The Plains Spadefoot

Spea bombifrons Cope 1863

Adult males reach 64 mm SUL; adult females reach 62 mm SUL. Body size is sexually dimorphic in some populations but not in others, usually with larger females. Its skin is mostly smooth and can have a gray, brown, or green tint with scattered dark blotches. Four indistinct stripes on the dorsum and lateral surfaces are usually present, and the middle two may form an hourglass shape. Tadpoles are tan, brown, or gray and have a transparent quality.  The eyes are dorsal. It is a plain-looking anuran with a distribution that does not overlap with other species of spadefoots.  The bottom of each rear foot has a wedge-shaped spade. As with many other anurans, tadpoles may exceed the size of the adult body length.  Tadpoles can reach 65 mm in total length but often metamorphose at 41 to 52 mm at 13 to 14 days of age. The call is a snore-like rasping. See the box on the call variation found in this species. The Plains Spadefoot differs from other Arizona spadefoots by having a bony boss (raised area) between the eyes and dorsolateral stripes.  It has a gray-brown or green-brown dorsum, with irregular dark markings; the belly is white.  Its skin has some tubercles.  Its eyes have a golden iris and elliptical pupils; the eyelid width is greater than the space between them. The bottom of each rear foot has a wedge-shaped spade.  Breeding males have keratinized nuptial pads on their first three digits.  Superficially it is most like the Chihuahuan Desert Spadefoot, with which it hybridizes in the Four Corners area of Arizona as well as in southeastern Arizona.  These hybrids may have a slight bony boss between the eyes.

Distribution and Habitat. The species occurs from Alberta, Manitoba, and Saskatchewan in Canada, as far east as Missouri and eastern Oklahoma, and southward to northwest Texas, Chihuahua, and Tamaulipas, Mexico.  Isolated populations occur in South Texas and Mexico.  It enters Arizona in the northeast and southeast corners of the state.  It is associated with loose, well-drained soils. The associated vegetation ranges from mixed-grass prairie, shrubland, and floodplains to desert grasslands. It occurs in ponderosa pine forests in the vicinity of Flagstaff and may reach elevations of 2134 m in Arizona (Aitchison and Tomoko 1974). Adults and juveniles use underground refugia and emerged at night to forage for food. Individuals will dig their burrows, but they will also use the tunnels of the Black-tailed Prairie Dog, gophers, ground squirrels (Kretzer and Cully 2001; Lomolino and Smith 2003). During the winter, they may dig 90 cm or more to escape freezing temperatures. Yet, there are reports of these anurans supercooling to -4C (Black 1970; Swanson and Graves 1995).It occurs in the Plains and Great Basin Grasslands (Holycross and Brennan 2006).Pierce et al. (2017) used Plains Spadefoots to contrast the population genetic effects of expansion into novel versus non-novel habitats. They evaluated gene flow from conspecifics and from heterospecifics via hybridization with a resident species and found that range expansion into novel habitat, relative to non-novel habitat, resulted in higher genetic differentiation, lower conspecific gene flow and bottlenecks. They also found that hybridizing with a resident species introduced genetic diversity in the novel habitat. The results suggest the evolution of species’ ranges can depend on the extent of differences in habitat between ancestral and newly occupied ranges. Furthermore, the results highlight the potential for hybridization with a resident species to enhance genetic diversity during expansions into novel habitat.

Fossils. Parmely et al. (2015) found fossil remains of Spea bombifrons at the Pratt Slide fossil site in Brown County, Nebraska. The site is dated at the late Clarendonian North American Land Mammal Age (late Miocene; ca. 10.5–9.5 MYA). The anuran fauna includes the extant genera Spea, Rana, Bufo, Hyla, and cf. Acris. The fauna was dominated by Rana (at least 39 individuals).Larvae. Farrar and Hey (1997) discovered Plains Spadefoot tadpoles occur as carnivorous and omnivorous morphs in flooded soybean and corn fields on the Missouri River flood plain of western Iowa. Carnivores have longer snouts, larger beaks with an upper cusp and lower notch, shorter intestines with fewer loops than the omnivores, and they feed on fairy shrimp. A similar polymorphism in Spea multiplicata increases survival because the carnivorous morph develops faster after consuming fairy shrimp and can metamorphose in rapidly drying ponds. However, the omnivore develops slower in deeper ponds, stores more fat and has better survival rates after metamorphosis. Fairy shrimp ingestion triggers development of the S. multiplicata carnivores, but whether this is also true for S. bombifrons remains to be seen. Pfennig et al. (2006) investigated the roles of resource availability and phenotypic plasticity in promoting ecological character displacement. They tested this hypothesis in two species of spadefoot toads (Spea bombifrons and S. multiplicata) whose tadpoles use phenotypic plasticity to develop into either an omnivorous morph, which specializes on detritus, or a physically distinctive carnivorous morph, which specializes on shrimp. Both species grow best on shrimp, but when reared together, S. bombifrons outcompetes S. multiplicata for shrimp and S. multiplicata outcompetes S. bombifrons for detritus. They found that when each species occurred alone in the field, they produced similar proportions of omnivores and carnivores. When the two species occurred together, they underwent ecological character displacement in larval development, with S. multiplicata producing mostly omnivores, and S. bombifrons producing mostly carnivores.

