AmphibiaWeb - Anaxyrus microscaphus


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Anaxyrus microscaphus (Cope, 1867)
Arizona Toad
family: Bufonidae
genus: Anaxyrus
Anaxyrus microscaphus
© 2013 William Flaxington (1 of 11)

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sound file   hear call (9048.2K WAV file)

[call details here]

Conservation Status (definitions)
IUCN Red List Status Account Least Concern (LC)
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National Status None
Regional Status None
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bookcover The following account is modified from Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo (©2005 by the Regents of the University of California), used with permission of University of California Press. The book is available from UC Press.

Bufo microscaphus Cope, 1867
Arizona Toad

Terry D. Schwaner1
Brian K. Sullivan2

We follow Gergus et al. (1997), Gergus (1998), and Crother et al. (2000) in recognizing Arizona toads as a distinct species.

1. Historical versus Current Distribution. Arizona toads (Bufo microscaphus) were originally described by Cope (1867), type locality: “Arizona…near the parallel of 35˚ and along the valley of the Colorado from Fort Mojave to Fort Yuma.” Shannon (1949) restricted the type locality to Fort Mohave, Mohave County, Arizona. Based on allozyme evidence, morphological diagnosability, and allopatry, Gergus (1998) argued that each of the three members of the B. microscaphus complex previously recognized as subspecies (B. m. microscaphus, B. m. californicus, and B. m. mexicanus; Price and Sullivan, 1988) should be recognized as full species. Arizona toads range from the Mogollon plateau of southwestern New Mexico westward to the Colorado and Virgin River basins of northwestern Arizona, southern Nevada, and southwestern Utah.

The current distribution of Arizona toads is probably largely similar to their historical distribution (e.g., A.P. Blair, 1955; Stebbins, 1985; Price and Sullivan, 1988; Sullivan, 1993, 1995; Hovingh, 1997). It appears that Arizona toads have declined at some sites following habitat disturbance; they are apparently being replaced by Woodhouse's toads (B. woodhousii) at some localities in Arizona (Sullivan, 1993) and southern Nevada (D. Bradford and colleagues, unpublished data).

2. Historical versus Current Abundance. Historical abundance is unknown; current abundance is presumably higher in areas not disturbed by damming, introduced predators, degrading land-use practices, and human use of recreational areas. In Arizona, 55 new localities have been documented for Arizona toads over the past 15 yr (1980–'95); given their presence at many historical sites, it appears a general decline has not occurred in this region (Sullivan, 1993; B.K.S., unpublished data).

Mark-recapture studies conducted at Lytle Preserve, Beaver Dam Wash, a tributary of the Virgin River in extreme southwestern Utah and southern Nevada, estimated 95% confidence limits of 436–972 adult toads in a 4-m wide, 1,500-m long road and stream transect in July–August 1996 (Schwaner et al., 1998). At Birch Creek, Zion National Park, in a 1,000 m long stream transect of similar width, 357–507 (95% confidence limits) adults were estimated in April–August 2000 (T.D.S., unpublished data). High densities at Lytle Preserve may be atypical compared to most other areas due to availability of irrigated fields for foraging following the breeding period. Schwaner (personal observations) found average monthly captures along the road and stream transect at Lytle Preserve were 167 (range = 31 in October and 330 in August). In a stream transect of similar proportions at Oak Creek, a tributary to the Virgin River in Zion National Park, southwestern Utah, average monthly adult captures were only 60 (range = 22 in May and 113 in June).

