Bufo cognatus Say, 1823
Great Plains Toad
Brent M. Graves1
James J. Krupa2
1. Historical versus Current Distribution. Great Plains toads (Bufo cognatus) occur throughout the U.S. Great Plains, from western Minnesota and Iowa, northwestern Missouri and the western 2/3 of Kansas, Oklahoma, and Texas, west to the Imperial Valley of California and up the Colorado River though eastern Nevada, Colorado, and eastern Wyoming and Montana. Their range extends south to central Mexico and north into the southwestern corner of Manitoba, across southern Saskatchewan and the southeastern corner of Alberta (Wright and Wright, 1949; Dixon, 1987, 2000; Johnson, 1987; Krupa, 1990; Collins and Collins, 1993; Oldfield and Moriarty, 1994). Great Plains toads generally are found at elevations < 1,900 m, but are found from 2,286–2,438 m in the San Luis Valley of Colorado (Hammerson, 1986, 1999; Degenhardt et al., 1996; Luce et al., 1997). Lannoo et al. (1994) compared their results to Bailey and Bailey (1941) and documented an apparent eastward range expansion of Great Plains toads in northwestern Iowa. King (1932) anecdotally described Great Plains toads as the most common toad of the southwest, while more recent accounts describe them as less common and localized (Stebbins, 1985; J.J.K., personal observations). Similarly, Great Plains toads were described as having a widespread distribution in eastern Montana (Black, 1970b, 1971b), but were later described as locally distributed in that state with large gaps in the known range (Reichel and Flath, 1995, cited in James, 1998). Busby and Parmelee (1996) did not find Great Plains toads in their survey of the Fort Riley Military Reservation in Kansas in 1993, although they were reported to be present in 1927. It is not known whether these patterns reflect long-term changes in species distribution, short-term population fluctuations, or inconsistent and superficial population monitoring (James, 1998).
2. Historical versus Current Abundance. Unknown, but Great Plains toads are common in many portions of their range (J.J.K., personal observations). For example, they were one of the three most common anurans in playa wetlands in the high plains of Texas (Anderson et al., 1999a). In contrast, Luce et al. (1997) suggest they are uncommon in Wyoming. They were described as common in Alberta in 1984, but were recommended for Potentially Threatened status in 1987 and Endangered status in 1988 (James, 1998). They are considered rare in Saskatchewan (Seburn, 1992) and in southwestern Minnesota (Lehtinen et al., 1999). Population densities are known to fluctuate widely in association with periodic droughts. Reproduction may not occur during years with insufficient rainfall (Sullivan and Fernandez, 1999). Furthermore, Great Plains toads are largely fossorial during times of inactivity, making them difficult to detect except during breeding periods (James, 1998).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Great Plains toads emerge from burrows below the frostline following heavy spring rains and move to wetland breeding areas (Ewert, 1969). Individuals may move > 1 km from breeding ponds (James, 1998), and mass, unidirectional migrations have been reported (Smith and Bragg, 1949; Bragg and Brooks, 1958), although it is not known if such movements are associated with feeding or reproduction. They exhibit an explosive breeding pattern (Wells, 1977; Sullivan, 1989a), but females have large follicles and reproduction may occur throughout most of the active season (Bragg and Smith, 1942; Long, 1987b; Krupa, 1989). Additionally, individual females may produce multiple clutches in a single year (Krupa, 1986a), suggesting that reproduction may be prolonged if conditions are appropriate. Reproductive activity is initiated by rainfall. The amount of rainfall necessary to trigger breeding averaged 4.4 cm in Oklahoma (Krupa, 1994), although chorus activation can be independent of rainfall (Brown and Pierce, 1967). Large choruses of 200–500 individuals have been recorded (Brown and Pierce, 1967; Degenhardt et al., 1996). Males call from vegetated shorelines to attract females or position themselves near other calling males to intercept females (satellite behavior; Sullivan, 1983; Krupa, 1989).
