Spea multiplicata (Cope, 1863)
Steven R. Morey1
There are two recognizable groups of North American spadefoot toads, Scaphiopus (Holbrook, 1836) and Spea (Cope, 1866[a]). With respect to those species that are referable to Spea, the literature is divided, with some authors following Bragg (1944, 1945b), Stebbins (1951, 1985), Blair (W.F., 1955c), Zweifel (1956b), and Hall (1998), who treat the two groups as subgenera. We follow B.C. Brown (1950), Smith (1950), Tanner (1989b), Wiens and Titus (1991), Maglia (1998, 1999), and Crother et al. (2000), who recognize the generic distinctness of Spea.
1. Historical versus Current Distribution. Prior to the mid 1970s, Mexican spadefoot toads (S. multiplicata) were widely recognized as a subspecies of western spadefoot toads (S. hammondii). Brown (H.A., 1976a) proposed that the populations east of California be recognized as a separate species, Scaphiopus multiplicatus Cope, 1863, citing marked differences in morphology, mating calls, and ecology. Patterns of allozyme variation (Sattler, 1980; Wiens and Titus, 1991) have subsequently supported the elevation of S. multiplicata to species status. Much of the literature on this species is under the name Scaphiopus hammondii. Mexican spadefoot toads are found from southwestern Kansas, western Oklahoma, and central Texas, through New Mexico, southern Colorado, southeastern Utah, Arizona, and in many Mexican states (Stebbins, 1951, 1985; Tanner, 1989b; Conant and Collins, 1991). Unexplained declines have not been observed. The current and historical distributions are similar, but there are minor differences because the current distribution reflects the effects of human activities. For example, Mexican spadefoot toads are now absent where urbanization and water development have destroyed places they once occupied. As with other western spadefoot toads, however, Mexican spadefoot toads readily breed in ephemeral artificial impoundments such as stock tanks and pools that form at the base of road and railroad grades. This has resulted in their colonization of some areas where suitable natural pools are rare or nonexistent.
2. Historical versus Current Abundance. Human activities have influenced abundance. Mexican spadefoot toads are probably more abundant than in the past wherever open country still exists and artificial ephemeral impoundments have been created where natural pools were rare or absent. Urbanization, water projects, and irrigated agriculture have, in contrast, altered some natural habitats so severely that Mexican spadefoot toads are less abundant than in the past or have been eliminated. Dimmitt and Ruibal (1980a) surveyed roads in the San Simon Valley, southeastern Arizona and southwestern New Mexico, on both dry and rainy nights and encountered Mexican spadefoot toads at a frequency of 0.5/100 km on dry nights and 17/100 km on rainy nights.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Adults are terrestrial and must move from underground refuges to reach breeding sites. Migrations are triggered when monsoon rainfall fills temporary pools, usually in July but sometimes later (Wright and Wright, 1949; Stebbins, 1954a; Degenhardt et al., 1996). Breeding is synchronous (Woodward, 1984; Sullivan, 1989a). Little is known about what portion of the population moves to breeding sites each year or how far individuals move to reach breeding sites.
ii. Breeding habitat. Most breeding occurs in temporary pools that form following monsoon rains. Breeding sites include ephemeral pools and playas, cienegas, tanks in rocky streambeds, isolated pools in temporary streams and arroyos, stock tanks, and pools that form at the base of road and railroad grades. Breeding can occur in pools that remain filled from 5 d to several months, and pools can dry and refill in the same season (Pfennig, 1990). In order to support metamorphosis, breeding pools must remain filled at least long enough for eggs to hatch and for the developing larvae to complete metamorphosis, which in nature takes about 3 wk. In the vicinity of Portal, Arizona, Pfennig (1990) observed that pools where eggs were deposited ranged in depth from 5–200 cm (mean = 46 cm) and had a surface to volume ratio of 1.35–55 (mean = 11.5). There is a risk of larval mortality due to pool drying. Pfennig (1990) observed that among 37 pools where breeding occurred, only 4 dried completely before any larvae transformed. The food supply available for larvae in desert pools can decline rapidly after filling (Loring et al., 1988).