3. Life History Features.

A. Breeding. Reproduction is aquatic. Amplectic position is axillary; oviposition is aided by the "basket method" (A. Brown et al., 2000). The advertisement call of Arizona toads is a trill, averaging 5.7 s in duration, with a dominant frequency of about 1.380 kHz and a pulse rate of roughly 46 pulses/s at 15 ˚C (Gergus et al., 1997).

i. Breeding migrations. In west-central Arizona (Sullivan, 1992b), and at Lytle Preserve in Utah (T.D.S., personal observations), calling males have been heard as early as February following warm days. Breeding begins in early to late February in Arizona, but in early to late March to early April in Utah or at higher elevations in Arizona (A.P. Blair, 1955; Sullivan, 1992b, 1995; T.D.S., personal observations). Breeding is not triggered by rain (B.K.S., personal observations), but more likely by warming nocturnal, ambient temperatures (air, 8–18 ˚C, water 12–18 ˚C; Sullivan, 1992b). Spring flooding delays breeding (Sullivan, 1992b; T.D.S., personal observations). Breeding choruses may be of short duration, lasting 10–12 d for less dense populations, and may be interrupted by flooding, only to resume following warmer, drier weather (Sullivan, 1992b).

Sullivan (1992b) observed female selection of calling males in Arizona toads. Working at Lytle Preserve, T.D.S. (personal observations) found only a few calling male Arizona toads among dense aggregations of non-calling and searching satellite males who invariably mated with approaching females. This behavior was much like Sullivan’s (1982a) description of mating in Great Plains toads (B. cognatus), except that females were either already clasped by a male before reaching the water or immediately clasped by any male sensing her presence in the water.

ii. Breeding habitat. Breeding sites are the edges of streams or shallows, backwashes, and side-pools where flow is minimal. In central Arizona, males often call from rocks along the edge of flowing streams and shallow rivers if pools or backwaters are unavailable (B.K.S., personal observations). Cottonwoods (Populus fremontii), willows (Salix spp.), and seep willows (Baccharis spp.) are common plants associated with lower elevation riparian areas used for breeding in Arizona and New Mexico. Factors explaining site selection at Oak Creek, Utah, in 1997 included areas of the stream with numerous side-pools and warmer diurnal water temperatures, perhaps facilitated by a reduction in tall trees and their canopy cover (Dahl et al., 2000).

B. Eggs.

i. Egg deposition sites. Riparian areas of streams, shallows, backwashes, and side-pools.

ii. Clutch size. Blair (A.P., 1955) indicated an average of 4,500 eggs/clutch. Schwaner et al. (1998) observed 227 egg masses laid in a 2-km section of stream at Lytle Preserve over a period of 16 d in March 1997. Although breeding ceased, some males continued to call until early June of that year (see Sullivan, 1992b). Eggs hatch in 3–6 d, depending on water temperature (Schwaner et al., 1998).

C. Larvae/Metamorphosis. Larvae and eggs are described in Stebbins (1951, 1985). Altig et al. (1998) provide a key to larvae of Arizona toads and their relatives. Sweet (1992) reported larval development and metamorphosis of arroyo toads from April–August (even September at higher elevation sites); Schwaner et al. (1998) indicated a shorter cycle, March–June or mid July, at Lytle Preserve, but longer cycles exist for populations in cooler water at higher elevations in the headwaters of the Beaver Dam Wash and Birch Creek, Zion National Park (T.D.S., unpublished data). Clearly, mating systems and developmental cycles vary according to environmental variables related to seasonal temperature and water cycles, which in turn are dependent in part on latitude and elevation. Most clutches of Arizona toads 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; T.D.S., unpublished data), although flooding in late spring (April–June) can also cause high mortality (T.D.S., B.K.S., personal observations).

D. Juvenile Habitat. Detailed observations are available only for the closely related arroyo toads, and so at this time the reader is referred to that account (Sweet and Sullivan, 2001).

E. Adult Habitat. Arizona toad populations in Arizona and Utah utilize sandy marginal zones or terraces with a mixture of dense willow clumps and open flats or flood channels situated within 100 m or so of the stream, plus adjacent terraces with cottonwoods or live oaks (see Sweet, 1992). At higher elevations (2,000 m) in Arizona and New Mexico, Arizona toads are still associated with riparian communities but may move more widely in associated forest biotic communities (e.g., Ponderosa pine) during summer rains (B.K.S., personal observations).