ii. Breeding habitat. Great Plains toads breed from March–September throughout their range and May–July in the north (Bragg 1937b, 1940a; Wright and Wright, 1949; Krupa, 1986b, 1994). Timing of reproduction is associated with the availability of temporary aquatic breeding habitat (Krupa, 1994), although breeding occurs in both temporary and permanent ponds (Woodward, 1987a). Brown and Pierce (1967) reported the highest intensity choruses in temporary floodings, with lower intensity choruses in apparently permanent ponds and the lowest intensity choruses in temporary lotic habitats. Other researchers have reported that Great Plains toads breed primarily in shallow temporary pools (Mackay et al., 1990; Krupa, 1994; Luce et al., 1997; Fischer et al., 1999). Some authors have emphasized that breeding will not occur in muddy, turbid habitats (Bragg and Smith, 1942, 1943; James, 1998). In Alberta, floodings produced by irrigation, as well as natural floodings, were used as breeding sites, but all were in regions of native vegetation rather than cultivated areas (Wershler and Smith, 1992, cited in James, 1998).
i. Egg deposition sites. Egg laying usually begins at dawn and is completed before noon, although oviposition can occur at other times (Krupa, 1994). Eggs are usually deposited in temporary floodings having clear water with little current (see "Breeding habitat" above). Breeding females may lay eggs in close proximity to each other (Krupa, 1994). Eggs in natural conditions normally hatch in 2–7 d (Hammerson, 1986; Johnson, 1987; Collins and Collins, 1993; Krupa, 1994; Luce et al., 1997). Eggs from a population in Maricopa County, Arizona, maintained under laboratory conditions hatched in 6–13 d at temperatures from 15.9–20.8 ˚C, in 4–5 d from 21.6–25.7 ˚C, and in 3 d at temperatures between 29.4–34.5 ˚C. Development was incomplete or did not occur at all at temperatures < 13.6 ˚C and > 39.1 ˚C. Water temperatures at natural breeding sites were from 15–28 ˚C (Ballinger and McKinney, 1966). Embryos at the mid gastrulation stage survived 6 h at 40.5 ˚C.
ii. Clutch size. Mean clutch size for 27 females in Oklahoma was 11,074, with a range of 1,342–45,054. Clutch size was positively and exponentially related to female size. Females are capable of laying multiple clutches over the course of a breeding season (Krupa, 1986a). Krupa (1994) reported that only 6.8% of males and 7.5% of females were captured at breeding ponds in multiple years of a 3-yr study. This suggests that either mortality is high, individuals move to new breeding habitats frequently, or reproduction occurs less frequently than annually. In Krupa's (1988) study, fertilization rate in nature was 89%. Mean egg diameter is 1.18 mm (Bragg, in Wright and Wright, 1949). Egg strings are usually single, sometimes double.
i. Length of larval stage. Tadpoles begin to metamorphose from 17–45 d following hatching (Bragg, 1937a; Hammerson, 1986; Degenhardt et al., 1996; Luce et al., 1997), at about 10 mm SVL (Johnson, 1987; Degenhardt et al., 1996). The larval period is shorter for clutches laid later in the spring or summer, probably as a result of higher water temperatures. However, higher temperatures increase evaporation and desiccation of temporary ponds reduces the area of available habitat. This increases larval density, which slows developmental rates (Krupa, 1994). Density dependent, chemically mediated growth suppression (Semlitsch and Caldwell, 1982) may also occur.
ii. Larval requirements. Temporary ponds are preferred because of the absence of many aquatic predators such as fish, and lotic habitats are extremely ephemeral in arid habitats. Desiccation of aquatic habitats either kills tadpoles outright or makes them more vulnerable to predation and may be the primary cause of larval mortality. Warmer water temperatures speed larval development, but increase the rate that ephemeral aquatic habitats evaporate. Low larval density enhances growth and development (Krupa, 1994).
a. Food. Great Plains toad tadpoles are similar to other bufonids in being suspension feeders that graze on organic and inorganic material associated with the submerged surfaces of plants, rocks, and other substrates. They are reported to feed on algae and decomposing invertebrate remains (Bragg, 1940a).
b. Cover. Great Plains toad tadpoles do not appear to be influenced by the use of cover. In wetlands, they tend to space themselves evenly or haphazardly, except to aggregate in warm, shallow waters in full sun (J.J.K., personal observations).