i. Egg deposition sites. Clutches are deposited in clusters on plant stems or rocks (Stebbins, 1985). Eggs hatch in 0.6–6.0 d depending on temperature (H.A. Brown, 1967a,b; Zweifel, 1968b), but usually in < 48 hr in nature.
ii. Clutch size. Clutches average just over 1,000 eggs (Woodward, 1987; Simovitch et al., 1991).
i. Length of larval stage. Larval period is flexible, but in the wild is usually 12–19 d (Pomeroy, 1981; Pfennig, 1990) and can be as long as 44 d (Pfennig et al., 1991). Under low food experimental conditions, larval period can exceed 50 d (Pfennig et al., 1991; Simovich et al., 1991).
ii. Larval requirements.
a. Food. Larvae eat organic detritus, algae, crustaceans, and tadpoles (Pomeroy, 1981; MacKay et al., 1990; Pfennig, 1990). There are distinct carnivorous and omnivorous larval forms. Carnivores eat mainly fairy shrimp (Thamnocephalus sp. and Streptocephalus sp.). Omnivores consume more detritus and algae and fewer fairy shrimp than carnivores (Pomeroy, 1981).
b. Cover. Larvae most often are found in turbid pools with little or no other cover. In some pools, areas with dried vegetation from the previous wet season are present. Some breeding sites are marshy.
iii. Larval polymorphisms. An inducible carnivorous morph, characterized by a broad head, large jaw muscles, a short gut, and rapid development can develop when larvae ingest fairy shrimp (Pomeroy, 1981). Carnivores are most likely to be found in rapidly shrinking pools with abundant fairy shrimp and low levels of organic debris (Pfennig, 1990). Larval condition, such as body size, plays a role in determining whether the carnivore morphology is expressed (Frankino and Pfennig, 2001). In the San Simon Valley, Arizona, and the Animas Valley, New Mexico, the proportion of carnivores ranges from 0–100% of the population (Pfennig, 1990).
iv. Features of metamorphosis. In nature, carnivorous larvae have more rapid development than omnivores and transform at smaller sizes and with lower body fat reserves. Because of their more rapid rate of development, carnivores have a better chance than omnivores of completing the larval stage before the pool dries. In longer-lived pools, omnivores, with comparatively slow development, transform at larger sizes and with higher amounts of stored body fat. Pfennig et al. (1991) demonstrated that larger size at metamorphosis was positively correlated with higher post-metamorphic survival in the laboratory, and there is abundant evidence for a terrestrial stage advantage conferred by large size at metamorphosis in spadefoot toads (Newman and Dunham, 1994; Newman, 1999; Morey and Reznick, 2001). Thus, there is a tradeoff between the larval carnivore’s advantage in rapidly shrinking pools and the omnivore’s post-metamorphic advantage in longer duration pools.
v. Post-metamorphic migrations. Post-metamorphic juveniles remain for a few days in the vicinity of the natal pool and are active on the surface as long as the soil remains moist. Eventually juveniles emigrate from the natal pool and unless showers moisten the soil, the search for suitable refuges must take place over dry soil. Little is known about how far post-metamorphic juveniles travel or how they survive the harsh conditions that are typical in the desert summer when these movements take place. Creusere and Whitford (1976) estimated that as few as 1% of juveniles survive to 6 wk post metamorphosis. They cite insufficient nutrition, predation, and desiccation among the important causes of mortality. Weintraub (1980) confirmed Creusere and Whitford's (1976) findings, and found that newly-metamorphosed Mexican spadefoots will use piles of cow dung as daytime retreats.
