Description

The Plains Spadefoot Toad or Central Plains Spadefoot is a small, rotund, toad-like frog, generally resembling true toads (Bufonidae) in body form. Spea bombifrons can grow up to 2¼ inches 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 smoother and thinner than that of Bufonidae species. 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

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (17 records).

Spea bombifrons is native to the more arid Plains region of western North America: the plains east of the Rocky Mountains from southern Alberta, Manitoba, and Saskatchewan to northwest Texas and Chihuahua, Mexico, east to Missouri and eastern Oklahoma and into eastern Arizona, with isolated populations in south Texas and Mexico (Stebbins 1985).

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
The habitat, breeding season, and breeding location are highly related. S. bombifrons is nocturnal, and spends most of its time underground, emerging only to breed in favorable moist conditions, or to feed (Lauzon 1999). During extended droughts, the frog does not remain underground indefinitely but emerges, especially during humid weather, to forage during the evening (Bragg 1965; Ruibal 1969). S. bombifrons burrows deeper into the soil and emerges less frequently as the summer progresses and the soil dries (Bragg 1965). It prefers loose soil, where it can easily burrow with the aid of its “spades” up to depths of almost 1 m (Baxter & Stone 1980; Russell & Bauer 1993; USGS 2002). They will most often burrow along the edge of a solid object or near a plant that provides security or shade (Bragg 1944). During the winter, it burrows deeply to avoid freezing and desiccation, as S. bombifrons does not tolerate freezing and must burrow below the frost line (Baxter & Stone 1980). This spadefoot toad surfaces quickly from its burrow and opportunistically migrates to breeding wetlands during periods of heavy rainfall and warm temperatures (Bragg 1965; Klassen 1998; Lauzon & Balagus 1998). The measured amount of rainfall required for emergence varies from 2.5 to 10.4 cm (Black 1970; Femmer 1978; Farrar & Hey 1995).

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 so far 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 preys on 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 introduced 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 2,000 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).

Larva
The tadpoles of the plains spadefoot can occur in two ecomorphotypes-- carnivorous and omnivorous forms. The carnivorous tadpoles have longer snouts, larger serrated beaks with an upper cusp and lower notch, shorter and less coiled intestines than the omnivorous form. These forms were discovered in the Missouri River flood plain, western Iowa, where tadpoles were found in flooded soybean and corn fields (Farrar and Hay 1997).

This is similar to the tadpole morphotypes of its relative Spea multiplicata, which also displays a carnivorous amd omnivorous tadpole forms (Farrar and Hay 1997). Hence developmental plasticity in has been well studied (e.g, Ledon-Rettig et al 2008).

Trends and Threats
This amphibian is not included on any list of rare, endangered, threatened, or sensitive species (Nature Conservancy 1999; RedList). The Nature Conservancy assigns S. bombifrons a global rank of G5, indicating that the species is “demonstrably secure” across its global range (Nature Conservancy 1999). The IUCN Redlist ranks this species as Least Concern.

Substantial population declines observed during extended droughts are more likely a result of non-breeding during suboptimal environmental conditions than true population declines (Cottonwood 1986).

The Province of Alberta places S. bombifrons on its Blue List of species that may be at risk of declining to non-viable levels within the Province’s boundaries (Alberta Wildlife Management Division 1996). In Canada, this species’ highly variable population size, the result of annual variability in the availability of breeding wetlands, do not provide evidence to suggest that the range is contracting (Lauzon 1999). The population in Alberta, Canada is considered stable but moderately threatened due to current land use practices (Lauzon 1999).

Relation to Humans
Alteration and destruction of habitat are considered the possible risk to habitats suitable for S. bombifrons. Availability of wintering and breeding habitats are the likely limiting factors of this amphibian; agriculture is primarily responsible for the loss of prairie land and is probably the most significant threat (Lauzon 1999). Cultivation and drainage of wetlands, as for cattle, as well as flood control, have also been identified as potential threats (Cottonwood 1986).

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
Intensified agriculture or grazing
Prolonged drought
Drainage of habitat
Dams changing river flow and/or covering habitat

Comments
This species was featured in news of the week October 3, 2022:

Depending on environmental conditions, tadpoles of the Spea genus can switch from omnivores to predaceous carnivores that display cannibalism similar to post-metamorphic feeding behavior. The switch has long been attributed to heterochrony, or the developmental shift in the timing of gene-expression. However, Ledón-Rettig (2021) tested this hypothesis against an alternative that a novel pattern of gene expression is behind the ecological behavioral shift. She quantified RNA expression in two morphotypes of Spea bombifrons at two points in their development, tadpole and juvenile, and found that gene expression of tadpoles were more similar to each other than the gene expression of juveniles, which were also more similar to each other than to tadpoles. However, predaceous tadpoles also expressed a different suite of genes than omnivorous tadpoles lending support to the alternative hypothesis of a novel pattern of gene expression. These findings do not discount a role for heterochrony, but provide evidence of the potential for multiple routes to novel behaviors. (Written by Ann Chang)

References

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Axys Environmental Consulting Ltd. (1996). 1996 Wildlife Surveys for the Proposed Express Pipeline Project. Prepared for Express Pipeline, a division of Alberta Energy Company Ltd., and TransCanada Pipelines Ltd., Calgary, Alberta.

Axys Environmental Consulting Ltd. (1997). 1997 Wildlife Surveys for the Proposed AEC Suffield Gas Pipeline Project. Prepared for Alberta Energy Company Ltd., Calgary, Alberta.

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Black, J.H. (1970). ''Amphibians of Montana.'' Montana Wildlife, Animals of Montana, Series 1, 1-32.