iii. Larval polymorphisms. Do not occur.
iv. Features of metamorphosis. Krupa (1986b, 1994) notes that the survival rate of tadpoles in natural ponds is so low that the emergence of newly metamorphosed animals is a rare event. However, large numbers of newly metamorphosed Great Plains toads have been reported (Bragg and Brooks, 1958; Graves et al., 1993), supporting the hypothesis of great year-to-year and geographic variation in reproductive success. Metamorphosis may be synchronous (Krupa, 1986b, 1994).
v. Post-metamorphic migrations. Newly metamorphosed toads remain near the natal pond for about 1 mo or until it dries. They commonly move in large numbers into agricultural fields where they find it easier to burrow into the soil (Smith and Bragg, 1949). Post-metamorphic Great Plains toads will form aggregations (Bragg and Brooks, 1958; Graves et al., 1993). Individuals are apparently attracted to each other, and this attraction is chemically mediated (Graves et al., 1993). Ewert (1969) found two first-year toads 1 km from the nearest oviposition site.
D. Juvenile Habitat. While adults are largely nocturnal, juveniles are active diurnally and remain near natal ponds. If pools dry while juveniles are small (30–35 mm), toads may move to agricultural fields.
E. Adult Habitat. Great Plains toads are found in habitats including short grass to tallgrass prairies, sandhills, desert mesquite, and desert scrub (Wright and Wright, 1949; Ewert, 1969; Stebbins, 1985; Lehtinen et al., 1999). Degenhardt et al. (1996) report that Great Plains toads are generally found in warmer grassland areas, rarely in upland woodlands. They are associated with temporary ponds, irrigation ditches, and bottom lands (Wright and Wright, 1949; Stebbins, 1985; Hammerson, 1986; Krupa, 1990; Fischer et al., 1999). Great Plains toads tolerate agriculture and drier conditions better than most bufonids (Degenhardt et al., 1996). They are also tolerant of urban conditions (Krupa, 1994). They are nocturnal, proficient burrowers (Ewert, 1969; Lannoo, 1996; James, 1998) and commonly form shallow superficial burrows that they occupy for 1–6 d (Ewert, 1969). They also form deeper burrows in the shape of an inverted question mark, with the toad positioned at the upper, terminal end (Tihen, 1937; in Collins and Collins, 1993). Ewert (1969) suggests that woods and cattail marshes are avoided. Mean preferred body temperature is 27.7 ˚C (Sievert, 1991), and ambient temperatures of 40 ˚C are lethal in < 40 min (Schmid, 1965a).
F. Home Range Size. May not establish home ranges, but some individuals return to the same breeding pond for many years (J.J.K., personal observations), and individuals may return to specific overwintering sites (Ewert, 1969). Great Plains toads in Minnesota moved about 462 m from overwintering sites to breeding sites and about 308 m from breeding sites to foraging areas (Ewert, 1969). Early summer daily movement distances were 0–3 m for 43% of observations, 3–31 m for 29% of observations, and > 31 m for 29% of observations. One animal moved 815 m in 1 d. During late summer and fall, the movements in these three distance categories were 69%, 7%, and 24%, respectively. Individuals tend to move gradually but directly from summer foraging areas toward overwintering sites, stopping to forage and remaining in shallow burrows (i.e., “forms”) for several days at a time. Hence, home ranges were described as distances (averaging at least 615 m with few < 308 m) rather than areas (Ewert, 1969).
G. Territories. Unknown.
H. Aestivation/Avoiding Dessication. Commonly use forms < 5 cm deep during short periods of inactivity (1–6 d; Ewert, 1969). Will aestivate deeper and for longer periods of time during hot, dry weather (Fischer et al., 1999). Ewert (1969) observed two toads dig to depths of 15 and 55 cm in July and August, respectively. The first remained burrowed for 14 d, while the second remained burrowed through the winter. Great Plains toads survive the loss of 42.9% ± 1.0% (mean ± SE) of initial body weight due to dehydration (Hillman, 1980). They absorb water from the soil at a soil moisture tension of 2.5 atmospheres (Walker and Whitford, 1970). They survive temperatures of 40 ˚C for 40 min, 43.5 ˚C for only 5 min (Schmid, 1965a).
