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The European, American, or Plains Spadefoot Toad is a small but rotund toad-like frog, generally resembling true toads (Bufonidae) in body form. Spea bombifrons can grow up to 2¼" in snout-vent length (Colorado Herpetological Society 2002). Its skin, which is relatively smooth, can range from gray to brown, sometimes with a greenish hue, and is scattered with darker spots or blotches. The skin is also smoother and thinner than that of Bufonidae. Along the dorsal and lateral surfaces run four vague light stripes, the middle two of which occasionally resemble an hourglass. There are many small tubercles on the otherwise smooth skin. The ventral surface is white and has no markings, although the throat of the male is bluish or grayish on the sides (USGS 2002). There is a raised, bony lump (boss) between the large eyes. The eye has an elliptical, vertical, cat-like pupil, and thus differs from the true toads, whose pupils are round. S. bombifrons also differs from Bufo in that it has no parotid glands (or, when present, they are indistinct) (USGS 2002). Like the other members of the family Pelobatidae, or the spadefoots, S. bombifrons is named for its large, well-developed, wedge-shaped, sharp-edged, black tubercle or metatarsal "spade" on the bottom of each hind foot. This bony element, which is capped with a keratinous cover, is used to burrow rear-first (USGS 2002).
Distribution and Habitat
Country distribution from AmphibiaWeb's database: Canada, Mexico, United States
U.S. state distribution from AmphibiaWeb's database: Arizona, Colorado, Iowa, Kansas, Missouri, Montana, North Dakota, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, Utah, Wyoming
Canadian province distribution from AmphibiaWeb's database: Alberta, Manitoba, Saskatchewan
Throughout the majority of its range, the distribution S. bombifrons is highly correlated with sandy, gravelly, or sandy loam soils, but it ranges from deserts in the American Southwest to aspen parkland in the Canadian prairies (Bragg 1944; Cook 1960; Black 1970; Huggins 1971; Femmer 1978; Collins 1982; Cook and Hatch 1964; Cottonwood Consultants 1986). S. bombifrons has been found in unvegetated sand dunes, sand dunes with willow and cottonwood, upland prairie, desert, short and mixedgrass prairie, mixed shrubland, fescue grassland, floodplains, and sagebrush (Bragg 1944; Black 1970; Huggins 1971; Stebbins 1985; Axys 1996 and 1997). East of the Rockies, S. bombifrons is found in shortgrass prairie and desert grasslands, where the soil is loose (USGS 2002).
Life History, Abundance, Activity, and Special Behaviors
Large choruses of these frogs have been associated with heavy rainfall, but the temporary wetlands that result from light rain or snow melt are known to attract small choruses as well (Lauzon 1999). S. bombifrons has been observed calling when daily maximum temperatures were 12.5°C to 23.5° and daily minimum temperatures were 7°C to 10.5°C (Klassen 1998), although breeding frogs have been observed in the southeastern extreme of their range at temperatures as low as 9°C (Bragg 1965). S. bombifrons has been observed calling at water temperatures of 10.5°C to 16°C (Lauzon 1999).
No studies have been conducted on the home range of S. bombifrons, but the related Eastern Spadefoot (Spea holbrooki holbrooki) has an average home range of 10.1 m2, with male home ranges generally larger than those of the female (Pearson 1955). S. bombifrons is known to be capable of migrating at least 1.6 km to breeding sites, and juveniles have been found more than 2 km from known breeding wetlands within weeks of metamorphosis (Landreth & Christensen 1971; Klassen 1998). This frog forages above ground, and its prey are a variety of insects (Lauzon 1999). In Oklahoma, this included flies, hymenopterans, moths, beetles, pentatomids, and various spiders (Bragg 1944). S. bombifrons is generally active from the start of its breeding season until the fall (Lauzon 1999). The breeding season is generally correlated with annual peaks in precipitation. In the center of its range, S. bombifrons breeds from May to August. In the southern end of its range, S. bombifrons breeds in July, after the summer rains. In the northern part, breeding may take place in late May at the earliest. If appropriate environmental conditions do not occur during the active season, breeding may not occur at all; conversely, S. bombifrons may breed more than once in a single year if conditions are particularly conducive (Bragg 1944 & 1965; Klassen 1998).
