Bufo alvarius Girard, 1859
Colorado River Toad
M.J. Fouquette Jr.1
Charles W. Painter2
1. Historical versus Current Distribution. Colorado River toads (Bufo alvarius) are one of the least known toads in the United States (Fouquette, 1970; Degenhardt et al., 1996. The type locality is Fort Yuma, Imperial County, California (Fouquette, 1968, 1970). In the United States, Colorado River toads range from extreme southwestern New Mexico (southwestern Hidalgo County only) throughout southern Arizona to southeastern California (Wright and Wright, 1949; Fouquette, 1970; Peters and McCoy, 1978), essentially restricted to the Sonoran Biotic Province (Dice, 1939). Stebbins (1951) suggested a possible extension north along the Colorado River bottomlands to southern Nevada and Utah. No records of this species are known for Nevada or Utah or for California north of the area of the type locality. Colorado River toads range from near sea level to 1600 m (Cole, 1962; Fouquette, 1970).
2. Historical versus Current Abundance. Both King (1932) and Cole (1962) reported Colorado River toads as common near Tucson, Arizona, and west to the Colorado River. Stebbins (1985) indicated these toads are widespread throughout the desert. Today, while Colorado River toads seem to be abundant at many desert localities in Arizona, they appear to have declined in New Mexico (Degenhardt et al., 1996) and California (Jennings and Hayes, 1994a).
3. Life History Features.
A. Breeding. Reproduction is aquatic. Males sometimes give advertisement calls, especially when few other males are present, but may rely more on mate searching (Sullivan and Malmos, 1994).
i. Breeding migrations. During dry, pre-monsoon periods, Colorado River toads seek shelter, often in rodent burrows. With the onset of summer monsoon rains, adults move overland to breeding sites. The extent of these overland migrations has not been investigated. Breeding usually occurs on one night, 2 or 3 d following a major rainfall event (Sullivan and Malmos, 1994).
ii. Breeding habitat. Seasonal and permanent pools (Fouquette, 1970); Musgrave and Cochran (1930) noted that irrigation ditches provide ideal breeding sites, and stock tanks are commonly utilized (Blair and Pettus, 1954; Degenhardt et al., 1996). According to the field notes of J.J. Thornber, adults appear before summer showers and congregate in seasonal pools once the rains begin (Ruthven, 1907; Fouquette, 1970). Stebbins (1985) notes that while breeding activity is stimulated by rainfall, Colorado River toads are not dependent on rainfall for breeding (supported by Arnold, 1943). Sullivan and Malmos (1994) found that all breeding activity occurred on one or two nights following rainfalls of over 25 mm; if rains exceed 75 mm, causing flooding, breeding activity may be delayed for two or three nights. Males exhibit one of two behaviors: they either call for females from shallow water (larger males) or they actively search for females. Sullivan and Malmos (1994) suggest that the number of males in a breeding chorus is inversely related to the likelihood that any single male will call. The breeding season itself can be long (D. Pettus, in Degenhardt et al., 1996).
i. Egg deposition sites. Shallow waters of seasonal and permanent pools.
ii. Clutch size. Average 7,500–8,000 eggs/female encased in a long, single tube of jelly, with a loose but distinct outline. Egg capsule gelatin is clear and not very adhesive; eggs are not partitioned, average 1.6 mm in diameter, and are packed between about 5–7/cm (12–18/in; Wright and Wright, 1949; see also Savage and Schuierer, 1961).
i. Length of larval stage. Not longer than 1 mo (Ruthven, 1907; Musgrave and Cochran, 1930). Tadpoles may reach 57 mm TL (Degenhardt et al., 1996).
ii. Larval requirements.
a. Food. Undescribed; presumably tadpoles are algivorous/omnivorous.
b. Cover. Undescribed.
iii. Larval polymorphisms. Unknown and unlikely.
iv. Features of metamorphosis. Undescribed.
v. Post-metamorphic migrations. Unknown.
