Plains Spadefoot (Spea bombifrons)
Plains Spadefoot, Apache Co., AZ. Photo by Jim Rorabaugh
The Plains Spadefoot is a moderate-sized anuran (< 64 mm SVL) with a blunt, rounded snout, large eyes with vertical pupils, and a dark, wedge-shaped tubercle on the underside of each hind foot. The tympanum is visible but small, less than half the size of the eye. Cranial crests and parotoid glands are absent. The dorsal color ranges from light gray to brown, sometimes with an olive or reddish tint, and usually with irregular dark markings. Light stripes may be present on the back and sides; the former may curve outward, forming an hourglass shape. A fleshy boss (raised area) lies between the eyes, but it may be indistinct or difficult to discern unless specimens of species, such as the Mexican Spadefoot, which lack the boss, are available for comparison. The Mexican Spadefoot is very similar in appearance to the Plains Spadefoot, but it smells like peanuts when handled, it is less likely to have light stripes or the hourglass figure on the dorsum, and as mentioned, it lacks the boss between the eyes. The calls are also very different. The advertisement call of the Plains Spadefoot is a quack or bleat repeated at intervals of 0.5 to 1.0 seconds, whereas the call of the Mexican Spadefoot is akin to the sound made by running a finger along the stiff teeth of a comb. In the 100-Mile Circle, the Plains Spadefoot occurs only in semi-desert grassland in the Sulphur Springs Valley. In Arizona, it also occurs elsewhere around the Chiricahua Mountains as well as on the Colorado Plateau in the northeastern portion of the state. Mexican and Plains Spadefoots occasionally hybridize. Hybrids can be recognized by their odd, intermediate advertisement calls.
This species is an explosive breeder, emerging from the ground soon after the first substantial summer storm to breed and feed at temporary rain pools. In most breeding aggregations, 90-100% of the breeding occurs in one night. Males typically call from the water’s edge or while floating in the pond. Females lay a clutch of up to 2,000 eggs that is deposited in masses of 10-250 eggs. The eggs hatch in a day or two and metamorphosis occurs in 2-4 weeks. Late stage tadpoles have relatively short tails, the eyes are oriented high on the head, and the large, blocky bodies are light in color with small dark markings and often golden flecks. They grow to about 48 mm total length prior to metamorphosis. Metamorphs resemble adults but measure about 19-21 mm SVL. Males and females reach maturity at about 38 and 40 mm SVL, respectively.
Tadpoles occur as both herbivorous and carnivorous morphs. The carnivores have more robust jaws and jaw musculature, they develop more rapidly, and metamorphose at a larger size than herbivorous morphs. The carnivorous morphs are often cannibalistic on other tadpoles. Metamorphosed spadefoots consume a variety of arthropods.
Skin secretions of this species can cause sneezing and a runny nose. The Plains Spadefoot is listed as a species of least concern on the IUCN’s Red List. There is no indication it has declined in Arizona, although it is vulnerable to development, including agriculture. However, it has benefited from the construction of ephemeral livestock tanks and other intended or unintended places that collect rain water, such as roadside ditches.
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Dimmitt, M.A. 1992. Biology of desert toads with emphasis on spadefoots. Pages 51-54 in Collected Papers of the Tucson Herpetological Society 1988-1991.
Farrar, E., and J. Hey. 2005. Spea bombifrons Cope, 1863 Plains Spadefoot. Pages 513-514 in M.J. Lannoo (editor), Amphibian Declines: The Conservation Status of United States Species. University of California Press, Berkeley.
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Author: Jim Rorabaugh