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Plains Spadefoot

Spea bombifrons

Family: Scaphiopodidae

COSEWIC status:
  • Not at Risk
SARA status:
  • No Status
IUCN status:
  • Least Concern

Description

The Plains Spadefoot can attain a body length of up to 6.4 cm. Individuals are grey, brown or olive with dark reddish-brown spots and often have four light lines running down the back. The colouration often matches the substrate of the areas in which they are found. The eyes are golden yellow with vertical pupils. The belly is white and males have a grey to bluish throat. Individuals have relatively smooth skin covered with tubercles (small bumps), a bump (called a boss) between the eyes, a blunt snout that is slightly upturned, short legs, webbed feet and a black ridge (spade) on the bottom of each hind foot that is used for burrowing. Larvae (tadpoles) have long tails with a large tail fin, a large body with a triangular-shaped head, upturned eyes that are located high on the head, and lack front legs (newly hatched tadpoles are legless). The body is grey, brown or tan with gold or brassy flecking and the tail fin is clear. The larvae can reach 6.5 cm in length before metamorphosis. The call of Plains Spadefoot is a loud, quacking trill.

Similar Species

The Plains Spadefoot may be confused with the Canadian, Great Plains and Western Toads. However, these “true toads” (family Bufonidae) have bumpier, drier skin, horizontal pupils and large parotoid glands behind the eyes. They also lack the “spade” on the hind feet.

Distribution

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The Plains Spadefoot occurs in the prairie regions of Canada in southeastern Alberta, southern Saskatchewan and extreme southwestern Manitoba. This species ranges throughout the Great Plains in the United States from Montana and western North Dakota south to southern Arizona and Texas. Its range extends into northern Mexico.

Habitat

This species inhabits grasslands and breeds in shallow, temporary pools created by rainwater, as well as anthropogenic features such as roadside ditches, irrigation ponds and flooded fields. Individuals generally occur in areas with loose, sandy soil in which they can burrow to seek shelter during the day and during extended dry periods. They hibernate underground below the frost line in deep burrows that they excavate or in mammal burrows.

Biology

In Canada, individuals hibernate during the winter and can be active from May until September or October. Breeding is explosive and occurs over a short period of just a few days following heavy rainfall in the spring or early summer. During breeding period, males call to attract females. Fertilization occurs externally in aquatic habitats with the male grasping the female (amplexus) as she lays eggs. The female lays up to 2,000 eggs in small clumps of 10–250 eggs, which are usually attached to vegetation or other debris in shallow water. The eggs are black on top and creamy white underneath and are surrounded by clear jelly envelopes. The eggs develop rapidly and hatch in 2–7 days and larvae transform into juveniles after 3–6 weeks. Individuals reach maturity at 1–2 years of age and can live for over 10 years. Adults and juveniles are primarily nocturnal and forage for a wide variety of invertebrates, including spiders, termites, beetles, flies, ants, caterpillars, moths, crickets and grasshoppers. Larvae can be omnivorous or carnivorous, each having different mouth morphology (i.e. carnivorous larvae have a large head and jaws and a sharp “beak”). Spadefoots are highly resistant to desiccation and can tolerate high levels of water loss. They also burrow underground and become dormant during extended dry periods, where they can absorb water directly from the soil in their burrows. Spadefoots secrete a noxious substance from their skin when threatened to deter predators.

Threats

The extensive conversion of prairie grasslands to agricultural uses — particularly tilled land — has resulted in widespread habitat loss and continues to pose a threat to this species in Canada. Drawing water for irrigation or other activities that lower the water table can render breeding habitats unsuitable. Roads can be a significant source of mortality when they bisect breeding sites or migration routes. Pesticides, herbicides and other environmental pollutants can be detrimental to frog and toad populations by causing direct mortality as well as developmental deformities. Pathogens, such as chytrid fungus and Ranavirus, can cause mass mortality of frog and toad populations. Climate change may also pose future threats to the Plains Spadefoot, particularly by increasing the frequency and severity of droughts.

Additional Information About This Species In Canada