Arizona Toad (Anaxyrus microscaphus)

Photo by Randy Babb

Arizona Toad, Gila Co., AZ. Photo by Randy Babb.

Location: Sonoran Desert

Image Gallery


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The Arizona Toad, Anaxyrus microscaphus (see image gallery), has geographically variable coloration but is often gray and beige dorsally, with hues ranging from rust, brown, pink, to dull yellow (Behler and King 1979, Stebbins 2003, Elliott et al. 2009). Juveniles are typically salmon or light olive with reddish to reddish-brown warts. There is usually a light stripe or patch on the head and back. The parotoid glands are oval and widely separated. Cranial crests are absent (Behler and King 1979, Stebbins 2003, Elliott et al. 2009). Male toads in Arizona range from 53-79 mm (2.1-3.1 inches) snout to vent length. Arizona Toads enter torpor presumably in September and become active in February (Schwaner and Sullivan 2005). Males begin calling for mates as early as February following warm days. Breeding begins in early February in Arizona. Breeding is not triggered by rain but rather by warm temperatures (Schwaner and Sullivan 2005). There are an average of 4,500 eggs per clutch (Blair 1955). Eggs hatch in 3 to 6 days depending on water temperature (Schwaner and Sullivan 2005).

The range of the Arizona Toad is rather fragmented. It occurs mainly in Arizona (Figure 1 in image gallery), but also ranges in southeastern Nevada and southwestern Utah (Sullivan 1993). There is limited distribution in the southeastern portion of California along the Colorado River, though it has likely been extirpated from the California side of the river (Sullivan 1993). Distribution in New Mexico extends from the Arizona boarder into the southwestern quarter of the state (Behler and King 1979, Stebbins 2003, Elliott et al. 2009).

Habitat associations in Arizona typically include riparian areas from lowlands, such as the Fort Mohave area, to the high upland pine-oak woodlands of the Arizona Colorado Plateau (Stebbins 2003). It can be found in the loose gravelly areas of streams in the more arid portions of its range. In the less arid portions of its range, it is often seen on sandy banks of quieter waters (Behler and King 1979). It is found in both seasonal and permanent streams in the arid lowlands and is associated with the rocky mountain streams in oak-pine forests (Elliott et al. 2009). Breeding habitat includes areas along the edges of streams, side-pools, and backwashes where flows are slow. In arid environments, cottonwoods, willows, and seep willows are commonly associated with breeding habitats (Schwaner and Sullivan 2005). Arizona Toads are typically active at night, emerging from sandy burrows at dusk.

The Arizona Toad was originally described as Bufo microscaphus by Cope (“1866” 1867:301; Figure 2 in image gallery). The type locality was designated as “Arizona . . . near the parallel of 35°, and along the valley of the Colorado from Fort Mojave to Fort Yuma.” Anaxyrus californicus and A. mexicanus were formerly included in Bufo microscaphus as subspecies, collectively known as the Southwestern Toad, but have been separated as distinct species by Gergus (1998). North American toads were removed from the genus Bufo and placed into Anaxyrus by Frost et al. (2006).

Hybridization with the Woodhouse’s Toad, Anaxyrus woodhousii, has compromised the genetic integrity of Arizona Toad populations, to a point where uncontaminated A. microscaphus populations no longer occur in some areas (Sullivan 1986, Schwaner and Sullivan 2005). Woodhouse’s Toad prefers aquatic areas with still or standing water, such as on golf courses and other areas of human disturbance (Stebbins 2003). Habitat alternation had led to further decline of the Arizona Toad while encouraging encroachment of the Woodhouse’s Toad, facilitating hybridization (Sullivan and Lamb 1988, Stebbins 2003). Crosses between the Woodhouse’s Toad and the Arizona Toad have led to hermaphrodites (Sullivan et al. 1996). Both species have a karyotype consisting of 2n=22. (Cole et al. 1968).

