Couch’s Spadefoot

Scaphiopus couchii Baird, 1854

Willis King (1932) described Couch’s Spadefoots in Tucson in the early 20th century.

“This spadefoot was found in greatest numbers after heavy rains in puddles and backwater along the Rillito River near University Farm.  The height of their breeding season was between July 11 and 15. Their call, which resembled most a plaintive bellow, began shortly after dark and continued until just before dawn. They call both from the edge of the muddy water and while swimming, the latter call being muffled: the male exerts his whole body in producing his call, drawing in his hind legs as the vocal pouch is protruded. The eggs varied from six to twenty-four in a clump and are fastened in irregular masses to any solid body in the water, just below the surface, hatching in one or two days. The little tadpoles grow rapidly and are sufficiently developed in a week to make their way into the mud at the bottom of the puddle and all have vanished by two weeks. The mucous from the spadefoot is irritating to any break in the human skin and has a pungent odor. The males out-number the females four to one, are more active, and are often found quite a distance from water.  By August 15, the species had disappeared”

Adults are a moderate-sized (less than 90 mm) anuran with a green or yellow-green dorsum.  The eyes are large, and the pupils are vertical.  A dark, sickle-shaped tubercle, distinctly longer than wide, is found on the inside of the ventral surface of each hind foot.  No cranial crests or parotoid glands are present.  Females have dark reticulations, while the slightly smaller males are lighter overall with faint dark spots, reduced reticulations, or an absence dark marking.  The other Arizona spadefoots are brown, tan, grayish, or grayish-green, and lack the reticulations.  The other spadefoots also have wedge-shaped tubercles on the hind feet (rather than sickle-shaped).Observing Couch’s Spadefoot is best done during or after a monsoon rain. Spadefoots are readily seen on blacktopped roads, and all age class may be present, even adults in amplexus can be observed on a wet road after a significant downpour.  They may also be seen crossing roads to reach breeding sites. After dark, walks in the desert before or after rainfall may reveal spadefoots feeding. Adults can reach 90 mm in SUL (Degenhardt et al. 1996), and calling Arizona males were reported to be 62–74 mm, and males in amplexus averaged 50.9–74.9 mm (Sullivan and Sullivan 1985) with a similar size range reported for California populations 48-82 mm (Jennings and Hayes, 1994). The tympanum is indistinct. The distance between the nares is less than the gap between the orbits; the long edge of the metatarsal tubercle is twice as long (or less) as broad, and the skin of the dorsum is tuberculate. Tadpoles are small (18–24 mm in total length) and black, with an even distribution of iridophores on the body and tail musculature. The labial tooth formula is usually 4/4. Newly transformed spadefoots are 9.5 to 12.9 mm.  Females are mottled with dark reticulations, while males are more uniform in color.  The dorsum has dark brown to black mottling on a yellow to green background; the belly is bright white. The iris is black brown with bronze flecking (Wright 1929).Distribution and Habitat. The species occurs from Texas and southwestern Oklahoma to New Mexico and Arizona, into extreme southeastern California, and southward to Nayarit, Zacatecas, and Queretaro, Mexico, as well as much of Baja California, Mexico. Within Arizona, Couch’s Spadefoot occurs in most of the southern third of the state, with disjunct populations north of the Mogollon Rim.  The elevational range is from sea level to 1,800 m in New Mexico (Degenhardt et al. 1996).Couch’s Spadefoots inhabit the Chihuahuan and Sonoran deserts, ranging from the short-grass prairie, mesquite savannas, and creosote flats to semi-desert grasslands and desert. They are often abundant on the alluvial fans extending away from mountain ranges and dunes where the soil is conducive to burrowing. Another landscape requirement is proximal clay soil that will hold water in temporary ponds for at least a week. Newman and Dunham (1994) investigated the microhabitats used by toadlets in Texas. They observed the first emergence from four ponds in late May to early June, about two weeks after the rain that triggered reproduction. For nearly 20 days, dispersing toadlets could be observed somewhere in the study area. By late June, about one month after the last substantial rain, toadlets could not be found. A series of rainstorms occurred in late July, and a new cohort of metamorphs began emerging in August from 10 ponds, none of which had produced metamorphs earlier in the summer. The authors searched for toadlets through-out the summer, but most observations occurred in the limited period beginning a week or two after heavy rains. The authors assumed that surviving toadlets were underground in cracks, holes, or burrows. Occurs in Sonoran Desertscrub (Lower Colorado River), Chihuahuan Desert Scrub, and Semidesert Grasslands (Brennan and Holycross 2006). Boeing et al (2014) found Scaphiopus couchii using creosote + succulent, mesquite + creosote + succulent in the Chihuahuan Desert. Wilkes (1963) reported this spadefoot from the burrow of the pocket gopher (Geomys bursarius) in southern Texas. Pocket gophers and other fossorial rodents act as ecological engineers when they create extensive burrow systems which alter plant community structures, create deep soils, and increase soil moisture levels. Their burrow systems are an optimal habitat for not just the rodents, but also for many amphibians and reptiles. Galan and Light (2017) documented 24 species of amphibians and reptiles using the burrows of this rodent. Feeding. Adults feed at night, but newly transformed juveniles will feed out in the open during the day.  