Great Plains Toad
Anaxyrus cognatus (Say in James 1822)
Larvae. Tadpoles are initially black on the dorsum with pale or gold flecking, then become paler and mottled brown with age; the dorsal pattern of large, paired blotches appears before metamorphosis is complete. The eyes are dorsal, and the dorsal fin is highly arched with some black dendritic lines. The upper mandible is highly arched, and labial tooth rows are usually 2/3, with oral papillae restricted to the sides of the mouth. Total length ranges from 25 to 35 millimeters.
Great Plains Toads in the higher altitudes of the San Luis Valley, Colorado, breed at smaller sizes than lowland animals (Hammerson, 1986) this may hold true for other high-altitude populations. Longevity for this species maybe ten or twenty years (James, 1998). The oldest individuals in a Sonoran Desert population were determined to be six years old by using skeletochronology (Sullivan and Fernandez, 1999).
Distribution and habitat. They range from sea level to 3,900 m ASL and are abundant in many areas of Arizona and can be found throughout most of the state. 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). In New Mexico, Degenhardt et al. (1996) report they are generally found in warmer grassland areas, rarely in upland woodlands. Most often they are associated with temporary ponds, irrigation ditches, and flood plains (Wright and Wright 1949, Stebbins 1985, Hammerson 1986, Krupa 1990, Fischer et al. 1999). They seem to tolerate agriculture and drier conditions better than other bufonids (Degenhardt et al., 1996). Urban habitats are also exploited (Krupa, 1994).
The Great Plains Toads forage in the habitat surrounding their burrows. Feeding is nocturnal, and the Biology is almost exclusively arthropods. The Biology of a New Mexico population was predominately flies, moths, beetles, true bugs, centipedes, mites, ants, and termites. Predators of adult toads include badgers, skunks, opossums, crows, hog-nosed snakes, and gartersnakes. Larvae are prey for birds, aquatic insect larvae, crayfish, and spadefoot tadpoles. As might be expected, tadpoles in permanent ponds suffer higher predation rates than those in temporary ponds, the result of more aquatic predators being present in permanent bodies of water.
Like other northern bufonids, Great Plains Toads are not freeze tolerant (Swanson et al. 1996) and survive the winter by spending it below the frostline (Collins and Collins 1993; Irwin 2003). Overwintering may start as early as 10 August to 16 September in Minnesota (Ewert 1969). A study done on over-wintering toad found they preferred sites that were elevated for overwintering and toads that survived the winter burrowed to maximum depths of 74–104 cm. When ground water rises the toads move up in the soil to avoid rising groundwater (Ewert, 1969), and they are to survival during short periods of anoxia (Armentrout and Rose,1971) as the water rises. Individual toads spend 63–77% of the year in dormancy (Ewert 1969).
Great Plains Toads are common in many portions of their range while in other areas they are rare. During a Texas study the species were one of the three most common anurans while in Wyoming they were uncommon (Luce et al. 1997; Anderson et al., 1999a) Populations can some time quickly change their abundance , the were common in Alberta Canada in 1984, but proposed as threatened three years later and endangered a year later 1988 (James 1998). In Pima County Arizona, this author never found them to be common but, in 2019 the monsoon failed, and none were observed compared to the previous years when dozens of specimens were observed.
Shallow burrows less than five centimeters deep are used during short periods of inactivity – one to six days (Ewert 1969). During extended cold, heat, and droughts they go deeper (Fischer et al. 1999) and the burrows are often shaped like a question mark with the toad at the bottom (Tihen 1937; in Collins and Collins 1993). Toads burrowing to 55 cm was reported by Ewert (1969). Great Plains Toads can survive the loss of 42.9% mean body mass due to dehydration (Hillman 1980). While in a burrow they can obtain water by absorption from the soil (Walker and Whitford 1970). High temperatures are lethal, Schmid (1965a) report this could survive temperatures of 40⁰C for forty minutes 43.5⁰C for only five minutes. Locations in Arizona have temperatures that often exceed 40⁰ C during the day.
This toad’s emergence from burrows is triggered by heavy rains and move to wetland breeding areas. Krupa (1994) noted 4.4 cm of rain was need in Oklahoma to trigger reproduction but male choruses may be independent of rainfall (Brown and Pierce, 1967). Individuals have been reported to move more than a kilometer to breeding sites, and mass, unidirectional migrations have been reported (Smith and Bragg 1949; Bragg and Brooks 1958; Ewert 1969; James 1998).
