoles and invertebrates. Cannibalism in breeding ponds is common. Omnivorous tadpoles eat organic matter, especially algae. (Farrar and Hey, 2006)

Adult plains spadefoot toads eat small invertebrates, such as hymenopterans, flies, moths, beetles, spiders, and stink bugs. (Farrar and Hey, 2006)

The cryptic coloration and burrowing habits of Spea bombifrons may protect many individuals from predation. They also have noxious skin secretions that may deter predators. Tadpoles gather in large feeding aggregations when they detect a predator, which may protect some individuals. (Farrar and Hey, 2006; Lauzon, 1999)

Hydrophilid beetle larvae (Hyrus triangularis), crustaceans (Apus), and other spadefoot toad tadpoles prey on tadpoles. Adults may be preyed on by barn owls (Tyto alba), Swainson's hawks (Buteo swainsoni), prairie rattlesnakes (Crotalus viridis), and burrowing rodents. (Farrar and Hey, 2006; Lauzon, 1999)

Plains spadefoot toads affect populations of their invertebrate prey. They also share breeding ponds with other spadefoot toad species (Spea) and with Great Plains toads (Anaxyrus cognatus). Plains spadefoot toad tadpoles will eat Anaxyrus cognatus tadpoles where they share breeding ponds. They also share breeding ponds with Woodhouse's toads, Anaxyrus woodhousii woodhousii. (Farrar and Hey, 2006; McCormick, 2006)

Plains spadefoot toads will outcompete Mexican spadefoot toads (Spea multiplicata) for fairy shrimp (Anostraca) where they co-occur. (Farrar and Hey, 2006)

Tadpoles are sometimes infected with Saprolegnia fungus. Adults have been found with intradermal mites (Acari) and may carry a monogean parasite, Pseudodiplorchis americanus. (Farrar and Hey, 2006)

Plains spadefoot toads are important members of the healthy ecosystems in which they live.

There is no known negative economic importance for humans.

Plains spadefoot toad populations seem to be healthy, for the most part. Local population declines are often the result of lack of breeding during drought years. In Alberta they are on the "blue list" of species at risk of decline because of non-viable population levels in the province. Plains spadefoot toads may be expanding their range in North America but local populations may be threatened by development that endangers breeding ponds, such as wetland draining and conversion of land to agriculture. The use of pesticides, herbicides, and the presence of other pollutants in water may also be a threat to these frogs. (Lauzon, 1999; McCormick, 2006)