Spea bombifrons lays its eggs in a variety of wetlands, but prefers flooded areas and temporary pools (USGS 2002). Its preference for sandy soils leaves S. bombifrons with limited breeding opportunities, since the soil drains rapidly and precludes the development of breeding wetlands. As a result, this spadefoot relies on fine-textured, less permeable soils within sandier habitats where temporary wetlands form and it is able to breed (Lauzon 1999). Breeding pond depths vary from 10 cm to more than 1 m. It tolerates varying amounts of vegetation, and native as well as tame habitats (Bragg 1965; Farrar & Hey 1995; Klassen 1998). Spawning S. bombifrons have been observed in partially flooded fields, roadside ditches, flooded dugouts, shallow temporary wetlands in fallow fields, temporary ponds in uplands, along streams, semi-permanent pools, oxbow lakes, stream meander channels, unvegetated sloughs, marshy depressions, flooded cultivated fields, temporary wetlands in pastures, river backwaters, and ditches (Cook 1965; Cottonwood 1986; Preston & Hatch 1986). These frogs have been observed breeding in non-native habitats such as a construction site (Femmer 1978), flooded soybean fields and cornfields (Farrar & Hey 1995), a flooded wheat field, and even driveways and bicycle paths (Didiuk).
This spadefoot breeds quickly to take advantage of its typically ephemeral wetland, and to allow the eggs and larvae as much time as possible to develop. The largest numbers have been observed during or the first night after heavy rains, with counts falling off dramatically thereafter (Bragg 1965). Generally, males migrate to the breeding wetlands before females, and generally outnumber them in a given time and place (Bragg 1945; Baxter & Stone 1980). The call of the male is loud, harsh, and may be heard at distances of up to 3 km (Lauzon 1999). The male spadefoot call greatly stimulates both sexes, and larger choruses attract more individuals of both sexes (Bragg 1945).
Females lay up to 2000 eggs in clutches of 10 to 250 each (Bragg 1965; Collins 1982). Whereas true toads lay their eggs in strings, S. bombifrons females lay eggs in spherical masses, attached to submerged vegetation (Baxter & Stone 1980). Suspended soil particles and other debris adhere to the sticky surface of the egg mass to help camouflage them from predators (Bragg 1965). The rate of egg development is temperature dependent, and under normal conditions spadefoot eggs will hatch in about two days (Bragg 1965). The lethal temperature limits for eggs are <10°C and >34°C (Justus 1977). Spadefoot toads (family Pelobatidae) as a group have the fastest development rate among amphibians (Bragg 1961). Larval development is also temperature dependent and thus allows the tadpoles to develop faster in conditions that evaporate the temporary breeding wetlands (Buchholz & Hayes 1996). In southern regions (in this study, Oklahoma), the time from hatching to metamorphosis may be as little as 14 days (Bragg 1967), with a range of 17 to 20 (in Missouri) days being more common (Femmer 1978); in the north (Alberta), metamorphosis has been observed 21 to 34 days after hatching, with some tadpoles requiring 60 days (Klassen 1998).
Trends and Threats
Relation to Humans
The extent to which fragmentation of prairie land and other habitats has affected toad movements between populations or the re-colonization of populations sinks is unknown (Didiuk 1997).
Other human activities, such as the widespread use of herbicides and pesticides, oil and gas exploration and development, and road kills have been implicated in the decline of related amphibians and those with similar ranges as S. bombifrons, but none has been studied specifically with regard to this species (Lauzon 1999).
Possible reasons for amphibian decline
General habitat alteration and loss
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Written by Charles P. McCormick (mccorm AT fas.harvard.edu), Harvard University
First submitted 2002-11-26
Edited by Meredith Mahoney and Charles P. McCormick (2008-02-03)
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