Scaphiopus couchii Baird, 1854(b)
Steven R. Morey1
1. Historical versus Current Distribution. Couch's spadefoot toads (Scaphiopus couchii) range from central Texas and southwestern Oklahoma, south and east from central New Mexico and Arizona, into Nayarit, Zacatecas, and Queretaro, Mexico, throughout much of Baja California, Mexico, and into extreme southeastern California (Stebbins, 1985; Conant and Collins, 1991; Degenhardt et al., 1996). A disjunct population occurs in southeastern Colorado (Livo, 1977). Couch's spadefoot toads have been eliminated wherever urban development and irrigated agriculture have destroyed areas where they once lived. However, as with other desert spadefoot toads, Couch's spadefoot toads readily breed in ephemeral artificial impoundments such as stock tanks and pools that form at the base of road and railroad grades. They have colonized many areas where natural pools are rare or nonexistent. Thus, the distribution differs somewhat from the historical pattern because it reflects the effects of habitat destruction and colonization of new areas.
2. Historical versus Current Abundance. Couch's spadefoot toads are probably more abundant than in the past wherever open country still exists and human activities have created ephemeral impoundments (Dimmitt, 1977). Examples of this can be found in southeastern California where, in some places, road and railroad construction has inadvertently increased the number of ephemeral pools, many of which have been colonized by Couch's spadefoot toads. The predominant change in historical versus current abundance, however, is that Couch's spadefoot toads are now absent wherever urban development and irrigated agriculture have destroyed places where they were once abundant. Dimmitt and Ruibal (1980a) encountered Couch's spadefoot toads at a frequency of 0.5/100 km on dry nights and 22/100 km on rainy nights in the San Simon Valley, southeastern Arizona and southwestern New Mexico.
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. Breeding is triggered when summer rainfall fills temporary pools. Breeding is markedly synchronous (Woodward, 1984; Sullivan, 1989a). Breeding usually occurs following heavy rains, from April–August (Wright and Wright, 1949; Stebbins, 1951), even into September in the western portions of the range (Mayhew, 1965). After the first bout of breeding, if showers refill a pool, a second bout of breeding can occur, but this usually involves a smaller number of breeding adults. Little is known about what portion of the population moves to breeding sites each year or how far individuals move to reach the breeding sites.
ii. Breeding habitat. Most breeding occurs in temporary pools that form following intense summer showers. Common breeding sites include ephemeral pools and playas, tanks in rocky streambeds, isolated pools in arroyos, stock tanks, and pools that form at the base of road and railroad grades. In order to support metamorphosis, the breeding pools must remain filled long enough to accommodate the period between egg deposition and hatching, usually about 1 d, and at least the minimum larval period, which is about 7–8 d in the wild (Newman, 1987, 1988; Morey, 1994). Couch's spadefoot toads breed in such ephemeral settings that their larvae seem constantly at risk of desiccating before the aquatic phase is complete. For example, in Big Bend National Park, Texas, Newman observed that desiccation was the primary cause of mortality among larvae in 49 of 81 pools surveyed. Likewise, in southeastern California, I observed that 8 of 13 pools surveyed dried completely on or before the day that the first larvae transformed (Morey, 1994).
i. Egg deposition sites. Eggs, described in Stebbins (1954a, 1985), are usually deposited on plant stems.
ii. Clutch size. Average clutch size > 3,000 eggs (Woodward, 1987a,b).
i. Length of larval stage. The larval period in Big Bend National Park, Texas, is 8–16 d and is positively correlated with pool duration (Newman, 1987, 1989). In southeastern California, the larval period is 7.5 d (range 7–8.5) and is not distinctly correlated with the duration of the natal pool (Morey, 1994). Newman (1994) showed with experiments that Couch's spadefoot toad larvae respond to low or decreasing food supplies by transforming earlier than larvae reared at higher food levels. Morey and Reznick (2000) demonstrated that the apparent acceleration of development among slow-growing larvae occurs because at low food levels, larvae transform as soon as they reach the minimum size that will support metamorphosis. Fast-growing larvae, on the other hand, delay metamorphosis beyond the minimum, presumably to capitalize on growth in the larval environment.
ii. Larval requirements.
a. Food. Diet of larvae is not well known. Larvae of other spadefoot toad species eat animals, plants, and organic detritus (Pomeroy, 1981; Pfennig, 1990).
b. Cover. Larvae most often occur in turbid pools with little or no other cover. Dried vegetation from the previous wet season is sometimes present.
iii. Larval polymorphisms. None reported.
