Arizona Toad

Arizona Toad, Anaxyrus microscaphus (Cope, 1867)

Arizona Toad, Anaraxyus microscaphusArizona Toad, Anaraxyus microscaphus

Arizona Toad, Anaraxyus microscaphus


Distribution of Anaraxyus microscaphus.Distribution of Anaraxyus microscaphus.

Distribution of Anaraxyus microscaphus.

Anaxyrus microscaphus (Cope, 1867)

Adults can reach 86 mm total length; males are smaller (70 mm maximum) than females (86 mm maximum).  The Arizona Toad presents as a typical toad, drab in color with short legs and a parotoid gland. It is a medium sized toad that is gray, red-brown, olive, or yellow. A pale stripe or bar is present between the eyes. The parotoid gland has an elongate oval shape, and a vertebral stripe and a large pair of scapular blotches are absent.  The venter is white.  Cranial crests are absent or poorly developed.

Larvae are gray olive with a lightly pigmented dorsum and a transparent tail with dark blotches.  It is most easily confused with the Woodhouse’s Toad that usually has a well-developed vertebral stripe, and with the Great Plains Toad that has a pair of scapular blotches. It will hybridize with Woodhouse’s Toad (see below).Voice. The advertisement call is a melodious trill lasting an average of averaging 5.7 seconds with a rising pitch; the dominant frequency is about 1.380 kHz and a pulse rate of roughly 46 pulses/s at 15 ˚C (Gergus et al. 1997).

Distribution, Habitat, and Hybridization. The species occurs from the Mogollon Plateau of southwestern New Mexico westward to the Colorado and Virgin River basins of northwestern Arizona, southern Nevada, and southwestern Utah.  It uses sandy soil areas bordering streams with dense willows, open flats, and flood channels within 100 m or so of the stream, as well as adjacent areas with cottonwoods or live oaks. Arizona Toads are in torpor from September to February.  Sullivan (1993) initiated a survey of the distribution of this toad in Arizona in 1990 and compare the historic and present distributions. It occurs in Interior Chaparral, Great Basin Conifer Woodland, and Petran Montane Conifer Forest (Holycross and Brennan 2006).Sullivan (1993) found populations of B. microscaphus in west central Arizona that appeared to be thriving. However, demographic data allowing an adequate evaluation of the status of these (or any other) populations are unavailable. During 1990-91, adults, juveniles, and larvae were noted at most of the historic localities (Bill Williams and Hassayampa drainages). Over most of this area there is no immediate threat from hybridization with B. woodhousii. However, documentation of B. woodhousii at Alamo Lake suggests that areas of hybridization immediately upstream and downstream from the impoundment may have been initiated recently following establishment of lentic habitats preferred for breeding by B. woodhousii (Sullivan 1986). He notes it is difficult to predict the outcome of such interactions, but in the absence of further modification of the riparian corridor it is reasonable to anticipate that B. woodhousii will remain confined to the general vicinity of Alamo Lake. It should be noted that in contrast to the report of Jones (1981), no B. woodhousii were found in 1990-92 on any of the tributaries of the Bill Williams River.  Although most of the sites visited were relatively disturbed (human recreational activity, cattle and burro grazing), other than at Alamo Lake, lotic habitats preferred by B. microscaphus for breeding predominated.

In the south-central portion of the state, B. microscaphus is apparently no longer present on the lower reaches of the Agua Fria River or Cave Creek. Along the Agua Fria River, it appears that B. woodhousii has replaced B. microscaphus to a point just upstream from Lake Pleasant proper (Table Mesa road crossing). With the documentation of vigorous reproductive activity by B. microscaphus at Black Canyon City in 1992, it seems reasonable to conclude that without additional habitat alteration, B. woodhousii will be unable to move further upstream from the Table Mesa Road site. The expansion of Waddell Dam should provide an excellent opportunity for testing this hypothesis.The Verde River Valley (Clarkdale to Camp Verde) is one area in which historic collections (all prior to 1960) documented Bufo microscaphus, but where only B. woodhousii are present today.