Transforming juveniles have difficulty capturing prey until the tail is almost completely lost.  Morey and Janes (1994) compared stomach contents of same-aged juveniles from two adjacent pools.  The population from one pool was completing metamorphosis, and individuals retained remnants of the larval tail.  Eighty percent of the toadlets had empty or near-empty stomachs, and the intestines still contained contents from the larval gut.  Fifty-one toadlets of the same age from a nearby pool developed faster and showed no sign of the tail.  About 70% percent of these had full stomachs, and to larval food in the intestines.  Newly transformed toads eat arachnids and insects, mainly Coleoptera, Collembola, Diptera, and Hymenoptera (Newman, 1999).Adults eat winged and nymph termites in high numbers when they are available.  Other prey includes a variety of arthropods (Whitaker et al. 1977; Dimmitt and Ruibal 1980b; Punzo, 1991); however, arthropods with well-known chemical defenses, were avoided.  Adult Couch’s spadefoot toads eat up to 55% of their body weight.  Assimilation efficiency suggests that this is enough food to provide energy reserves enough to last about one year (Dimmitt and Ruibal, 1980b).Predation. Water scavenger beetle larvae, Barred Tiger Salamander larvae, carnivorous Mexican Spadefoot tadpoles, mud turtles, Wester Diamondback Rattlesnakes, corvid birds, and skunks have been reported as predators (Wright and Wright, 1955, Woodward 1983, Newman 1987, Moore 2005). Bonnie et al. (1999) observed the ant Aphaenogaster cockerelli attempting to feed on new metamorphs of this spadefoot, this included an 8.7 mm ant, attempting to carry a 10 mm spadefoot. Clanuch et al. (2018) reported the capture of nine American Bullfrog, Lithobates catesbeianus, at Willow Pond, Cochise County, Arizona. A population of American Bullfrogs has been established in this area for at least 30 years, despite active eradication efforts. Upon dissection, one adult male was found with two partially digested adult female Couch’s Spadefoot Toad in its stomach. Lithobates catesbeianus is known to feed on other spadefoots however, this is the first report of L. catesbeianus feeding on S. couchii. Egg predation was reported by Dayton and Jung (1999). They observed ant predation upon Scaphiopus eggs. At an ephemeral pool in Big Bend National Park, Texas, they observed ants (Forelius mccooki) walking along a blade of grass onto the gelatinous coating of a Couch’s Spadefoot (Scaphiopus couchii) egg mass on the water surface. The ants ate through the gelatinous envelope and were harvesting the ovum before returning to their nest. Parasitism. Tinsley (1990) noted that Scaphiopus couchii, experience only one significant parasite infection, the monogenean Pseudodiplorchis americanus. The parasite is transmitted during host spawning and provides a natural system for testing the influence of parasite burden on host mating success. The ten-month hibernation involves total starvation during which the blood-feeding parasites reduce fat reserves and hematocrit. The spadefoot emerges and spawns on the first night after rainfall, before they replenish depleted reserves. Male chorusing is energetically very demanding and mate selection is limited to a seven-hour nocturnal assembly and is determined by female choice. Parasite transmission triggered by host sexual activity results in 100% prevalence and high intensities of infection amongst males. About 50% of the spadefoots lose their burdens entirely but the rest carry chronic infections throughout hibernation. Field data show a consistent reduction in intensity each year strongly suggesting resistance. Parasite infection is pathogenic and creates extra stress during hibernation; therefore, to the extent that elimination of infection is heritable, toads entering spawning assemblies with heavy burdens should make poor mates. However, extensive field studies show no correlation between mate success and parasite burden. Although infection can prejudice survival, it is only one of several inter-related factors (including feeding success, tolerance of hibernation). The condition of successful and unsuccessful males in spawning assemblies indicates that all exceed a threshold at which parasite-induced pathology is significant. Males which are debilitated by infection—or other factors—are selected against before mate choice begins. Tocque (1993) followed up on Tinsley’s work. In one area of southeastern Arizona, infected spadefoots emerging from hibernation had significantly smaller fat bodies than those individuals uninfected. However, fat body weights increased during foraging, and after two weeks, there was no measurable effect of adult parasite infection. The difference observed in freshly emerged spadefoots probably reflects the actual parasite drain on the fat reserves during hibernation, representing about seven percent of the annual lipid requirement. Ovary weight also increased during foraging, but it was not possible to determine if the reproductive investment was lower in infected animals. It is possible that spadefoots would not reproduce or may not survive hibernation due to this parasite.