I. Seasonal Migrations. Movements from overwintering sites to breeding sites were at least 462 m (Ewert, 1969). Following breeding, Great Plains toads moved to feeding sites from 300–1,300 m away from wetlands (Ewert, 1969; Fischer et al., 1999). Toads moved from 100–1,100 m between breeding sites and overwintering sites (Ewert, 1969).
J. Torpor (Hibernation). As with all northern bufonids, Great Plains toads are not freeze tolerant (Swanson et al., 1996) and overwinter by burrowing below the frostline (Collins and Collins, 1993; Irwin, 2003). Toads entered overwintering burrows between 10 August–16 September in Minnesota (Ewert, 1969). Sixteen toads overwintered in roadbanks and six survived the winter, while two overwintered in grasslands and both died. Those that overwintered in the road bank did so near the crest. A preference for elevated overwintering sites may be general, but these habitats are limited. Toads that survived burrowed to maximum depths of 74–104 cm. Toads move up in the soil to avoid rising groundwater (Ewert, 1969), but have physiological responses to aid survival during short periods of anoxia (Armentrout and Rose, 1971). Individual toads spend 63–77% of the year in dormancy (Ewert, 1969). Spring emergence and breeding are triggered by heavy rains (Fischer et al., 1999), although Brown and Pierce (1967) found breeding to occur independent of rainfall, and Ewert (1969) observed emergence from hibernation to occur over at least a 5-wk period.
K. Interspecific Associations/Exclusions. In Arizona, Great Plains toads are found in watering troughs with Colorado River toads (Bufo alvarius) and are found calling on pond banks in association with spadefoot toads (Scaphiopus sp.; A.I. and R.D. Ortenburger, in Wright and Wright, 1949). In an Arizona playa, they breed concurrently with New Mexico spadefoot toads (S. multiplicatus; MacKay et al., 1990). In Utah, Great Plains toads breed in association with Woodhouse's toads (B. woodhousii; V.M. Tanner, in Wright and Wright, 1949). In Colorado and Iowa, they breed in association with plains spadefoot toads (Spea bombifrons), plains leopard frogs (Rana blairi), Woodhouse's toads (Bufo woodhousii), and striped chorus frogs (Pseudacris triseriata). In Oklahoma, Great Plains toads are found in association with tiger salamanders (Ambystoma tigrinum), western narrow-mouthed toads (Gastrophryne olivacea), spotted chorus frogs (Pseudacris clarkii), and Strecker's chorus frogs (P. streckeri) in early spring (J.J.K., personal observations). In South Dakota, Great Plains toads, Woodhouse’s toads, chorus frogs, and plains spadefoot toads will breed concurrently in the same ponds (Flowers and Graves, 1995; B.M.G., personal observations). Great Plains toads hybridize with American toads (B. americanus), Woodhouse's toads (Collins and Collins, 1993; Gergus et al., 1999), red-spotted toads (B. punctatus; Sullivan, 1990; Degenhardt et al., 1996), Texas toads (B. speciosus; Rogers, 1973), Canadian toads (B. hemiophrys; Brown and Ewert, 1971), and Colorado River toads (Gergus et al., 1999).
L. Age/Size at Reproductive Maturity. Males range from 47–103 mm SVL, females from 49–115 mm (Wright and Wright, 1949; Krupa, 1990, 1994; Collins and Collins, 1993). Hammerson (1986) reported that Great Plains toads begin to breed when 2–5 yr old. In Arizona, reproductive maturity for both sexes is achieved at 2 yr (Sullivan and Fernandez, 1999). Both Krupa (1994) and Degenhardt et al. (1996) note that males first breed at 56 mm SVL, females at about 60 mm. Great Plains toads in the higher altitudes of the San Luis Valley, Colorado, breed at smaller sizes than lowland animals (Hammerson, 1986).
M. Longevity. Great Plains toads commonly live 10 yr, perhaps 20 (James, 1998; J.J.K., personal observations). The captive longevity record is 10 yr, 8 mo, 10 d (Snider and Bowler, 1992; Collins and Collins, 1993). The oldest individuals in a Sonoran Desert population were 6 yr, as determined by skeletochronology (Sullivan and Fernandez, 1999).