F. Home Range Size. Juvenile (young of the year, > 30 mm SVL) Arizona toads migrated from streamside to irrigated fields (at distances of 50–200 m) at Lytle Preserve, Utah, in August 1996–'98; adult females migrated to the fields shortly after breeding in March–April 1997–'98; and adult males appeared in the fields from May–June 1997–'98 (T.D.S., personal observations).

G. Territories. At sites in central Arizona in 1990 and 1991, male Arizona toads were grouped in choruses of 2–15 individuals (but usually groups of 2–4) along a 50–100 m stretch of river with interindividual spacing of about 1 m (Sullivan, 1992b). Subsequently, following a series of wet years, males still maintained interindividual distances of about 1 m, but densities were considerably higher (1997) with as many as 40 males observed in a chorus along an approximately 80 m stretch of the Agua Fria River (B.K.S., personal observations). Sweet (1992) reported a minimum distance between adult, calling male arroyo toads of 1–8 m, with no evidence of agonistic or territorial behavior, although the densities of males/pool were typically low (1–3 males). Neither author reported active searching or satellite male behavior. However, T.D.S. (personal observations) saw several groups of up to 15 adult male Arizona toads along stretches of the Beaver Dam Wash, 1997–'99, at average distances between males of 0.5–1.5 m. Only a few males/group called regularly, but movement of a male caused immediate clasping from an adjacent male, whereupon the clasped male gave a release call. The clasping male then released its hold whereupon the released male called; the clasping male then moved 0.5–1 m away. Subsequent movements often prompted the released male to approach and clasp the previously clasping male resulting in the same stereotyped behavior. This behavior often brought other adjacent males into the area so that male–male interactions occurred frequently and repeatedly among the same group of males.

H. Aestivation/Avoiding Dessication. Unknown. Adults are nocturnal, emerging from sandy burrows at dusk (Sweet, 1992; T.D.S., B.K.S., personal observations).

I. Seasonal Migrations. None observed or reported.

J. Topor (Hibernation). Presumably September–February; activity for Arizona toads at Lytle Preserve in Utah can begin in February and last to September (T.D.S., personal observations).

K. Interspecific Associations/Exclusions. Woodhouse's toads hybridize with Arizona toads at a number of sites in Arizona and at the junction of tributaries to the main course of the Virgin River in southwestern Utah, southern Nevada, and northwestern Arizona (Shannon, 1949; Stebbins, 1951; A.P. Blair, 1955; Sullivan, 1986b, 1995; Sullivan and Lamb, 1988). Blair (A.P., 1955) reported clasped Arizona toads and red-spotted toads (B. punctatus) without hybridization. Arizona toads have been observed breeding with Great Basin spadefoot toads (Spea intermontana), red-spotted toads, canyon treefrogs (Hyla arenicolor), and American bullfrogs (Rana catesbeiana) in southwestern Utah (T.D.S., personal observations), and with Great Plains toads (B. cognatus), red-spotted toads, Woodhouse's toads, canyon treefrogs, and lowland leopard frogs (R. yavapaiensis) in central Arizona (B.K.S., personal observations).

L. Age/Size at Reproductive Maturity. In central Arizona, males at a breeding aggregation along the Hassayampa River (n = 26 over 2 yr) ranged from 53–79 mm SVL (B.K.S., personal observations). The smallest male Arizona toads in 30 male-female clasping pairs observed at Lytle Preserve in Utah was 53 mm SVL; the smallest female was 56 mm SVL (T.D.S., unpublished data).

M. Longevity. Size frequency distributions, growth rates, and skeletochronology all indicated only four generations of Arizona toads at Lytle Preserve (Schwaner et al., 1998). However, these measures suggest five generations at Birch Creek and Zion National Park (T.D.S., unpublished data).

N. Feeding Behavior. No comprehensive analysis of feeding behavior has been conducted. Larvae presumably feed on algae and small, unicellular organisms attached to local substrates.