iv. Features of metamorphosis. As with other spadefoot toad species, body size at metamorphosis is variable. For example, in Big Bend National Park, Texas, the average body length at metamorphosis ranges from 9.5–12.9 mm (Newman, 1989). In southeastern California, body mass at metamorphosis is 0.5 grams, range 0.2–0.8 (Morey, 1994). A vivid example of the way environment can influence size at metamorphosis was described by Morey and Janes (1994), who compared size and body fat reserves in newly metamorphosed Couch's spadefoot toads that developed in adjacent pools. Both pools filled following a summer shower. One of the pools dried 9 d later, the other remained full much longer. There was no mortality due to drying, even in the short-lived pool, but toadlets from the longer-lived pool were almost four times more massive and had nearly three times more stored body fat than toadlets from the shorter-lived pool. Morey and Reznick (2001) showed that the effects of this type of size variation on characteristics such as survival, growth, and behavior, persist at least several months after metamorphosis. Newman and Dunham (1994) showed that larger toadlets have lower mass-specific rates of water loss. They postulated an advantage for larger toadlets after they leave the natal pond because they can survive longer in dry areas and may have more time to locate suitable refuges. Newman (1999) also showed experimentally that larger toadlets were better at capturing prey (pinhead crickets) than smaller toadlets. In 1992, I observed in several pools that larvae from bouts of breeding other than the first one of the season have slower growth rates and transform at only about half the size of the larvae from parents that bred with the first pond filling (Morey, 1994). There is some uncertainty about what factors account for the size difference. The first larval cohort may deplete the food resources available to later larval cohorts (Seale, 1980; Loring et al., 1988). This is certainly possible because Newman (1987) demonstrated that food is limiting by manipulating the food supply available to larvae in the wild. Woodward (1987b) suggested that some of the size differences in toadlets from the first versus later cohorts is attributable to quality differences between adults participating in early versus later breeding bouts. In pools where second bouts of breeding occur, there are generally large numbers of tadpole shrimp Triops sp. about the same size as Couch's spadefoot toad larvae. Tadpole shrimp are active compared to Couch's spadefoot toad larvae; under these conditions, almost all the Couch's spadefoot toad larvae have damaged tails that appear to be nipped severely, probably by the tadpole shrimp. The accumulation of predators and competitors in desert pools that remain full a long time, or refill, is a well-known phenomenon (Pomeroy, 1981). Harassment and possibly competition for food by tadpole shrimp, which usually does not occur significantly in the first larval cohort, could easily account for the size differences I observed.
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 they travel or how they survive the harsh conditions that are typical in the desert summer when these movements take place. Mayhew (1965) observed newly transformed juveniles dispersing across sand dunes 400 m from and 30 m in elevation above a breeding pool. Newman and Dunham (1994) suggest that small juveniles may be incapable of burrowing and rely instead on finding refuge in cracks and holes, such as mammal burrows.
D. Juvenile Habitat. Once they leave the margin of the natal pool, the habitat characteristics of juveniles are probably similar to adults. On rainy nights, adults and juveniles can be encountered together on roads. An exception to the similarity of juvenile and adult habitat characteristics might be in the selection of refuge sites. If, as Newman and Dunham (1994) suggest, small juveniles are unable to burrow, then they must rely on cracks and mammal burrows for refuges, whereas adults almost always dig their own burrows (Ruibal et al., 1969).
E. Adult Habitat. Mesquite and mesquite-yucca, short-grass plains, and creosote desert (Bragg, 1944; Stebbins, 1951, 1985; Mayhew, 1965), as long as temporary rain-filled pools exist. Degenhardt et al. (1996) consider sandy, well-drained soils an important habitat element in New Mexico.
F. Home Range Size. Unknown.
G. Territories. Unknown. There is little evidence of agonistic or territorial behavior. Males seem to maintain individual space while chorusing. Spadefoot toads are solitary during periods of inactivity in burrows (Ruibal et al., 1969). Woodward (1982a) found positive size-assortive mating with larger males mating most often. The pattern was not attributable to female choice, so the result may best be explained by male-male interactions in the form of scramble competition, as postulated by Wells (1977).
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), which they dig themselves (Ruibal et al., 1969). Couch's spadefoot toad burrows sometimes coincide with mammal burrows (McClanahan, 1967). 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 it 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 that spadefoot toads do not burrow into the drying mud of a breeding site.
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 period of summer showers, quiet periods are spent in shallow soil-filled summer burrows 1.3–10 cm deep (McClanahan, 1967; Ruibal et al., 1969), often under dense vegetation (Mayhew, 1965).
K. Interspecific Associations/Exclusions. Amphibian communities in the desert southwest tend to be simple compared to those in the eastern United States (Woodward and Mitchell, 1991; Dayton and Fitzgerald, 2001). In southeastern California, for example, Couch's spadefoot toads usually breed alone. Only rarely in this part of the range do red-spotted toads (Bufo punctatus) breed in the same pools as Couch's spadefoot toads. Even in more complex desert anuran communities, only 1–3 species usually make up the breeding assemblage in any one pool (Woodward and Mitchell, 1991, and references therein). The other amphibians that may be encountered at Couch's spadefoot toad breeding sites are usually other spadefoot toads and toads in the genus Bufo. Even predators tend to be scarce (Newman, 1987; Woodward and Mitchell, 1991). Among temporary pool breeders in the desert southwest (e.g., Scaphiopus, Spea, Bufo), community structure is probably influenced by intraspecific interactions, such as competition for food, which in turn can influence the outcome of interspecific interactions such as competition and, occasionally, predation (Woodward, 1982b, 1983b, 1987a; Dayton and Fitzgerald, 2001).