Defenses. Like most amphibians Couch’s Spadefoot produces irritating, noxious molecules in their skin. In humans these molecules cause a runny nose, sneezing, and will likely irritate other moist membranes (Degenhardt et al 1996).Sexual Dimorphism. Vásquez and Pfennig (2008) investigated sexual dimorphism in this spadefoot noting that females commonly use multiple or complex traits to choose mates. The significance of multiple or complex signals remains controversial and largely unknown. Different elements of multiple or complex signals may convey independent pieces of information about different aspects of a prospective mate (the “multiple messages” hypothesis). Alternatively, multiple or complex signals could provide redundant information about the same aspect of a prospective mate (the “redundant” or “back-up” signal hypothesis). The authors investigated these alternatives using spadefoot toads, Scaphiopus couchii. Spadefoot toads primarily use calls to attract their mates, but males also exhibit sexually dimorphic coloration. They investigated whether male coloration is indicative of male size, condition, or infection status by a socially transmitted monogenean flatworm. Vásquez and Pfennig (2008) found that male coloration and dorsal patterning predicts male size and condition but not infection status. Moreover, when females were presented with a choice between a bright male model and a dark male model, the females preferred the bright model. Because aspects of males’ calls are also associated with male size and condition, they conclude that coloration is a potentially redundant indicator of male phenotype. Thus, coloration could enhance mate choice in conjunction with male calling behavior by providing females with a long-distance cue that could enable them to identify prospective mates in a noisy chorus environment where the discrimination of individual calls is often difficult. Generally, such redundant signals may facilitate mate choice by enhancing the quality and accuracy of information females receive regarding prospective mates.

Reproduction. Ephemeral pools that last at least seven or eight days are need for successful reproduction, pools that earlier will result in unsuccessful reproduction. Pools that last longer will produce larger larvae and metamorphs. Clutch sizes can exceed 3000 eggs (Woodward 1987).The advertisement call sounds like,  “loud resonant yē-ŏw repeated at intervals” (Strecker, 1908) and as a “plaintive bellow” by King (1932), Stebbins (1962) described it as  the anxious bleating of a sheep, lasting 0.75 to 1.25 seconds with an interval of 3 to 5 seconds between calls. Figure 2-x is an audiospectragram for Couch’s Spadefoot.