D. Juvenile Habitat. Similar to adults.
E. Adult Habitat. Colorado River toads occur primarily in deserts, including mesquite-creosote bush lowlands, but are also found in arid grasslands, rocky riparian zones with sycamore and cottonwoods, and oak-walnut woodlands in mountain canyons (Schmidt, 1953; Fouquette, 1970; Stebbins, 1985; Holycross et al., 1999).
Largely nocturnal, they will take refuge in rodent burrows (Lowe, 1964), rocky outcrops (D. Beck, unpublished data), or in hollows under watering troughs (Wright and Wright, 1949); they breed in seasonal pools (Fouquette, 1970). Wright and Wright (1949) describe Colorado River toads as semiaquatic, found only in wet places around cattle watering troughs and in seasonal wetlands where Couch's spadefoot toads (Scaphiopus couchii) and western spadefoot toads (Spea multiplicata) breed. Wright and Wright (1949) cite E.A. Mearns' 1907 description: "Nothing was seen nor heard of them until the advent of early summer rains, which formed a large shallow lake … These large toads then filled the air with their loud cries until a deafening roar was produced." Likely this description actually was of a chorus of either Great plains toads (Bufo cognatus) or spadefoot toads (Scaphiopus or Spea), as the calls of Colorado River toads are not very loud and choruses are small (Blair and Pettus, 1954; Sullivan and Malmos, 1994); while the call may carry > 100 m (Degenhardt et al., 1996; Sullivan and Malmos, 1994), their choruses would hardly be considered deafening.
F. Home Range Size. Dan Beck (unpublished data) radio-tracked an adult Colorado River toad for a period of 390 d in the Tucson Mountains, Pima County, Arizona. During that time activity centered around the release site, although movements of > 400 m were noted in a single day.
G. Territories. Undescribed.
H. Aestivation/Avoiding Dessication. Degenhardt et al. (1996) suggest that large size and smooth skin may predispose Colorado River toads to desiccation, but there is no experimental evidence for this. Indeed, larger size also means lower surface to volume ratio, thus relatively lower expectation of evaporation. There is no direct evidence for aestivation by the species, and it is not likely that they utilize any form of torpor to any greater extent than other species of Bufo. Dan Beck (unpublished data) radio-tracked an adult Colorado River toad that remained in the same burrow under a railroad tie from 26 September 1988–17 June 1989. Body temperature during that time ranged from 11.7 ˚C–29.7 ˚C. It is possible that during part of that period below the surface the toad may have been in a state of torpor or aestivation.
I. Seasonal Migrations. May migrate several hundred meters from permanent to seasonal pools (Wright and Wright, 1949) following heavy rains.
J. Torpor (Hibernation). Unknown, but see "Aestivation/Avoiding Dessication" above.
K. Interspecific Associations/Exclusions. Will associate with allotopic spadefoot toads (Scaphiopus or Spea spp.) in seasonal wetlands (Wright and Wright, 1949). Found with Great Plains toads in association with watering troughs (Wright and Wright, 1949), and at stock tanks with Great Plains toads, red-spotted toads (B. punctatus), and Couch's spadefoot toads (Scaphiopus couchii; Blair and Pettus, 1954; Sullivan and Malmos, 1994). Gergus et al. (1999) reported hybridization between B. alvarius and B. woodhousii in central Arizona, and Painter (2001, personal observations) documented the first known calling B. woodhousii males within the habitat of B. alvarius in New Mexico, thus increasing the possibility of a breakdown in reproductive isolation.
L. Age/Size at Reproductive Maturity. Males, 80–156 mm; females, 87–178 mm (Wright and Wright, 1949). Adults may be as large as 190 mm (Heringhi, 1969, in Fouquette, 1970; Degenhardt et al., 1996). Sullivan and Fernandez (1999) studied populations in which adult males were mostly 2–4 yr of age.
M. Longevity. Conant and Hudson (1949) cited a record of 2 yr (and 0 mo) in captive toads, while Snider and Bowler (1992) reported a wild-caught adult lived 15 yr, 5 mo, 16 d in captivity; however, based on data from other bufonids, longevity in nature is likely to be at least 4–5 yr.