O. Predators. Killdeer (Charadrius vociferus ) and intermountain wandering garter snakes (Thamnophis elegans vagrans) preyed on Arizona toad larvae in southwestern Utah, and raccoons killed and consumed many adult male toads during the breeding season (T.D.S., personal observations). Predation on breeding males and females by small mammals has also been observed for populations in central Arizona (B.K.S., personal observations).

P. Anti-Predator Mechanisms. Eggs are presumably distasteful to snakes (see Sweet, 1992). Adults in breeding choruses are quick to retreat underwater at slight disturbances (T.D.S, personal observations). Parotoid glands produce steroids, possibly rendering adults unpalatable to some predators (Duellman and Trueb, 1986).

Q. Diseases. There is no evidence of disease among field populations.

R. Parasites. Goldberg et al. (1996c) described the helminths of Arizona toads, Woodhouse's toads, and their hybrids from a site in central Arizona.

4. Conservation. Populations of Arizona toads in central Arizona are threatened by habitat destruction and interspecific hybridization (Sullivan, 1986, 1993). Damming along the Agua Fria River has reduced lotic habitats favored by Arizona toads for breeding and provided lentic environments favored by Woodhouse’s toads. Interspecific hybridization is occurring in these areas and appears to be unidirectional (Malmos et al., 2001), with Woodhouse's toad females mated by Arizona toad males. Historically, these species hybridized at the junctions of several tributaries of the Virgin River, where branch streams and washes entered the main course of the river, and along stretches between these locations (Blair, 1955; Sullivan, 1995). Most recently, T.D.S. and B.K.S. (unpublished data) found genetic evidence for hybrid swarms at these locations, and introgression of B. woodhousii (16S rRNA) genes into putatively “pure” B. microscaphus populations 100 km upstream from the junction of the Beaver Dam Wash and the Virgin River. They also found genetic evidence for hybridization (marker alleles for aspartate amino transferase; Sullivan and Lamb, 1988) in populations near the junction of Ash-Laverkin Creek and the Virgin River, near Zion National Park. Although anecdotal, based on observations in a single season, morphotypes representing the range of phenotypes of the two species and their hybrids bred throughout March 2001 (T.D.S., unpublished data) at the junction of the Beaver Dam Wash and the Virgin River. Populations of putatively “pure” B. woodhousii did not begin to breed until the beginning of April 2001 at Mesquite, only 20 km away and at the same elevation. This evidence suggests that effects of hybridization are not localized and perhaps behavioral as well as ecological, that genetic barriers to introgression have broken down, and most, if not all, hybrid zones are in areas of human disturbance (dams and golf courses along tributaries and rivers). Southwestern Utah, northwestern Arizona, and southeastern Nevada contain large and continuous populations of B. microscaphus. However, the integrity of these populations is challenged by hybridization, at least in part due to human activities. As reported in the local media, human population growth in this area is expected to increase from tens of thousands to hundreds of thousands of people (many retirees) in the next 30 yr.

Acknowledgments. T.D.S. thanks the Utah Division of Wildlife Resources for permits, Brigham Young University for use of the facilities at Lytle Preserve, Zion National Park for access to Oak Creek populations of toads, and Wes Adams, Anna Brown, Brooke Christensen, D. Rijeana Hadley, and Kim Jenkins for undergraduate research assistance. B.K.S. acknowledges support of the Arizona Game and Fish Department, and the assistance of Rob Bowker, Mike Demlong, Matt Kwiatkowski, Erik Gergus, and Keith Malmos.

1Terry D. Schwaner
Department of Biology
Southern Utah University
Cedar City, Utah 84720

Present address:
Department of Biology
North Georgia College & State University
Dahlonega, Georgia 30597

2Brian K. Sullivan
Department of Life Sciences
Arizona State University West
P.O. Box 37100
Phoenix, Arizona 85069

Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.

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