N. Feeding Behavior. Adults feed primarily at night, feeding on arthropods almost exclusively (Smith and Bragg, 1949). Primary prey include insects such as lepidopterans, dipterans, hymenopterans, and coleopterans, as well as other small invertebrates including centipedes and mites (Smith and Bragg, 1949; Dimmitt and Ruibal, 1980b; Hammerson, 1986; Luce et al., 1997). Ants and termites (Hymenoptera) predominated in the diets of animals examined by Dimmitt and Ruibal (1980b), whereas coleopterans were the most common prey of toads in New Mexico (Anderson et al., 1999b) and Oklahoma (Smith and Bragg, 1949). Great Plains toads forage daily in surrounding habitat, then return to agricultural fields to burrow (Smith and Bragg, 1949). Post-metamorphic toads feed day and night during the first month after metamorphosis, and feed almost exclusively on arthropods (only 1 out of 1,412 stomach content items was an annelid; Smith and Bragg, 1949). In Oklahoma, juveniles fed on acarinans (mites), hymenopterans, and coleopterans (beetles), with a decrease in mites and an increase in hymenopterans in the first month of growth (Smith and Bragg, 1949). In South Dakota, collembolans (springtails) and mites were eaten after metamorphosis, with a shift in preference to coleopterans and hymenopterans within 1 mo (Smith and Bragg, 1949; Flowers and Graves, 1995).
O. Predators. Adults are eaten by larger mammals, birds, and snakes, including badgers, skunks (Mustelidae), opossums (Didelphis marsupialis; Jense and Linder, 1970; J.J.K., personal observations), western hog-nosed snakes (Heterodon nasicus; Wershler and Smith, 1992, cited in James, 1998; J.J.K., personal observations), plains garter snakes (Thamnophis radix; Flowers and Graves, 1997), and crows (Bragg, 1940a). Larvae are eaten by birds, insect larvae, and spadefoot toad (Scaphiopus spp.) tadpoles (Bragg, 1940a). Woodward (1983) found that tadpoles in permanent ponds suffered higher predation rates than those in temporary ponds. Ewert (1969) reported that 3 of 21 road crossings by Great Plains toads resulted in fatal interactions with automobiles.
P. Anti-Predator Mechanisms. Nocturnal activity patterns, cryptic coloration, and parotoid gland secretions reduce predation. Great Plains toads inflate when attacked by western hog-nosed snakes (J.J.K., personal observations). Juveniles avoid areas containing chemical cues from plains garter snakes (Flowers and Graves, 1997).
Q. Diseases. Mortality from “red-leg disease” (i.e., Pseudomonas bacterial infection) is known (Ewert, 1969). Cutaneous, hepatic, respiratory, and intestinal infections with Mycobacterium marinum can be fatal (Shively et al., 1981).
While reports of malformations have appeared with some regularity in recent years among many anuran species (Lannoo, 2000), malformations were not reported for Great Plains toads in four recent surveys conducted within the species' range (Converse et al., 2000; Helgen et al., 2000; Johnson et al., 2002; Lannoo et al., in press).
R. Parasites. Nematodes and cestodes may occur in the gastrointestinal tract and lungs (Goldberg and Bursey, 1991a; Goldberg et al., 1995). Ulmer (1970) examined two Great Plains toads from Iowa, and found no trematodes; Ulmer and James (1976) examined four Great Plains toads from Iowa and found no cestodes.
4. Conservation. Great Plains toads are fossorial and difficult to monitor except during their explosive breeding bouts. In some portions of their range, populations may be scattered and isolated compared to the historical condition. There is at least one report of a range expansion to the east (see "Historical versus Current Distribution" above). It is not known whether these observations reflect long-term changes in species status, short-term population fluctuations, or inconsistent and superficial population monitoring. They currently receive no federal or state protection in the United States.
1Brent M. Graves
Department of Biology
Northern Michigan University
Marquette, Michigan 49855
2James J. Krupa
University of Kentucky
School of Biological Sciences
Lexington, Kentucky 40506
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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