Feeding is nocturnal. The diet is mostly arthropods (Smith and Bragg, 1949). Studies done to date suggest prey includes insects (lepidopterans, dipterans, hymenopterans, and coleopterans), centipedes and mites (Smith and Bragg, 1949; Dimmitt and Ruibal, 1980b; Hammerson 1986; Luce et al. 1997). Ants and termites predominated in the diet (Dimmitt and Ruibal 1980b); beetles were the most common prey in New Mexico and Oklahoma studies (Anderson et al., 1999b; Smith and Bragg, 1949). When using burrows in agricultural fields they will forage in surrounding habitat, returning to agricultural fields to burrow (Smith and Bragg, 1949). Toadlets feed day and night during the first month after metamorphosis, and feed almost exclusively on arthropods. In Oklahoma, juveniles fed on mites, true bugs, and beetles, with a decrease in mites and an increase in hymenopterans in the first month of growth (Smith and Bragg, 1949). In South Dakota, Springtails and mites were eaten after metamorphosis, followed by a shift to beetles and hymenopterans with a month of growth (Smith and Bragg, 1949; Flowers and Graves, 1995).
It occurs in Semidesert Grasslands and Plains and Great Basin Grasslands, (Holycross and Brennan 2006).
Reproduction. In Arizona, reproductive maturity for both sexes occur in the second year after metamorphosis (Sullivan and Fernandez, 1999). In arid landscapes they are an explosive breeder (Wells, 1977; Sullivan, 1989a). Large follicles in females suggests reproduction may occur throughout most of the active season with some females possibly producing multiple clutches per season (Bragg and Smith 1942; Long 1987b; Krupa 1989). Choruses of 200–500 individuals have been recorded (Brown and Pierce, 1967; Degenhardt et al. 1996) but more often the choruses are smaller, consisting of six to ten males. Males usually position themselves along vegetated shorelines to attract females. However, males will also take calling stations that are away from shorelines when the water is relatively shallow. Some males position themselves near other calling males to intercept females (satellite behavior; Sullivan 1983; Krupa 1989).
Eggs are black above, white below, and about 1.2 to 1.3 millimeters in diameter, usually in a single row in long strings of two-layered jelly that is constricted between individual eggs (Bragg 1973; Livezey and Wright 1974)..
As reported by previous authors, Goldberg (2018) found the mean SUL of females was significantly larger than that of males. He found two stages were present in the spawning cycle: (1) females that were ready to spawn with mature oocytes and; (2) females not in spawning condition, they contained mostly early diplotene oocytes. The presence of ready to spawn females before commencement of the summer monsoon season suggested a readiness for females to breed as soon as rainfall begins. The non-spawning females from August and September contained predominantly early diplotene oocytes. Their presence in the August and September samples indicate the spawning cycle was concluded for the year. The smallest mature A. cognatus measured 61 mm SUL and was collected in July. Reproduction occurs between April to September in southern states versus May to July in northern states. Goldberg’s data from southern Arizona fits the monthly sequence for southern states. Krupa (1986) provided information of multiple clutch production based on indirect observations. Additional evidence for multiple clutching in A. cognatus would come from the presence of postovulatory follicles (previous clutch) and mature oocytes (subsequent spawning) in the same ovary.
Breeding habitats include both temporary and permanent ponds (Woodward, 1987). Chorus intensity increases in areas with temporary flooding’s, while lower intensity choruses are associated with permanent ponds and the lowest intensity choruses in temporary, stagnant-water habitats Brown and Pierce (1967).
Turbid water may deter toads from reproducing under some circumstances (Bragg and Smith 1942, 1943; James 1998). Temporary ponds lack many aquatic predators but are extremely ephemeral in arid habitats. Warm water speed larval development but increase the rate that ephemeral aquatic habitats evaporate. Desiccation of breeding pools may kill tadpoles or makes them more vulnerable to predation. Low density of larvae enhances growth and development (Krupa 1994). The tadpoles are 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).
Tadpoles initiate metamorphose 17–45 days following hatching (Bragg 1937a; Hammerson 1986; Degenhardt et al. 1996; Luce et al. 1997), at about 10 mm SUL (Johnson 1987; Degenhardt et al. 1996). Thus, the larval period is shorter for clutches laid when higher water temperatures are more likely. Higher temperatures, however, increase evaporation and desiccation rates of temporary ponds and reduce the area and volume of available habitat. This increases larval density, slowing developmental (Krupa 1994) this is due to molecules produced by the tadpole and suppress the growth of other tadpoles (Semlitsch and Caldwell 1982).
Egg laying often starts in the morning with sunrise and continues until noon. Females may lay eggs near each other’s clutches (Krupa, 1994). Hatching requires two to seven days (Hammerson 1986; Johnson 1987; Collins and Collins 1993; Krupa, 1994; Luce et al., 1997). Incubation times vary with water temperature. Eggs from a population in Maricopa County, Arizona, maintained under laboratory conditions hatched in six to thirteen days at temperatures between 15.9–20.8 ˚C, in four to five days between 21.6–25.7 ˚C, and in three days at temperatures between 29.4–34.5 ˚C. Development was incomplete or did not occur at all at temperatures less than 13.6 ˚C and more than 39.1 ˚C. Water temperatures at natural breeding sites were from 15–28 ˚C (Ballinger and McKinney 1966).