The increase in lentic habitats associated with agricultural activities in the Verde Valley presumably has favored the establishment of populations of B. woodhousii. It is reasonable to conclude that pure populations of B. microscaphus remain intact along the relatively undisturbed and inaccessible sections of the Verde River (e.g., Perkinsville) and its tributaries (e.g., Sycamore Creek). For example, a “pure” breeding aggregation of B. microscaphus was observed at Perkinsville on the Verde River on 22 May 1989. However, the extent to which B. woodhousii has gained access to the major tributaries of the Verde (e.g., Wet Beaver Creek, West Clear Creek, Oak Creek) remains unclear; B. woodhousii is currently present in the lower reaches of all these streams. The abundance of larvae at Cedar, Eagle, and Bonita creeks, as well as the Blue River, indicates that populations of B. microscaphus occur throughout the historic range in this part of the state. Bufo woodhousii occurs in the Gila River proper and is thought to hybridize with B. microscaphus near the mouths of both Bonita and Eagle creeks (Sullivan 1986). The present results suggest that B. woodhousii has not moved further upstream into the tributaries of the Gila River, but additional study will be necessary to adequately assess the extent of hybridization in this region. Sixteen years after this was published (Schwaner and Sullivan, 2009) reported the following.

Conservation of species subject to hybridization strongly urge long-term studies to determine the extent and the direction of hybridization and introgression, especially in relation to human-induced habitat alterations. They extended previous work on hybridization between the Arizona Toad (Bufo microscaphus) and Woodhouse’s Toad (B. woodhousii) along Beaver Dam Wash by evaluating morphological and genetic status of four populations. Populations occur at sites from the high elevation headwaters to the hybrid zone at the confluence with the Virgin River in extreme southwestern Utah and extreme northwestern Arizona. Hybrid indices for individuals at the confluence shifted from predominantly microscaphus-like in 1949–1953 samples, to predominantly woodhousii-like in 1991-1992 samples, and back to microscaphus-like in 2001 samples. Nuclear and mitochondrial markers identified 49 individuals from the confluence site as “parental” types, “F1” hybrids, or reciprocal backcrossed hybrids. Although observed and expected frequencies of individuals in each of six cytonuclear categories were similar, numbers of hybrids with the woodhousii cytotype were significantly greater than those with the microscaphus cytotype. By contrast, hybrid indices of toads upstream (45–97 km) from the confluence were predominantly microscaphus-like in the 2001 samples, like earlier reports. Nonetheless, individuals with woodhousii mtDNA and microscaphus nuclear markers were found at sites 45.0 and 64.4 km, but not 96 km, upstream from the confluence of Beaver Dam Wash and the Virgin River. These results indicate that the confluence site is a hybrid swarm, and that introgression of woodhousii mtDNA into putatively “pure” microscaphus populations occurs for a considerable distance upstream along Beaver Dam Wash.

Six years later Sullivan et al. (2015) found microscaphus occupied the entire Agua Fria River drainage in central Arizona until relatively recently. By the 1980s, woodhousii, colonized the lower reaches of the Agua Fria and replaced B. microscaphus at some sites. They tested the hypothesis that habitat disturbance drives replacement of microscaphus by woodhousii, via hybridization, by examining shifts in the distribution of these toads following the expansion of the Waddell Dam on the lower Agua Fria River in the early 1990s. As of 2010, the high elevation headwaters of the Agua Fria River were still occupied by microscaphus, the lower reaches near the confluence with the Gila River were occupied by  woodhousii, and along the middle reaches, hybridization between these two toads occurred at the same three sites as documented in the early 1990s. Contrary to expectations, evidence of hybridization along middle reaches of the river is largely unchanged-  microscaphus has not been replaced by  woodhousii at any additional sites nor is there any evidence of introgression of woodhousii mtDNA into putatively pure microscaphus populations upstream of hybrid sites. Hybrids between Anaraxyus microscaphus and A. woodhousii have be confirmed in Arizona by Wooten et al. (2018), but the two species have not been found to hybridize in New Mexico (Ryan et al. 2017).Wooten et al. (2019) followed up on the hybrids noting that the ecological consequences of impoundment construction on riparian systems has profoundly affected a variety of organisms, including many amphibians. They used microsatellite loci to evaluate 260 individuals representing 10 total populations constituting woodhousii, microscaphus, and putative hybrids along the Agua Fria River in Arizona during two time periods (1992–97 and 2009–10). Consistent with prior work with these two anurans documenting unidirectional replacement or genetic introgression, they predicted that microsatellites would provide evidence of directional introgression of woodhousii into microscaphus. The putative hybrid populations exhibited the highest number of alleles, and microscaphus exhibited the lowest number of alleles. The authors found that the genetic identity of microscaphus remains distinct from woodhousii and the hybrids, suggesting that the genetic structure of the corresponding populations has remained intact.  Anaraxyus woodhousii has not replaced microscaphus along the Agua Fria River beyond those habitats directly associated with impoundment construction.