Most breeding is confined to July and August in Arizona, but it may start earlier and last longer in some years and at different localities.  After the first monsoon storms fill temporary pools, and the first reproductive event occurs, subsequent breeding events follow with subsequent rainfalls. Spadefoots may be sexually mature at two or three years of age, and most individuals sampled at breeding sites are five-to-ten years old. Males call from shallow water and arch their head and body upward, exposing a bright white gular region.  Females are attracted to the sound and possibly to the light reflected off the male’s bright white vocal sac. Spadefoots have an amplexus position with the male clasping the female around the waist (pelvic amplexus).  As the female releases her eggs, the male releases sperm, and the fertilized eggs are deposited on vegetation, or left in the water if the pools lack plants.  Females produce clutches of 350-3000 ova in strings that are surrounded by a thin layer of gelatin that is easily folded into what looks like a mass called a wrapped rosary. The eggs become entangled in the vegetation as the adults move during oviposition. Eggs hatch in 1 to 1.5 days and the tadpoles grow quickly, metmorphosis may occur in as little at seven days or as long as forty days at body lengths of 7-13 mm. The tadpoles are benthic and swim with exaggerated beats of their tail (Altig and McDiarmid 2015). The tadpoles feed on detritus. Goldberg (2018) examined 80 S. couchii collected 1959 to 1985 from Pima County, Arizona consisting of 39 adult males (mean snout-vent length, SUL = 60.7 mm ± 4.3 SD, range = 54–75 mm); 28 adult females (mean SUL = 61.7 mm ± 6.9 SD, range = 51-83 mm), three subadult males (mean SUL = 43.3 mm ± 3.5 SD, range = 40–47 mm), one subadult female (SUL = 47 mm), nine unsexed subadults (mean SUL = 35.8 mm ± 7.1 SD, range = 25–45 mm). He found no significant difference between the mean SUL of adult males versus females (t = 0.68, df = 65, P = 0.50). All 25 males collected in July contained abundant sperm. Half of the 14 males from August-September contained residual sperm indicating the peak of reproduction had passed. Some mating could have occurred in August. The smallest reproductively active male (spermiogenesis in progress) measured 54 mm (SUL) and was from July. Wright and Wright (1949) reported males of S. couchii were mature at 48 mm SUL. The ovaries of S. couchii indicated two stages were present in the spawning cycle of S. couchii (1) ready to spawn individuals had mature oocytes predominated; (2) not in spawning condition individuals, had ovaries with degenerating (atretic) oocytes. The smallest reproductively active female (yoked oocytes) measured 55 mm (SUL) and was from July. One slightly smaller female from August (SUL = 51 mm,) contained some small yoked oocytes and was an adult, although it was not in spawning condition. Wright and Wright (1949) reported S. couchii females were mature at 50 mm SUL. Reproductive activity in Scaphiopus couchii is restricted to short periods following warm-season rains. Goldberg’s data support findings of summer breeding during the monsoon and includes evidence that reproduction occurs after the peak of summer precipitation as indicated by S. couchii males with abundant sperm in August and September and most August females in spawning condition. Mayhew (1965) presented evidence that S. couchii reproduced in September in California. Furthermore, Hardy and McDiarmid (1969) reported S. couchii breeding commenced in Sinaloa, Mexico, after the first heavy rains and continued through September. Tinsley and Tocque (1995) analyzed growth rings in the bones of S. couchii collected during seven consecutive seasons (1986–92) and found females may first breed at three years of age and males at two years, with a maximum longevity of about 13 years for females and 11 years for males. Sixty-five percent of breeding animals are at least five years old, 33% are seven years and older and 5% may live for more than ten years. Studies on the age structure of breeding populations have revealed a series of dominant cohorts which originate in wet summers particularly favorable for reproduction; each cohort can be followed for up to four successive years at which time it is replaced by the next dominant age‐group. The annual skeletal ring record shows that individual growth rates are highly variable, and the width of bone produced each year can be correlated with the amount of summer rainfall (and hence feeding opportunities). During their 12 years of fieldwork, there was a virtual alternation between wet (1981, 1983, 1984, 1986, 1988, 1990, 1991) and dry (1982, 1985, 1987, 1989, 1992) summers, and 49% of animals show corresponding alternation of thick and thin growth bands. Despite a pre‐metamorphic development period of only 10 days, tadpole mortality is high in ephemeral ponds, and post‐metamorphs are vulnerable during the very restricted period in which they can accumulate reserves for hibernation; however, evidence from the age‐distribution of breeding populations indicates good survival after maturity. Analysis of individual growth rates shows that, in general, faster‐growing animals are not often represented in the older cohorts and that slower‐growing animals tend to live the longest. Buchholz and Hayes (2000) compared the larval periods of couchii and the related spadefoot Spea multiplicata to determine if their larval periods differ when reared under comparable laboratory conditions. Their results established that these two species vary in larval period length and size at metamorphosis. Comparing Scaphiopus couchi and Spea multiplicata larvae across temperatures, Buchholz and Hayes (2000) found Scaphiopus couchii metamorphosed significantly earlier than Spea multiplicata. At 24⁰C, Scaphiopus couchii metamorphosed 5.9 days earlier than Spea multiplicata. At 28⁰C, Scaphiopus couchii transformed 4.1 days prior. At 32⁰C, Scaphiopus couchii metamorphosed 3.5 days earlier.  Metamorphosing in as little as eight days, Scaphiopus couchii has the shortest larval period reported for any anuran (Bragg 1961; Newman 1989). Because tadpole growth and development are exceptionally phenotypically plastic (Alford and Harris 1988), the hypothesize that tadpoles of any species grown under the same conditions as couchii would metamorphose as quickly. Alternatively, specialized developmental and physiological mechanisms may allow rapid metamorphosis in couchii compared to other species. Adults excavate their burrows and spend eight-to-ten months 20 to 90 cm below the surface.  The over-winter sites are relatively close to breeding sites.  The time spent below the surface may be significantly extended during droughts that span years.  They survive extended dry periods by reducing water loss with several layers of shed skins and accumulating concentrations of urea in their body fluids to create an osmotic gradient and pull water from the soil into their bodies. Couch’s Spadefoots have been observed sitting in the opening of their burrow, waiting to ambush prey.  They feed during rainy and humid nights.    Tadpoles have numerous predators, including larval beetles, larval tiger salamanders, carnivorous Mexican Spadefoot larvae, mud turtles, grackles, and skunks.  Juvenile and adult frogs are prey for a variety of amphibians, snakes, birds, and mammals.  Several authors have commented on the irritation caused by Couch’s Spadefoot skin secretions when it contacts mucus membranes or broken human skin (Stebbins and Cohen, 1995; Waye and Shewchuk, 1995; Degenhardt et al., 1996). The secretions from this species, like many anurans are noxious and have evolved to deter predators. These molecules may cause sneezing and skin irritation in some humans. Goldberg (2018) found postovulatory follicles (evidence of recent spawning) in females with concurrent abundant mature oocytes suggesting S. couchii may spawn more than once in the same summer.