Diet. An opportunistic predator on a variety of arthropods, including beetles, crickets, grasshoppers, ants, spiders, termites, moths, caterpillars, etc. (Bragg, 1944; Whitaker et al., 1977; Collins and Collins, 1993; Anderson et al., 1999b).Reproduction. Spea bombifrons is nocturnal and fossorial, and spends most of the year underground, emerging to reproduce and feed when conditions are favorable (Lauzon 1999). During extended droughts, the spadefoot does not remain underground indefinitely but emerges during humid weather, to forage at night (Bragg 1965; Ruibal et al. 1969).Burrows are deep into the soil and the Plains Spadefoot emerges less often as the soil dries (Bragg 1965). It prefers loose soil, where it can easily burrow up to depths of almost one meter (Baxter & Stone 1980, Russell & Bauer 1993, USGS 2002). Bragg (1944) noted that burrows are frequently located along the edge of a solid object or near a plant that provides security or shade. At latitudes with winter temperatures below freezing it burrows deeply to avoid the low temperatures and desiccation. The burrows need to extend below the frost line (Baxter & Stone 1980).During the monsoon spadefoots surface and migrates to breeding wetlands during periods of heavy rainfall and warm temperatures (Bragg 1965, Klassen 1998, Lauzon and Balagus 1998). The amount of rainfall required to trigger emergence varies from 2.5 to 10.4 cm (Black 1970, Femmer 1978, Farrar and Hey 1995). Droughts may cause populations to skip reproduction for one or more seasons.  However, it may breed more than once in a single year when conditions are favorable.

Choruses can be heard at considerable distances, and this stimulates both sexes to move to the breeding pools. Males migrate to the breeding wetlands before females and outnumber them (Bragg 1945; Baxter & Stone 1980). The advertisement call is loud and maybe heard at distances of up to three kilometers (Lauzon and Balagus 1998). The calls of the male spadefoot trigger members of both sexes to move to the breeding sites; the larger choruses attract more individuals of both sexes (Bragg 1945).The combination of preferences for temporary pools and sandy soils limits the number of potential breeding sites. Breeding sites   include cattle tanks, playa lakes, roadside ditches, construction site, upland ponds, oxbows, sloughs, marshy depressions (Femmer 1979, Farrar & Hey 1995).Single clutches contain 10 to 250 eggs, but females lay multiple clutches that may total 2,000 eggs per season (Collins and Collins, 1993).  The egg mass is spherical and attached to submerged vegetation. Soil particles often stick to the egg mass and camouflage the eggs from potential predators. Like other spadefoots, egg development is temperature-dependent, and eggs hatch in about two days.  Temperatures below 10°C and above 34°C tend to be lethal for eggs and larvae. Eggs are deposited on the substrate or attached to partly submerged vegetation or other objects (Collins and Collins, 1993). Hatching takes place in 20 hours at 30 ˚C (Justus et al. 1977).Goldberg (2019) examined 99 Spea multiplicata from New Mexico consisting of 65 adult males ( mean SUL = 46.6 mm, range = 34–56 mm), 24 adult females (mean SUL = 47.2 mm, range = 40–58 mm and 10 unsexed juveniles (mean SUL = 29.5 mm, range = 22–33 mm). There was no sexual dimorphism in size between male and female SULs of S. multiplicata.

All 14 male S. multiplicata from June and 91 % from July contained large quantity of sperm. Five of sixteen (31%) of males from August held reduced amount of sperm, suggesting the period of reproduction was ending. The greater number of males over females is typical of explosive breeding anurans (Woodward 1984). The two smallest mature males measured 34 mm SUL and 35 mm SUL and were collected in August. The two smallest mature females (ready to spawn) measured 40 mm SUL and 41 mm SUL and were collected in June. The number of times a female may spawn in a reproductive season is not known. However, the presence of mature oocytes with concurrent postovulatory follicles indicates females may spawn more than once in the same breeding season. Two of these contained very few or no mature oocytes and would not have spawned again in the current year.