Diet. Adults feed on insects, with larvae probably feeding on detritus.  Known predators include killdeer, gartersnakes, and raccoons.  Anti-predator mechanisms have not been studied, but parotoid gland secretions presumably function as they do in other toads.

Reproduction. Unlike many other Arizona amphibians, breeding is not triggered by rainfall; rather, warm nocturnal temperatures stimulate reproduction from February to early April in Arizona (Blair, 1955; Sullivan, 1992b, 1995).  The call attracts females to breeding sites and choruses may last 10 to 12 days for small populations.  Choruses may be interrupted by flooding after heavy rains, but the males will resume calling with warmer, drier weather. In west-central Arizona calling males have been heard as early as February following warm days (Sullivan, 1992b).Calling sites are often at the edges of streams or shallow pools where the water is slowly flowing. The streams have typical riparian-edge vegetation of cottonwoods, willows, and seep willows.  Some populations, have relatively few males call; instead, non-calling males actively search for females, and pairs go into amplexus before reaching the water.  Some males intercept females as they approach vocalizing males.  Breeding males have a body length of at least 53 mm, and the smallest breeding females have body lengths of 56 mm (Schwaner and Sullivan 2005). Distances between calling males. Arizona Toads were grouped in choruses of 2–15 individuals along a 50–100 m stretches of river with individuals spaced about 1 m apart (Sullivan 1992b). In years that were wetter males still maintained distances of about 1 m, but densities were higher (Sullivan 1997).Blair (1955) reported an average of 4,500 eggs/clutch. Schwaner et al. (1998) observed 227 egg masses laid in a two-kilometer section of stream over a period of 16 days in March. Although breeding ceased, some males continued to call until early June of that year (Sullivan, 1992b). Eggs incubate and hatch in 3–6 days depending on water temperature (Schwaner et al., 1998).Larval development and metamorphosis occur from April–August and may extend into September at higher elevation sites (Sweet 1992). However, shorter and longer reproductive cycles may occur locally (Schwaner et al. 1998). Mating systems and developmental cycles vary according to temperature and water cycles, which in turn are related to latitude and altitude. Arizona populations reportedly have a high survival rate, with close to 100% of the embryos hatching and reaching the dispersal phase.


Through the course of larval life, metamorphosis, and early juvenile stages, predation appears to be the chief cause of mortality (Sweet, 1992) but stochastic events such as flooding may cause high mortality (Schwaner and Sullivan 2005).Longevity. Size frequency distributions, growth rates, and skeletochronology all suggested only four generations of Arizona toads at Lytle Preserve (Schwaner et al., 1998), and five generations at Birch Creek and Zion National Park Schwaner and Sullivan (2005).Predators and Defense. Schwaner and Sullivan (2005) report Killdeer and the Wandering Garter Snakes as predators on the tadpoles. Raccoons and other small mammals kill and consumed adult toads during the breeding season. Chemical defenses are likely present in the eggs and the skins of larvae and adults; adults in breeding choruses retreat underwater at the slightest disturbance (Duellman and Trueb 1986, Sweet 1992, Schwaner and Sullivan 2005).

Conservation.  Arizona Toads are threatened by habitat destruction and possibly with interspecific hybridization (but see the section on hybridization above). Southwestern Utah, northwestern Arizona, and southeastern Nevada contain large and continuous populations of A. microscaphus. However, the integrity of these populations is challenged by human activities. Human population growth in this area is expected to increase from tens of thousands to hundreds of thousands of people in the upcoming decades.

Taxonomy. Cope (1867:301) described Bufo microscaphus based upon three syntypes (USNM 4106 (now lost), USNM4184, and USNM 132901). USNM 4184 was designated as the lectotype by Shannon (1949:307). Type localities are “Territory of Arizona…chiefly near the parallel of 35°, and along the valley of the Colorado from Fort Mojave to Fort Yuma” and “upper Colorado region.” Shannon, (1949:307) restricted the type locality to “Fort Mohave, Mohave County, Arizona”, USA, based on lectotype selection. Garman (1884:43) implied the combination Bufo lentiginosus microscaphus. Shannon (1949:301) used the combination Bufo woodhousii microscaphus. Stebbins (1951:274) used the combination Bufo microscaphus microscaphus. Frost et al. (2006:363) placed it in the genus Anaxyrus and called it Anaxyrus microscaphus. And, Fouquette and Dubois (2014:305) used the combination Bufo (Anaxyrus) microscaphus.