Bragg, A.N. (1944). ''The spadefoot toads in Oklahoma with our summary of our knowledge of the group.'' American Naturalist, 78, 517-533.

Bragg, A.N. (1961). ''A theory of the origin of spade-footed toads deduced principally by a study of their habits.'' Animal Behaviour, 9, 178-186.

Bragg, A.N. (1965). ''The spadefoot toads in Oklahoma with our summary of our knowledge of the group: II.'' American Naturalist, 79, 52-72.

Bragg, A.N. (1965). Gnomes of the Night: the Spadefoot Toads. University of Pennsylvania Press, Philadelphia, PA.

Bragg, A.N. (1967). ''Recent studies on spadefoot toads.'' Bios, 38, 75-84.

Buchholz, D.R. and Hayes, T.B. (1996). ''Comparative larval biology in spadefoot toads.'' American Zoologist, 36, 97A.

Collins, J.T. (1982). Amphibians and Reptiles in Kansas, 2nd Edition. University of Kansas Museum of Natural History, Lawrence, KS.

Colorado Herpetological Society (2002). ''Plains Spadefoot Toad (Spea bombifrons).'' Guide to the Reptiles and Amphibians of Colorado. Colorado Herpetological Society.

Cook, F.R. (1960). ''New localities for the Plains Spadefoot Toad, Tiger Salamander, and the Great Plains Toad in the Canadian Prairies.'' Copeia, 1960(4), 363-364.

Cook, F.R. (1965). ''Additions to the known range of some amphibians and reptiles in Saskatchewan.'' Canadian Field-Naturalist, 79, 112-120.

Cook, F.R. and Hatch, D.R.M. (1964). ''A spadefoot toad from Manitoba.'' Canadian Field-Naturalist, 78, 60-61.

Corn, P.S. (1994). ''What we know and don't know about amphibian declines in the West.'' Sustainable Ecological Systems: Implementing an Ecological Approach to Land Management. USDA Forest Service, Fort Collins, CO, 59-67.

Cottonwood Consultants (1986). An Overview of Reptiles and Amphibians in Alberta's Grassland and Parkland Natural Regions. Prepared for the Wild West Program, World Wildlife Fund Canada, Toronto, Ontario.

Didiuk, A. (1997). ''Status of amphibians in Saskatchewan.'' Amphibians in Decline: Canadian Studies of a Global Problem. D.M. Green, eds., Society for the Study of Amphibians and Reptiles, Saint Louis, Missouri, 110-116.

Farrar, E.S. and Hey, J.D. (1995). Plains Spadefoot (Scaphiopus bombifrons) Distribution, Breeding Habitat Characterization, and Natural History Studies in Western Iowa: 1995 Studies. Wildlife Diversity Program Grant Report 1995, Boone, IA.

Femmer, S.R. (1978). Distribution and Life History Studies of Scaphiopus bombifrons Cope and Bufo cognatus Say in Missouri. M.A. Thesis, University of Missouri, Columbia, MO.

Huggins, D.G. (1971). ''Scaphiopus bombifrons Cope, a species new to Iowa.'' Journal of Herpetology, 5(3/4), 216.

Justus, J.T., Sandomir, M., Urquhart, T., and Ewan, B.O. (1977). ''Developmental rates of two species of toads from the desert southwest.'' Copeia, 1977(3), 592-594.

Klassen, M.A. (1998). ''Observations on the breeding and development of the Plains Spadefoot, Scaphiopus bombifrons, in southern Africa.'' Canadian Field-Naturalist, 112, 387-392.

Landreth, H.F. and Christensen, M.T. (1971). ''Orientation of the Plains Spadefoot Toad, Spea bombifrons, to Solar Cues.'' Herpetologica, 27, 454-461.

Lauzon, R.D. (1999). ''Status of the Plains Spadefoot (Spea bombifrons) in Alberta.'' Wildlife Status Report No. 25. Alberta Environment, Fisheries, and Wildlife Management Division, and Alberta Conservation Association, Edmonton, Alberta..

Lauzon, R.D. and Balagus, P. (1998). ''New Records from the Northern Range of the Plains Spadefoot Toad, Spea bombifrons, in Alberta.'' Can. Field-Nat., 112, 506-509.

Ledon-Rettig, C.C., Pfennig, D.W. and Nascone-Yoder, N. (2008). "Ancestral variation and the potential for genetic accommodation in larval amphibians: implications for the evolution of novel feeding strategies." Evolution & Development, 10, 316-325. [link]

Nature Conservancy (1999). The Natural Heritage Network.

Pearson, P.G. (1955). ''Population ecology of the spadefoot toad, Scaphiopus h. holbrooki (Harlan).'' Ecological Monographs, 25(3), 233-267.

Preston, W.B. and Hatch, D.R.M. (1986). ''The Plains Spadefoot, Scaphiopus bombifrons, in Manitoba.'' Canadian Field-Naturalist, 100(11), 123-125.

Ruibal, R., Tevis, L., Jr, and Roig, V. (1969). ''The terrestrial ecology of the spadefoot toad Scaphiopus hammondii.'' Copeia, 1969(3), 571-584.

Russell, A.P. and Bauer, A.M. (1993). The Amphibians and Reptiles of Alberta. University of Calgary Press, Calgary, Alberta.

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Species Account Citation: AmphibiaWeb 2022 Spea bombifrons: Plains Spadefoot Toad <https://amphibiaweb.org/species/5278> University of California, Berkeley, CA, USA. Accessed Dec 12, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 12 Dec 2024.

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