Larval Ecology. Newman (1989) studied the Tornillo Flat population in Big Bend National Park, Texas, and found larvae metamorphosed only from low—density ponds, or ponds that refilled before drying. With enough food S. couchii larvae developed very rapidly (eight to sixteen days). Rapid development maximized the probability of completing development before the pond dried but resulted in small size at metamorphosis.  raised larvae in a factorial field experiment manipulating food level, larval density, and pond duration to test if timing of metamorphosis can be altered by pond drying and to determine if the response to drying is affected by growth history. Both pond duration and growth history affected timing of metamorphosis, but pond duration explained most of the variation in timing of metamorphosis. For a given pond duration, larvae in high—food ponds metamorphosed earlier than did larvae in low—food, low—density ponds. Short pond duration accelerated metamorphosis at a smaller size and with relatively smaller hindlimbs. Larvae may actively avoid desiccation by metamorphosing earlier from short—duration ponds, but probably benefit by delaying metamorphosis in longer duration ponds. The similar relations between pond drying and metamorphosis in experimental ponds and natural ponds indicate that developmental plasticity may play an important role in the ecology of this population. Dayton and Wapo (2002) observed instances of tadpoles feeding on the eggs of conspecifics and duplicated the behavior in the lab. Dayton and Fitzgerald (2001) tested the hypotheses that non-overlapping use of breeding sites by species that use ephemeral breeding sites was due to activity rates of tadpoles that in turn reflect their competitive ability and susceptibility to predation. They found tadpoles of S. couchii were significantly more active and more susceptible to predation than were tadpoles of other species (Gastrophryne, Anaxyrus) that use temporary pools for reproduction. Davis and LaDuc (2017) observed two large aggregations of Scaphiopus couchii tadpoles were found in a shallow, ephemeral pool along a dirt road on Jeff Davis County, Texas. Each aggregation consisted of approximately 700 tadpoles that were visible at the surface of the water. However, turbid water and the presence of individuals at the bottom of the pool prevented a more accurate assessment of the number of tadpoles in each aggregation. Surrounding these large aggregations were smaller aggregations of 8–20 tadpoles. In sum, the total size of each of these aggregations likely exceeded 1000 tadpoles. Most individuals were feeding at the water surface in a vertical position. Aggregations have been observed in other spadefoots (Bragg 1965), this appears to be the first detailed description of aggregation behavior in the field for S. couchii.