N. Feeding Behavior. Colorado River toads are active foragers. Prey includes beetles, grasshoppers, wasps, centipedes, millipedes, ants, termites, solpugids, spiders, snails, scorpions, Great Plains toads, Couch's spadefoot toads, small lizards, and mice (King, 1932; Arnold, 1943; Gates, 1957; Cole, 1962; Degenhardt et al., 1996). Colorado River toads will eat almost any prey they can subdue and ingest, including those with defensive stinging capabilities (Cole, 1962; Degenhardt et al., 1996).
O. Predators. Raccoons (Procyon lotor; Wright, 1966) and probably birds, other mammals, and reptiles; adults may be safe from most predators other than raccoons due to the toxicity of their parotoid secretions (see "Anti-Predator Mechanisms" below). The secretory product from the parotoid glands is strongly hallucinogenic (Weil and Davis, 1994), making this species attractive to exploitation by members of certain drug subcultures (Most, 1984).
P. Anti-Predator Mechanisms. Colorado River toads are poisonous (Musgrave and Cochran, 1930; Hanson and Vial, 1956) and exhibit defensive behaviors that include assuming a butting pose with their parotoid glands directed toward the intruder (Hanson and Vial, 1956; Stebbins, 1985). In addition to having paratoid glands, Colorado River toads possess glands resembling paratoid glands on the dorsal surfaces of both fore- and hindlimbs (Fouquette, 1970). Musgrave (in Musgrave and Cochran, 1930) witnessed a fox terrier become paralyzed and die after picking up one of these toads in its mouth; a "police dog" (= German shepherd) became paralyzed and distressed for most of an hour after mere facial contact (see also Noble, 1931; Hanson and Vial, 1956; Wright, 1966). Erspamer et al. (1967) and Cei et al. (1968) report the presence of presumably toxic indolealklamines from the skin and paratoid glands of Colorado River toads. Cannon and Hostetler (1976) and McGill and Brindley (1978) report that the secretory product from the parotoid glands of these toads contains steroid-derived bufogenins.
Wright (1966) reported a raccoon slitting the bellies of Colorado River toads, avoiding the glandular skin and eating the entrails; other predators may not have learned this means of feeding on these toads. Musgrave and Cochran (1930) report that Colorado River toads will "inflate to a remarkable degree and remain in this condition for some time" when disturbed (see photo in Hanson and Vial, 1956).
Q. Diseases. Unknown.
R. Parasites. Goldberg and Bursey (1991a) reported four nematodes from the gastrointestinal tract of the Colorado River toad (Aplectana itzocamensis, Physaloptera sp., Physocephalus sp., and Oswaldocruzia pipiens). They also found one species of cestode (Nematotaenia dispar) as well as a species of nematode from the lungs (Rhabdias americanus).
4. Conservation. Colorado River toads are listed as Endangered by the New Mexico Department of Fish and Game (Degenhardt et al., 1996). Jennings and Hayes (1994a) commented on the status of Colorado River toads and noted that they appear to be extirpated from most or all sites in California and " … some investigators have suggested that Colorado River toads are imperiled throughout much of [their] range …" In southern Arizona, these toads seem to be abundant at many desert localities.
Acknowledgments. Thanks to David Bradford, who commented on an earlier draft of this manuscript. Dan Beck provided unpublished field notes from a radio telemetry study.
1M.J. Fouquette Jr.
Department of Biology
Arizona State University
P.O. Box 871501
Tempe, Arizona 85287-1501
2Charles W. Painter
Endangered Species Program
New Mexico Department of Game and Fish
P.O. Box 25112
Santa Fe, New Mexico 87504
USGS-Patuxent Wildlife Research Center
12100 Beech Forest Road
Laurel, Maryland 20708-4031
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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Citation: AmphibiaWeb. 2020. <http://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 26 Oct 2020.
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