Montana Field Guide

Plains Spadefoot - Spea bombifrons

Adult plains spadefoot are gray or brown with darker mottling on the back and white on the belly. The back may be covered with smallish tubercles tipped in yellow or orange, and often present as a rough hourglass-shaped marking. Some individuals have indistinct longitudinal streaking. In adults the pupils are vertically elongate in bright light; there is a hard lump or "boss" between the eyes, slightly anterior of an imaginary midline connecting the eyes. Prominent parotoid glands posterior to the eyes are absent. A single hard and dark wedge-shaped spade is present on each hind foot. Maximum snout-vent length (SVL) is about 6.0 centimeters. Males have dark patches on the inner 2-3 digits of the forelimbs during breeding, and have an expanded bi-lobed vocal sac. The male breeding call is a brief snore.

Tadpoles may be brown or green to whitish on the back, or mottled gray to dull olive-yellow, sometimes with a bluish iridescence. The belly is an iridescent golden color; the gut coil is not visible through the body wall. The dorsal fin is clear or with sparse yellowish flecking; the anus is at the base of the tail on the midline. The body shape is globular, with the eyes positioned dorsally, and total length is usually up to 7.0 centimeters. The mandibles are frequently cusped; labial tooth rows are 0/0 to 6/6, but most often 3/4 or 4/4. Oral papillae completely encircle the mouth. Eggs are black above and white below, about 1.5 to 1.6 millimeters in diameter and surrounded by two jelly layers, and deposited in elliptical masses of 10 to 250 eggs.

No other adult frog or toad in Montana has a combination of vertical pupils, bony "boss" between the eyes, large black tubercles or spades on the hind feet, and lack of prominent parotoid glands. No other tadpoles have a combination of a normal (not sucker-like) mouth completely surrounded by oral papillae, and a midline anal vent at the base of the tail. No other Montana amphibian has small (less than 4.0 millimeters) pigmented eggs surrounded in two jelly layers that are laid singly or in short linear or globular clusters. The Great Basin spadefoot (Scaphiopus inermontanus) is known from southeastern Idaho and may occur in southwestern Montana; it has a low and pliable lump between the eyes, while the plains spadefoot's lump is high and hard.

Montana Range

No information specific to Montana plains spadefoot is available. Elsewhere adults are known to migrate up to several hundred meters between breeding pools and nonbreeding terrestrial habitats. During breeding, they may move 60 to 150 meters during each night as they leave the breeding ponds and move inland (Hammerson 1999).

Little specific habitat information is available. This species is usually found in areas with soft sandy/gravelly soils near permanent or temporary bodies of water. For much of each year it lives largely inactively in burrows of its own construction or occupies rodent burrows, and enters water only to breed. Following heavy rains, adults have been reported in water up to 30 centimeters deep in flooded wagon wheel ruts, temporary rain pools formed in wide flat-bottom coulees, water tanks, and badland seep ponds, and tadpoles and toadlets have been observed in stock ponds and small ephemeral reservoirs, usually in sagebrush-grassland habitats (Cope 1879, Mosimann and Rabb 1952, Dood 1980, Reichel 1995, Hendricks 1999, Hossack et al. 2003).

Metamorphosed toads in Colorado eat various small terrestrial arthropods, including spiders, terrestrial amphipods, snails, earthworms, centipedes, and nine orders of insects, especially moths, caterpillars, and ground beetles (Whitaker et al. 1977), and rarely small vertebrates (Hammerson 1999). In the playa wetlands of northwest Texas, carabid beetles were the most common food (Anderson et al. 1999). Larvae eat suspended matter, organic debris, algae, and plant tissue. Large larvae may develop into predatory tadpoles that eat relatively large aquatic invertebrates, such as fairy shrimp and insect larvae, as well as other amphibian larvae, including their own species (Hammerson 1999). Food habits in Montana have not been studied.

Plains spadefoots are mostly active as adults at dusk and night when air temperatures are between 12 and 26 degrees C. Adults in Alberta, Wyoming, and Colorado may be active anytime during May through August and less frequently later (Baxter and Stone 1985, Russell and Bauer 1993, Klassen 1998, Hammerson 1999), depending on rains and the presence of standing water. In Montana, adults have been observed during May to August (Mosimann and Rabb 1952, Hendricks 1999, Hossack et al. 2003). When conditions are such that adults retreat underground, the spades on the hind feet are used to dig backwards into the soil until pockets of moist soil are encountered, sometimes at depths of almost a meter. Tadpoles may be able to tolerate brief periods of almost total evaporation of their pools (Russell and Bauer 1993).

Predators of adults and juveniles include birds (Swainson's Hawk, Burrowing Owl, Black-crowned Night-heron), and western rattlesnakes; tadpole predators include garter snakes, the tadpole shrimp (Triops), and cannibal spadefoot tadpoles (Baxter and Stone 1985, Hammerson 1999). Predators in Montana are not reported. Threatened adults inflate with air and squat with the head bent downward, and often release fluid from the vent if handled; large tadpoles may produce a clicking sound with their mouthparts if removed from water (Hammerson 1999).

Little specific information is available. Adult choruses have been heard from late May through early August. Tadpoles from legless to 4-legged stages have been reported during late June of the same year in Carbon County (Hendricks 1999), and fully transformed juveniles have been found in the same area during a different year in late August (P. Hendricks personal observation, Maxell et al. 2003). Recently transformed juveniles have been reported along the Missouri River in late August (Cope 1879).

Breeding occurs in Colorado after heavy rains of 1.8 centimeters or more at air temperatures greater than 10 degrees C. (Hammerson 1999). Breeding choruses usually last about 2 or 3 days, with adults calling from edges of the pools or while floating; large choruses can be heard from a distance of 3 kilometers or more. Females lay up to 2700 eggs during a single breeding event, often in several egg masses each attached to submerged vegetation or other objects. Adults usually leave the water once breeding and egg-laying are finished, but may remain nearby for several days if rain continues. Rains occurring after the first round of breeding may stimulate more breeding, presumably from individuals that did not breed the first time. Adults probably reach maturity when 2 to 3 years old. Iowa males are mature when 3.1 to 3.8 centimeters snout-vent length (SVL), females when 3.2 to 4.0 centimeters SVL; breeding adults in Colorado are typically 4.0 to 6.0 centimeters SVL (Hammerson 1999).

Eggs hatch in 2 to 3 days. Tadpoles develop rapidly, and complete metamorphosis in about 36 to 40 days (21 to 75 days) in Colorado (Hammerson 1999). In Alberta, time to metamorphosis was a minimum of 21 to 34 days and a maximum of more than 60 days (Klassen 1998). Transforming toadlets emerged from water may still have tails.

No special management needs are currently recognized. However, at permanent and semi-permanent water bodies (reservoirs and stock ponds) where breeding has been observed, portions of the shoreline where emergent vegetation might develop could be fenced to create exclosures that protect breeding adults, eggs and tadpoles from trampling and the removal of emergent cover by livestock; trampled juveniles have been found at some stock ponds (P. Hendricks personal observation). Another option would be the creation of ponds designed for use by prairie amphibians as breeding sites, with the perimeter surrounded by fencing to prevent access by livestock. Game fish should not be introduced to any of these ponds.