Hybridization. Plains Spadefoot females that live with Mexican Spadefoots (S. multiplicata) alter their mate choice when they are using the same deep-water breeding sites.  Plains Spadefoot females seem to prefer to mate with male Chihuahuan Desert Spadefoots, and apparently, the hybrid offspring has an advantage over the purebred offspring in that more of the hybrid tadpoles become carnivorous (Pfennig 2007).Schmidt and Pfennig (2015) investigated female mate choice using spadefoot toad hybrids of Spea multiplicata and Spea bombifrons. They assessed the mate preferences of female hybrid spadefoot toads for sterile hybrid males vs. pure‐species males in two alternative habitat types in which spadefoots breed: deep or shallow water. They found that, in deep water, hybrid females preferred the calls of sterile hybrid males to those of S. multiplicata males. Thus, maladaptive hybrid mate preferences could serve as an isolating mechanism. However, in shallow water, the preference for hybrid male calls was not expressed. Moreover, hybrid females did not prefer hybrid calls to those of S. bombifrons in either environment. Because hybrid female mate choice was context‐dependent, its efficacy as a reproductive isolating mechanism will depend on both the environment in which females choose their mates as well as the relative frequencies of males in each population. Thus, reproductive isolation between species, as well as habitat specific patterns of gene flow between species, might depend critically on the nature of hybrid mate preferences and the way in which they vary across environments.

Garcia et al. (2015) evaluated whether leptin, a protein hormone involved in the regulation of appetite, might affect the expression of condition-dependent mate choice decisions. They administered leptin to the spadefoot toad, Spea bombifrons, which exhibit condition-dependent mate choice for males of their own species versus congeneric males of the Mexican Spadefoot, S. multiplicata. Poor-condition Plains Spadefoot are more likely than are good-condition individuals to prefer Mexican Spadefoot males, but only in environments where hybridization between the two species is beneficial. The authors found that the leptin treatment reduced appetite in Plains Spadefoot adults, as was expected from leptin’s known effects on appetite. However, although they predicted that leptin would reduce female preferences for heterospecific males, they found the opposite. The leptin treatment generated a consistent, repeatable preference for heterospecifics in an environment where females generally prefer conspecifics regardless of condition. These results indicate that leptin has the potential to affect female mate choice, but that it might do so in non-intuitive ways.

The slow and fast calling Spea bombifrons populations. Advertisement calls in frogs are important because they determine which female will mate with which male. Pierce (1976) summarized the situation, this account is based on his summary. Blair (1955) described the mating call of Spea bombifrons from Swisher Co., Texas, as a trilled call, with a mean trill rate of 40–55 trills per second at a water temperature of 18⁰ C. The calls ranged from 0.49 to 0.72 seconds in duration. Bogert (1960) recorded the mating calls of S. bombifrons in the San Simon Valley near Rodeo, New Mexico, and from Chihuahua, Chihuahua, Mexico, and described their calls as trilled, with a trill rate of approximately 250 trills per second at 20 C. The calls averaged 0.12 seconds in duration.

Bogert hypothesized that Blair’s S. bombifrons might have been a hybrid between S. bombifrons and S. hammondi. Or, that two mating call types of S. bombifrons might be present; a slow trill frog at the eastern edge of the range, and a fast trill frog at the western edge of the range. Bogert also proposed that call differences between S. bombifrons and S. hammondi might have occurred in the western portion of the range of S. bombifrons, but not in the eastern part.

The first hypothesis suggested by Bogert is unlikely. Blair’s slow trill S. bombifrons have now been recorded extensively in western Texas, southeastern New Mexico, Wyoming, and northwestern New Mexico. Only slow trill S. bombifrons have been recorded in these areas, no fast trill S. bombifrons have been detected. Thus, it appears possible that two distinct mating call types of S. bombifrons are present in the southwestern United States.  As the data indicate the major differences between the two call types are in the duration of the calls, the inter-call interval, and the trill rate. However, differences also occur in the dominant frequency and trills per call.

When compared with slow trill S. bombifrons, the calls of fast trill populations are shorter in duration, are repeated with greater frequency and possess a much faster trill rate. When the effect of temperature on the mating call is considered, these mating call parameters for slow trill and fast trill S. bombifrons agree with the data of Blair and Bogert. The two call types of S. bombifrons are certainly distinct and may represent two species even though the frogs are morphologically similar.