Mean clutch size for 27 females from Oklahoma was 11,074, with a range of 1,342–45,054 eggs. Clutch size is positively and exponentially related to female body size. Females can produce multiple clutches over the course of a breeding season. A three-year study found only 6.8% of males and 7.5% of females were captured at breeding ponds in multiple years (Krupa 1994) suggesting a high mortality, individuals do not have site fidelity or females do not reproduce every year. In one study the fertilization rate in nature was 89% (Krupa 1988). Mean egg diameter is 1.18 mm (Bragg, in Wright and Wright 1949). Egg strings are usually single, sometimes double.
Large numbers of metamorphosed Great Plains Toads have been reported at various sites suggesting that metamorphosis is synchronized. (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).
Newly metamorphosed toads (about 30-35 mm SUL) stay near the natal pond for about a month or until it dries. They them move in large numbers into agricultural fields where burrow construction is easily accomplished (Smith and Bragg 1949). Post-metamorphic Great Plains toads will aggregate (Bragg and Brooks 1958; Graves et al. 1993). Individuals are apparently chemically attracted to each other (Graves et al. 1993). Two first-year toads were found a kilometer from the nearest oviposition site (Ewert 1969).
This species may not establish a home range, but some individuals return to the same breeding pond for many years and individuals may return to specific overwintering sites (Ewert 1969). Great Plains Toads moved 462 m between breeding sites and over-wintering sites and 308 m from breeding sites to foraging sites in one study in Minnesota (Ewert 1969). One individual moved 815 m in a single day. Individuals tend to move slowly but directly from summer foraging areas to overwintering sites, stopping to forage and take refuge in shallow burrows for several days at a time.
In Arizona, Great Plains Toads are found in watering troughs, ephemeral ponds, roadside ditches with Sonoran Desert Toads and are found calling on pond banks in association with Couch’s Spadefoot and Green Toads.
Hybrids. This species has been reported to hybridize in nature with A. hemiophrys (Brown and Ewert 1971; A. woodhousii (Gergus et al. 1999). Laboratory hybrids have been produced with at least eight other bufonid species.
Predators on these toads are numerous and include mammals (badgers, skunks, opossums), birds (crows), and snakes (Western Hog-nosed Snakes, Plains Garter Snakes) (Bragg 1940a). The tadpoles are eaten by birds, aquatic insect larvae, and spadefoot tadpoles (Bragg 1940a). Their defense system against predation includes nocturnal activity, cryptic coloration, and toxic parotoid gland secretions. The toads will also inflate their body when attacked. Juveniles avoid areas containing chemical cues from Plains Garter Snakes (Flowers and Graves 1997).
Parasites. Mortality from red-leg disease is known (Ewert, 1969). Cutaneous, hepatic, respiratory, and intestinal infections with Mycobacterium marinum can be fatal (Shively et al., 1981). Bd was not found in this species in New Mexico (Christman and Jennings, 2018).
Conservation. This is a species considered to be of Least Concern. Because much of their life is fossorial Great Plains toads are difficult to survey except during their time on the surface when reproducing. They currently have no federal or state protection in the United States.
Taxonomy. Bufo cognatus was described by Say in James (1822:55) based upon a holotype reported to be in the “Philadelphia Museum.” The specimen was apparently lost or destroyed (Kellogg 1932, Bull. U.S. Natl. Mus., 160: 43). The type locality was given as, “The alluvial margins of the [Arkansas] river,” Prowers County, Colorado, USA. The type locality was corrected by Dundee (1996:83) to “3 miles west of Holly, Prowers County”, Colorado, USA. Cope (1862:358) called this toad, Chilophryne cognata; later Cope (1863:50) placed it in the genus Incilius and called it Incilius cognatus. He (Cope 1875:29) moved it back to Bufo and considered it Bufo lentiginosus cognatus. Cope (1879:437) described Bufo dipternus based on syntypes ANSP 19769–71, according to Malnate, (1971) and he apparently report their loss is in error (Smith and Taylor, 1948:41). The type locality for B. dipternus was “On the plains…of northern Montana … north of the Missouri river east of Fort Benton”. The type locality was restricted to “Fort Benton, Chouteau County”, Montana, USA by Smith and Taylor (1950:358). Schmidt (1953) reported it as “North of Missouri River, east of Fort Benton, Chouteau County, Montana.” Cope (1886:23) placed B. dipternus in the synonymy of B. cognatus. Camp (1915:331) recognized Bufo cognatus cognatus. Frost et al., 2006:363 move it to Anaxyrus, and called it A. cognatus Bufo (Anaxyrus) cognatus.