D. Juvenile Habitat. Once they leave the margin of the natal pool, the habitats of juveniles are probably similar to adults. On rainy nights, adults and juveniles can be encountered together on roads. One aspect of juveniles that is not well known is their selection of refuge sites. In excavations, juveniles have not been encountered in burrows as commonly as adults. Newman and Dunham (1994) suggested that tiny Couch's spadefoot toad juveniles are unable to burrow on their own. If this applies to Mexican spadefoot toads, then small juveniles may rely more on cracks and mammal burrows for refuges than adults, which almost always dig their own burrows (Ruibal et al., 1969).
E. Adult Habitat. Mexican spadefoot toads occur in a wide range of arid and semi-arid habitat types, as long as breeding pools exist. They are often found where the soil is sandy or gravelly (Stebbins, 1985). In the vicinity of Portal, Arizona, Pfennig (1990) found Mexican spadefoot toads breeding in pools in semidesert grasslands or Chihuahuan desert scrub. In New Mexico, Mexican spadefoot toads are found in grasslands, sagebrush flats, semi-arid shrublands, river valleys, and agricultural lands (Degenhardt et al., 1996).
F. Home Range Size. Unknown.
G. Territories. There is little evidence of agonistic or territorial behavior. Males seem to maintain an individual space while chorusing. Adults are solitary during periods of inactivity in burrows (Ruibal et al., 1969).
H. Aestivation/Avoiding Dessication. Surface activity is restricted to short periods following summer showers (Ruibal et al., 1969; Dimmitt and Ruibal, 1980a). Thus, much time is spent in underground retreats. In the desert southwest, spadefoot toads spend 8–10 mo in soil-filled “winter” burrows (20–90 cm in depth) that they dig themselves (Ruibal et al., 1969). Spadefoot toads survive periods of osmotic stress during long periods of dormancy in burrows by accumulating urea in their body fluids. This allows them to absorb water from the surrounding soil, as long as the soil has a water potential higher than that of the body fluids (Shoemaker et al., 1969; Jones, 1980). By flooding and excavating pools in Arizona, Ruibal et al. (1969) demonstrated experimentally that spadefoot toads do not burrow into the drying mud of breeding sites.
I. Seasonal Migrations. Not known for subadults. Adults make seasonal movements to and from breeding pools. These movements are nocturnal, but little is known about the distance between breeding pools and the winter burrow or about what proportion of the adult population moves to and from the breeding site each year.
J. Torpor. During the season of summer showers, periods of activity are spent in shallow, soil-filled summer burrows, 1.3–10 cm deep (Ruibal et al., 1969).
K. Interspecific Associations/Exclusions. In areas of sympatry, matings between Mexican spadefoot toads and other spadefoot toads—usually plains spadefoot toads (S. bombifrons) or Couch's spadefoot toads—can occur. Matings between Mexican spadefoot toads and Couch's spadefoot toads are uncommon and believed to result in inviable eggs. However, hybridization between Mexican spadefoot toads and plains spadefoot toads is well known (H.A. Brown, 1976a; Sattler, 1985); in some areas, such as the San Simon Valley, Arizona, high frequencies of F1 hybrids and backcross progeny occur, with hybrid adults representing up to 31% of the breeding congress at some pools (Simovich et al., 1991; Simovich, 1994). Hybrid males are sterile; fertile hybrid females produce only about half as many eggs as females of either parental species. Simovich et al. (1991) showed that under controlled conditions, hybrid larvae had better survivorship and developed faster than larvae of either parental species, so there may be a survival advantage for hybrids under some conditions in the wild. Pfennig (2000) conducted phonotaxis experiments on female Mexican spadefoot toads to assess mate choice and found that females from pools with other spadefoot toad species made choices that ensured conspecific matings.
