The Mexican Spadefoot

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Mexican Spadefoot

Mexican Spadefoot

Spea multiplicata (Cope, 1863)

Adults may reach 65 mm SUL; the Mexican Spadefoot has red-tipped tubercles scattered over the dorsum, it lacks the bony boss between the eyes, and their diameter is wider than the space between them. The dorsum color is uniformly brown or dark gray with small dark spots or larger blotches with red-tipped tubercles scattered over the skin, and dorsolateral stripes are indistinct. A short wedge-shaped spade is present on each heel. The iris is variegated and a pale copper color. Male vocal sacs are heavily pigmented (Conant and Collins, 1991).

Voice. One author described the call as a loud purr of a cat with the metallic sound of grinding gears; another describes it as a metallic snore that is a long trill that lasts 0.75–1.5 seconds.

The tadpole’s body is broadest just behind the eyes, and it tapers gradually towards the bottom of the tadpole and tapers sharply towards the top. Tadpoles have a short snout and a tail that is about 1.25–1.33 times the head-body length. The dorsal fin originates posteriorly on the body. The eyes are close together and dorsally oriented on the head; the anus is medial, emerging in the base of the ventral fin. The spiracle is low on the left side, below the lateral axis of the body (Stebbins 1962).

Distribution and Habitat. The species occurs from western Oklahoma to Arizona and southward to Guerrero and Oaxaca, Mexico.  The Mexican Spadefoot subspecies is commonly known as the Chihuahuan Desert Spadefoot, Spea multiplicata stagnalis occurs in the eastern half of Arizona and uses a wide range of arid and semi-arid habitats if breeding pools are available.  They are associated with sandy or gravel soils in semi-desert grasslands, sagebrush flats, semi-arid shrublands, river valleys, and agricultural land.  In Arizona, adults are subterranean most of the year, coming to the surface in July with the monsoon rains, and then returning to their burrows in September. It occurs in Chihuahuan Desert Scrub, Great Basin Desert Scrub, Semidesert Grasslands, Plains and Great Basin Grasslands, (Brennan and Holycross 2006). Boeing et al (2014) found Spea multiplicata using mesquite, grasslands, mesquite succulent, mesquite +creosote and playa habitats in the Chihuahuan Desert.

Diet. The top two food items consumed by New Mexico Spadefoots in playas were Carabidae and Curculionidae (Anderson et al. 1999). Like many anurans, S. multiplicata is a generalized arthropod predator that focuses on ground-dwelling species. Insects and spiders comprise over 90% of their total Biology, with no significant differences in the Biology by sex or season. Arthropods with noxious chemical defenses, such as blister beetles, velvet ants, stink bugs, and millipedes, are usually avoided, but it will occasionally feed on centipedes and scorpions. Studies suggest S. multiplicata may require seven feedings before it has accumulated the fat reserves needed to survive for a year (Degenhardt et al. 1996). Zack and Johnson (2008) analyzed the stomach contents of 65 Great Basin Spadefoot collected in an area of irrigation runoff in south central Washington State. The spadefoots were collected using pitfall traps. The spadefoots consumed at least 56 different arthropod taxa belonging to the orders Coleoptera, Diptera, Hemiptera, Hymenoptera, Lepidoptera, Neuroptera, Orthoptera, Trichoptera, Collembola, and Araneae. Ants and darkling beetles were among the most common prey. Feeding appeared to be very generalized with the spadefoots accepting almost anything they could capture and subdue.

Reproduction. Like most other spadefoots, reproduction is associated with monsoon rains that fill pools in low-lying areas. The average breeding period duration is only about 1.6 days. Males call when floating on the surface of the water.  Adult females lay an average of 1,070 eggs.

Females produce clutches of 300-500 ova, distributed as clusters of two to twenty-six eggs in strings that are in submerged vegetation.  Eggs hatch in 48 to 96 hours and the tadpoles  have a total length of 5-7, metamorphosis may occurs in one to six days and metamorphosis requires 10-45 days (Altig and McDiarmid, 2015).