Conservation. The IUCN considers this a species of Least Concern, it is widespread, abundant, and there are few reasons to consider this species threatened. Morey (2005) looked at historical versus current abundance and suggests Couch’s spadefoot toads are probably more abundant today than in the past wherever open country still exists, and human activities have created temporary impoundments (Dimmitt, 1977).  He points to southeastern California, where, in some places, road and railroad construction has inadvertently increased the number of temporary pools, many of which have been colonized by Couch’s Spadefoots.  The change in historical versus current abundance, however, is that Couch’s Spadefoot is now absent wherever urban development and irrigated agriculture have destroyed habitats.  Dimmitt and Ruibal (1980a) found 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. Couch’s Spadefoots do depend on vernal pools for reproduction. These pools lack fish, which would consume eggs and larvae.

Fossil Record. Mead (2005) summarized the fossil record for Couch’s Spadefoot from the Deadman Cave and Wellton Hills. The remains from Deadman Cave are not accurately dated but are recovered with typical late Pleistocene fauna. The remains from packrat middens are of early to middle Holocene age. Fossils attributed to Scaphiopus/Spea are recorded from Papago Springs Cave (Czaplewski et al., 1999).Taxonomy and Systematics. Jean Louis Berlandier was a French/Swiss naturalist (Lawson, 2012) who settled in Matamoras, México and made a collection of flora and fauna in the early 19th century. Darius Couch, a US soldier, businessman, and naturalist, purchased Berlandier’s collection with his own money and gave it to the Smithsonian. At the Smithsonian, Spencer Baird (1854) described Scaphiopus couchi based upon specimens from the Berlandier collection. Baird reported the type localities as Coahuila and Tamaulipas México. Cope (1863) described Scaphiopus varius and later (Cope 1866) considered it a subspecies of S. couchii (S. couchii varius), but then returned it to full species status (1875). Boulenger (1882) treated varius as a synonym of couchii, and Cope (1889) agreed with this arrangement. Scaphiopus rectifrenis Cope 1863 was also relegated to subspecific status under S. varius by Cope (1875), it was later recognized as a separate species by Boulenger (1882) and synonymized with S. couchii by Cope (1889). Smith and Sanders (1952) resurrected the name rectifrenis as a subspecific designation for western populations of couchii, and Langebartel and Smith (1954) and Fugler and Webb (1956) referred Mexican specimens to this race. However, Zweifel (1956) suggested that the recognition of geographic races should await a detailed analytic study, and subsequent authors have followed this course. Spea laticeps Cope, 1893, was ignored in the literature until Chrapliwy and Malnate (1961) showed it to be a synonym of S. couchii.

Molecular Work. Garcia-Paris et al. (2003) discovered substantial genetic divergence (6.2%) in the 16S gene between S. couchii from Rodeo, New Mexico, and Baja, Mexico, suggesting Scaphiopus couchii is likely multiple cryptic species.