L. Age/Size at Reproductive Maturity. Age at sexual maturity is probably 2–3 yr for males. Woodward used skeletochronology and estimated the average age of breeding males was 2.8 yr (Woodward, 1982). I reared newly transformed juveniles of closely related western spadefoot toads and found that under high-food conditions, most males developed secondary sexual characters by the beginning of the first breeding season following metamorphosis. At the same age, females reared under similar conditions had small ovaries that had not reached the vitellogenic stage of the first ovarian cycle. Thus, it seems reasonable that female southern spadefoot toads probably mature at a slightly older average age than males. Adult body size is 37–64 mm (Stebbins, 1985). Females may be a bit larger, on average, than males. For example, a series of adult males described by Woodward (1982) ranged from 38–56 mm. During breeding aggregations, Sullivan and Sullivan (1985) collected males that ranged from 41–59 mm. A series of adult females ranged from 42–60 mm (Long, 1989).
M. Longevity. Unknown. Captive wild-caught adult spadefoot toads can live several years in captivity (Snider and Bowler, 1992). In studies on Couch’s spadefoot toads, Tinsley and Tocque (1995) used skeletochronology to estimate that 5% of the breeding population was > 10 yr old. They made estimates of a maximum longevity of about 13 yr for females and 11 yr for males.
N. Feeding Behavior. Adults are mainly nocturnal, but newly transformed juveniles will feed in the open during the day. Transforming juveniles capture prey with great difficulty until the tail is almost completely resorbed. Adults eat winged and nymphal termites in great numbers when they are available. Other common prey are beetles (Coleoptera, especially Carabidae and Curculionidae), lygaeid bugs (Hemiptera), ants (Formicidae), grasshoppers and crickets (Orthoptera), and spiders (Arachnida; Whitaker et al., 1977; Dimmitt and Ruibal, 1980b; Punzo, 1991a; Anderson et al., 1999b).
O. Predators. Predators on the larvae of desert spadefoot toads include larval water scavenging beetles (Hydrophilus sp.), larval tiger salamanders (Ambystoma tigrinum), carnivorous conspecific larvae, yellow mud turtles (Kinosternon flavescens), grackles (Quiscalus sp.), and skunks (Spilogale putorius; Wright and Wright, 1949; Woodward, 1983; Newman, 1987). In the summer of 1981, Marie Simovich and Clay Sassaman found spadefoot toad larvae among the stomach contents of 25 of the 35 juvenile American bullfrogs (Rana catesbeiana) they examined from a single pool near Portal, Arizona. Woodward and Mitchell (1991) suggest that, because desert predators are well established in permanent water, proximity to permanent water may increase the level of predation in nearby temporary pools. They also suggest that irrigation practices in the desert may increase the influence of predation in structuring desert anuran communities.
P. Anti-Predator Mechanisms. As with other spadefoot toads, Mexican spadefoot toads can produce volatile skin secretions that cause sneezing and a runny nose in some humans; while these secretions are most noticeable after an injury or struggle, a smaller amount can be produced during handling. Several authors (e.g., Stebbins, 1951; Stebbins and Cohen, 1995; Waye and Shewchuk, 1995; Degenhardt et al., 1996) have commented on the irritating quality of the secretions if they come into contact with the eyes, nose, or broken skin. It is probable that the secretions are noxious and repulse some predators.
Q. Diseases. Unknown.
R. Parasites. Spadefoot toads are host to polystomatid monogenean trematode parasites (Tinsley and Earle, 1983). In the wild, infections from these trematodes apparently do not lead to substantial health problems (Tinsley, 1995).
4. Conservation. Human activities have influenced abundance. Urbanization, water projects, and irrigated agriculture have altered some natural habitats so severely that Mexican spadefoot toads are less abundant than in the past or have been eliminated. Unexplained declines have not been observed. In Colorado, Mexican spadefoot toads are listed as a Species of Special Concern. As with other western spadefoot toads, however, Mexican spadefoot toads readily breed in ephemeral artificial impoundments such as stock tanks and pools that form at the base of road and railroad grades. This has resulted in their colonization of some areas where suitable natural pools are rare or nonexistent.
Acknowledgments. Thanks to Sean Barry and Brian Sullivan for comments on an earlier version of this account.
1Steven R. Morey
U.S. Fish and Wildlife Service
911 Northeast 11th Avenue
Portland, Oregon 97232
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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