Predators of Arizona Toads include birds, such as Killdeer (Charadrius vociferus) and reptiles, such as Wandering Gartersnakes (Thamnophis elegans vagrans). Raccoons (Procyon lotor) commonly consume Arizona Toads during the breeding season (Schwaner and Sullivan 2005). Reportedly, eggs of the Arizona Toad are distasteful to snakes. The parotoid glands produce steroids that likely make Arizona Toads unpalatable to some predators (Duellman and Trueb 1986). The gastrointestinal tracts, lungs, and urinary bladders from 77 Anaxyrus microscaphus, 61 Anaxyrus woodhousii, and 8 of their hybrids were examined for helminthes by Goldberg et al. (1996). One species of trematode (Glypthelmins quieta), 1 species of cestode (Distoichometra bufonis), and 5 species of nematodes (Aplectana incerta, A. itzocanensis, Rhabdias americanus, Physaloptera sp., and Physocephalus sp.) were found. Hybrids harbored fewer parasites than either parent species (Goldberg et al. 1996).

Literature Cited

Behler, J. L., and F. W. King. 1979. Field Guide to North American Reptiles and Amphibians. National Audubon Society. Alfred A. Knopf, New York.

Blair, A. P. 1955. Distribution, variation, and hybridization in a relict toad (Bufo microscaphus) in southwestern Utah. American Museum Novitates 1722:1-38.

Cole, C. J., C. H. Lowe, and J. W. Wright. 1968. Karyotypes of eight species of toads (Genus Bufo) in North America. Copeia 1968:96-100.

Cope, E. D. “1866” 1867. On the Reptilia and Batrachia of the Sonoran Province of the Nearctic Region. Proceedings of the Academy of Natural Sciences of Philadelphia 18:300-314.

Duellman, W. E., and L. Trueb. 1986. Biology of Amphibians. McGraw-Hill Book Company, San Francisco, CA.

Elliott, L., C. Gerhardt, and C. Davidson. 2009. The Frogs and Toads of North America. A Compreensive Guide to their Identification, Behavior, and Calls. Houghton Mifflin Harcourt, New York.

Frost, D. R., et al. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297:1-291.

Gergus, E. W. A. 1998. Systematics of the Bufo microscaphus complex: allozyme evidence. Herpetologica 54:317-325.

Goldberg, S. R., C. R. Bursey, K. B. Malmos, B. K. Sullivan, and H. Cheam. 1996. Helminths of the Southwestern Toad, Bufo microscaphus, Woodhouse’s Toad, Bufo woodhousii (Bufonidae), and their hybrids from central Arizona. Great Basin Naturalist 56:369-374.

Schwaner, T. D., and B. K. Sullivan. 2005. Bufo microscaphus Cope, 1867 “1866” Arizona Toad. Pages 422-424 in: M. Lannoo, editor. Amphibian Declines: The Conservation Status of United States Species. University of California Press, Berkeley.

Stebbins, R. C. 2003. Western Reptiles and Amphibians. Third Edition. Peterson Field Guides. Houghton Mifflin Company, New York.

Sullivan, B. K. 1986. Hybridization between the toads Bufo microscaphus and Bufo woodhousei in Arizona: morphological variation. Journal of Herpetology 20:11-21.

Sullivan, B. K. 1993. Distribution of the Southwestern Toad (Bufo microscaphus) in Arizona. Great Basin Naturalist 53:402-406.

Sullivan, B. K., and T. Lamb. 1988. Hybridization between the toads Bufo microscaphus and Bufo woodhousii in Arizona: variation in release calls and allozymes. Herpetologica 44:325-333.

Sullivan, B. K., C. R. Propper, M. J. Demlong, and L. A. Harvey. 1996. Natural hermaphroditic toad (Bufo microscaphus × Bufo woodhousii). Copeia 1996:470-472.

Author: Howard Clark

Originally published in the Sonoran Herpetologist 24(2):14-15.


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