L. Age/Size at Reproductive Maturity. Sullivan and Fernandez (1999) used skeletochronology to estimate that most breeding Couch's spadefoot toads are 2–3 yr old. Tinsley and Tocque (1995) also used skeletochronology and estimated that the majority of the breeding population is 5–10 yr old. Stebbins (1985) reports adult body lengths of 56–87 mm. The largest individuals are usually females. Wright and Wright (1949) give the length of adult males as 48–70 mm, adult females as 50–80 mm. A series of 13 calling males measured by Sullivan and Sullivan (1985) ranged from 62–84 mm (mean = 70 mm).
M. Longevity. Using skeletochronology, Tinsley and Tocque (1995) estimated that 5% of the breeding population was > 10 yr old. They estimated a maximum longevity of about 13 yr for females and 11 yr for males. A wild-caught adult survived almost 7 yr in captivity (Snider and Bowler, 1992).
N. Feeding Behavior. Adults are nocturnal, but newly transformed juveniles will feed out in the open during the day. Transforming juveniles capture prey with great difficulty until the tail is almost completely resorbed. Morey and Janes (1994) illustrated this when they compared stomach contents of same-aged juveniles from two adjacent pools. The population from one pool was just completing metamorphosis, and individuals still retained remnants of the larval tail (mean = 1.35 mm, n = 50). Eighty percent of the toadlets from this pool had empty stomachs or only a small amount of food in the stomach, and the intestines still contained contents from the larval gut. Fifty-one same-age toadlets from an adjacent pool developed slightly faster and retained no sign of the larval tail. Seventy percent of these had full stomachs, and the intestines had been cleared of larval food. Newly transformed toadlets eat arachnids and insects, mainly Coleoptera, Collembola, Diptera, and Hymenoptera (Newman, 1999). Adults eat winged and nymphal termites in great numbers when they are available. Other common prey are beetles, especially Carabidae, lygaeid bugs, ants, grasshoppers, crickets, and spiders (Whitaker et al., 1977; Dimmitt and Ruibal, 1980b; Punzo, 1991a). Arthropods with well-known chemical defenses, such as blister beetles, velvet ants, stink bugs, and millipedes, seem to be avoided. Adult Couch's spadefoot toads have an impressive stomach capacity and in experiments voluntarily eat up to 55% of their body weight. Calculations of assimilation efficiency suggest that this is enough food to provide energy reserves sufficient to last ≥ 1 yr (Dimmitt and Ruibal, 1980b).
O. Predators. Predators on larvae include larval water scavenging beetles (Hydrophilus sp.), larval tiger salamanders (Ambystoma tigrinum), carnivorous New Mexico spadefoot toad larvae (Spea multiplicata), yellow mud turtles (Kinosternon flavescens), grackles (Quiscalus sp.), and skunks (Spilogale putorius; Wright and Wright, 1949; Woodward, 1983a,b; Newman, 1987). In the summer of 2000, Mike Westphal discovered a juvenile western diamondback rattlesnake (Crotalus atrox) in the process of swallowing a subadult Couch's spadefoot toad on a road in the San Simon Valley, 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, Couch's spadefoot toads can produce volatile skin secretions that cause sneezing and a runny nose in some humans. These secretions are most noticeable after an injury or struggle. Smaller amounts can be produced during handling. Several authors (e.g., 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. Couch's spadefoot toads are host to polystomatid monogenean trematode parasites (Tinsley and Earle, 1983). These flukes invade adult toads at breeding pools where they enter through the nostrils and migrate into the lungs and then into the urinary bladder, where they mature and reproduce while the toad is aestivating. When adult toads return to breed the following year, encapsulated fluke larvae are released. Tocque (1993) showed that at the beginning of the active period, infected adults of both sexes had slightly smaller fat bodies than uninfected adults, but the difference disappeared after 2 wk of foraging. Infections from these trematodes apparently do not lead to substantial disease outbreaks in the wild (Tinsley, 1995), and females do not avoid parasitized mates (Pfennig and Tinsley, 2002).
4. Conservation. Couch's spadefoot toads are now absent wherever urban development and irrigated agriculture have destroyed historical habitats. They are listed as a Species of Special Concern in California (Jennings and Hayes, 1994a) and Colorado (http://wildlife.state.co.us).
In contrast, Couch's spadefoot toads will readily breed in ephemeral artificial impoundments such as stock tanks and pools that form at the base of road and railroad grades. They have colonized many areas where natural pools are rare or nonexistent, and therefore they are probably more abundant than in the past wherever open country exists and humans have created ephemeral impoundments (Dimmitt, 1977).
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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