Mexican Spadefoots have two tadpole morphs: a carnivorous morph and an omnivorous morph.  The carnivorous morph feeds mostly on fairy shrimp (Anostraca sp.), while the omnivorous morph feeds on detritus, algae, and the occasional fairy shrimp.  Carnivorous morphs have a broad head, large jaw muscles, a short gut, and rapid development.  They occur in rapidly shrinking pools with abundant fairy shrimp and low levels of organic debris.  As might be expected, carnivorous larvae develop more quickly than the omnivores and transform at a smaller size and with less body fat than the omnivorous morph.  A large body at metamorphosis is a survival advantage.  Thus, there is a trade-off between the carnivorous morph’s small body size advantage in rapidly drying pools and the omnivorous morph’s post-metamorphic advantage in ponds that do not dry up.  Interestingly, being a carnivorous or an omnivorous morph is plastic, and the tadpoles can change their feeding strategies based on the available food supply.

The factors inducing a tadpole to become carnivorous were experimentally studied by Levis et al. (2015).  They reared tadpoles on different diets (detritus, shrimp, Chihuahuan Desert Spadefoot tadpoles, and Couch’s Spadefoot tadpoles) and found that diets containing Couch’s Spadefoot tadpoles produced more carnivorous Mexican Spadefoots than diets without them.  Couch’s Spadefoot tadpoles make excellent food for the carnivorous multiplicata tadpoles.

Chihuahuan Desert Spadefoots give off noxious secretions, which become airborne and have a smell that has been described as somewhat like pungent peanuts.  These fumes can be a strong irritant to humans.

Hybridizing Spadefoots, Climate Change, and Survival. Understanding how new species arrive is central to biology. Mate choice on the part of either or both sexes is critical to the speciation process. Mate choice can prevent hybridization and produce reproductive isolation between potentially interbreeding groups. At the same time, when hybridization occurs, hybrid female mate choice may play a role in reproductive isolation by affecting hybrid fitness and contributing to a pattern of gene flow between species. Schmidt and Pfennig (2016) investigated if hybrid mate choice behavior could serve as such an isolating mechanism using the spadefoot hybrids of Spea multiplicata and Spea bombifrons. They assessed the mate preferences of female hybrid spadefoot for sterile hybrid males vs. pure-species males in two alternative habitat types in which spadefoots breed: deep water or shallow water.

They found hybrid females in deep water preferred the calls of sterile hybrid males to those of S. multiplicata males. Therefore, 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 male calls to those of S. bombifrons in either environment. Thus, hybrid female mate choice was environmentally dependent.  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.

Shifting climates will drastically alter environmental signals, potentially resulting in environments where hybridization is more likely.  The frequency of hybrids between Mexican Spadefoot (Spea multiplicata) and Plains Spadefoot (Spea bombifrons) is inversely correlated with the size of the ephemeral ponds in which they breed (Pfennig and Simovich 2002). Experimental work has shown that this pattern can be explained by the increased preference for heterospecific Mexican Spadefoots (Spea multiplicata) males expressed by female Plains Spadefoots in shallow water (Pfennig 2007).

Hybrid offspring develop faster relative to pure species; thus, hybridization is adaptive in ponds that would dry before pure species offspring could complete metamorphosis (Pfennig and Simovich 2002). As pond depth is affected by temperature and precipitation, the hotter, drier summers predicted for the western United States where the spadefoots hybrid zone is located (Seager et al. 2007) may increase hybridization between these species (Chunco et al. 2012).

The frequency of hybrids between Mexican spadefoot toads (Spea multiplicata) and Plains spadefoot toads (Spea bombifrons) is inversely correlated with the size of the ephemeral ponds in which they breed (Pfennig and Simovich 2002). Experimental work has shown that this pattern can be explained by the increased preference for heterospecific Mexican Spadefoot (Spea multiplicata) males expressed by female Plains Spadefoot in low water conditions (Pfennig 2007). Because hybrid offspring develop faster relative to pure species, hybridization is adaptive in ponds that would dry before pure species offspring completed metamorphosis (Pfennig and Simovich 2002). As pond depth is affected by temperature and precipitation, the hotter, drier summers predicted for the western United States where the spadefoot hybrid zone is located (Seager et al. 2007) may result in increased hybridization between these species (Chunco et al. 2012, Chunco 2014).