Bogert recorded fast trill S. bombifrons from Rodeo, New Mexico, and from Chihuahua, Chihuahua, Mexico. No other published accounts of fast trill populations of this species have been reported up to this time. Recently, however, additional recordings of fast trill populations have been made in order to define more completely the range of this call type. At the present time fast trill populations of S. bombifrons are known to occur from southeastern Arizona and northern Chihuahua, Mexico, across southern New Mexico to the vicinity of McNary, Texas. Thus, the two call types have been recorded within 25 miles of one another in southcentral New Mexico near Carrizozo. The two call types may be sympatric in this area or they may be geographically separate from each other throughout the range. Fast trill spadefoots are present in large numbers in southeastern Arizona and southwestern New Mexico. However, toward the eastern limits of their range in southeastern New Mexico the fast trill spadefoots appear to be greatly reduced in numbers. It may be that the fast trill populations are spreading eastward and are reduced in size, particularly at the northeastern edge of their range. If the two call types do exist in sympatry in these regions, it is questionable whether much interaction is occurring between them due to the small population size of the fast trill call type.

Based on mating call analysis two distinct call types of S. bombifrons are evident. The range of the fast trill call type is much more extensive than was reported by Bogert. Stirman and Pfennig (2018) noted that males of many species aggregate in large groups where they signal to attract females and the large aggregations create intense competition for mates, and the simultaneous signaling by many individuals can impair any given male’s ability to attract females. In response to this situation, male signals can be modified, so that the detectability of the signal is enhanced. The way in which signals are modified varies among even closely related species. The authors address this issue using male spadefoot toads (Spea multiplicata and Spea bombifrons), which call to attract females. Using data from natural populations, the authors examined if, and how, male calls of three different call types (S. multiplicata with a slow call, S. bombifrons with a slow call, and S. bombifrons with a fast call) varied depending on competition with other males. They found that in both call types consisting of slow calls, call pulse rate decreased with increasing competition. By contrast, in the call type consisting of fast calls, call rate decreased with increasing competition. They also found that the relationship between competition and male call effort—a measure of the energy that males expend in calling––differed between the call types. Such variation in male signals in response to competition can have important implications for explaining diversity in male signals and patterns of sexual selection. While some of the other Arizona spadefoot species breed with the first monsoonal rains, the events that stimulate the Plains Spadefoot migrations are not as clear.  Surface activity occurs during the early warm evenings of spring, and while it is most encountered during an evening rain, diurnal activity is known.

Reproduction occurs from April to July and is variable from year to year and not synchronized.  Eggs usually require two to four days to hatch, and metamorphosis takes another 36–60 days, depending upon temperature.  The clutch size is between 300–1,000 eggs. The tadpoles are omnivorous, feeding on detritus and carrion, including the carcasses of conspecifics, and they prey upon the embryos of Woodhouse’s Toad.  The tadpoles may also be cannibals.  When tadpoles of the Plains Spadefoot and the Great Basin Spadefoot occur together, they produce different morphs (Pfennig and Murphy 2002).  The Plains Spadefoot increases the number of carnivorous tadpoles that feed mostly on shrimp, while the Great Basin Spadefoot produces few carnivorous tadpoles and increases the number of omnivorous tadpoles that feed primarily on detritus.  Males mature at a slightly smaller size than females, probably in the first or second year after metamorphosis.  Females are probably not sexually mature until the second or third breeding season after metamorphosis.

Parasites. Goldberg and Bursey (2002) found five (14%) of 35 Spea bombifrons harbored helminths: one male with immature individuals of the trematode Polystoma nearcticum; one male with gravid individuals of the nematode Aplectana incerta; one female and one male with gravid individuals of the nematode Aplectana itzocanensis; and one female with one larva of Physaloptera sp. (Nematoda).Predators of tadpoles include water scavenger beetle larva, Triops shrimp, gartersnakes, and cannibalistic tadpoles.  Predators of adults include rattlesnakes, hawks, and rodents.

Defense. They have an odor that has been compared to popcorn or roasted peanuts.  If the skin secretion contacts a mucous membrane, it can cause a burning sensation.  Predators on Great Basin Spadefoots are numerous and include trout, gartersnakes, and rattlesnakes.Taxonomy. Cope (1863) described Scaphiopus bombifrons based upon three syntypes: USNM 3704 (Fort Union), 3520 (Platte River), and 3703 (Llano Estacado). USNM 3704 was designated a lectotype by Fouquette and Dubois (2014) with a type locality at Fort Union, on Missouri River, lat. 48° N; On Platte River, 200 miles west of Fort Kearney; Llano Estacado, Texas. Invalidly restricted to Fort Union, North Dakota, USA, by Schmidt (1953:59) and validly by lectotype designation by Fouquette and Dubois (2014). Cope (1866) placed the species in the genus Spea, but Boulenger (1882:435) returned it to the genus Scaphiopus; Cope (1889:306) applied the combination Scaphiopus hammondii bombifrons; Tanner (1939:11) used the combination Scaphiopus (Spea) bombifrons, and Firschein (1950:75) used the combination Spea bombifrons.