Reproductive behavior is also strongly influenced by the availability of suitable mates. Both males and females of many species are more likely to engage in heterospecific mating if conspecific mates are rare (Hubbs 1955; Avise and Saunders 1984). Climate change may directly impact mate availability by two mechanisms. First, relative frequencies of species may change due to climate-mediated shifts in ranges or differential effects of climate on population dynamics.

 Conservation. The Mexican Spadefoot is considered a species of Least Concern. However, Dinehardt et al. (2010) assessed acute and chronic toxicity of two widely used herbicides to larval Mexican Spadefoot (Spea multiplicata) and the Plains Spadefoot (S. bombifrons) from cropland and native grassland playas. Roundup WeatherMAX® (WM) toxicity estimates (48- and 216-h LC50; 48-h LC1) for both species were like environmental concentrations expected from accidental overspray. Chronic (30-day) exposure to WM at predicted environmental levels (2.0 and 2.8 mg glyphosate acid equivalents/L) reduced the survival of both species. Ignite® 280 SL (IG) toxicity estimates (48-h LC50 and LC1) for both species were above predicted environmental concentrations of 1.0 mg glufosinate/L. Chronic exposure to predicted environmental levels of IG did not reduce the survival of either species. Toxicity test results suggest that at predicted environmental concentrations IG would not cause extensive mortalities among larval spadefoots. However, WM may cause widespread mortality among larvae of these species. Cooper and Peterson (1994) report they found no representative members of this species at the Idaho National Engineering Laboratory during a 1994 survey but note that they had been found in a 1975 survey.

Fossil Record. Mead (2005) summarized the fossil record for the Mexican Spadefoot in Arizona. Remains are known from Deadman Cave and Picacho Peak. The fossils reported here were originally listed as belonging to Scaphiopus hammondi.

Taxonomy and Systematics. Cope (1863:52) described Scaphiopus multiplicatus based on a holotype (USNM 3694) from the type locality: “Valley of Mexico.” The type locality was restricted to “Coyoacán,” Distrito Federal, Mexico, by Smith and Taylor (1950:329), this restriction considered invalid because of not being based on evidence by Fouquette and Dubois (2014). Cope (1866:81) used the combination Spea multiplicata and later described Spea stagnalis (Cope in Yarrow, 1875:525) but did not list the type specimens. Yarrow (1882:177) stated the stagnalis syntypes were USNM 8558 (3 specimens). Cochran (1961:78), considered USNM 8188 (2 specimens), 8563, and 25335 to be syntypes. The type locality was in northwestern New Mexico [USA]; and was given as “Alto dos Utas, N. Mex.”, the USA by Yarrow, (1882:177). Cope (1887:12) later placed S. stagnalis in the synonymy of Spea hammondii.

Brocchi (1887:23) described Scaphiopus dugesi based upon the syntypes: MNHNP 1886.287–288, Cochran (1961:78) considered USNM 16205–07 to be syntypes. The type locality was listed as “Mexique”; and later corrected to “Guanajuato (Mexique)” by Brocchi, 1881:25); it was then given as “Silao de la Victoria, Guanajuato, Mexico” by Cochran (1961:77). Smith and Taylor (1950:329) restricted the type locality to “Coyoacán,” Distrito Federal, Mexico. Cope (1887:12) placed this name in the synonymy of Spea hammondi.

Boulenger (1882: 436) uses the combination Scaphiopus multiplicatus as well as the combination of Scaphiopus stagnalis recognizing both names as valid. Kellogg (1932:19) used the combination Scaphopus hammondii multiplicatus. Firschein (1950:76) used the combination Spea hammondii multiplicata. Spea multiplicata was used by Taylor (1952:794). Scaphiopus (Spea) multiplicatus was employed by Brown (1976:14); Spea hammondi stagnalis, Spea hammondi multiplicata, Spea hammondii stagnalis were used by Tanner (1989:55-58). Wiens and Titus (199:28) used the name Spea multiplicata.