Bufo retiformis Sanders and Smith, 1951
Sonoran Green Toad
Sean M. Blomquist1
1. Historical versus Current Distribution. Sonoran green toads (Bufo retiformis) are known only from Pima and Pinal counties in south-central Arizona and extend south through west-central Sonora to north of Guaymas, Mexico (Hulse, 1978; Stebbins, 1985). The range of Sonoran green toads in the United States extends from San Cristobal Wash and Organ Pipe Cactus National Monument, east to San Xavier Mission and the Altar Valley, and north to Waterman Wash near Mobile, Arizona (Nickerson and Mays, 1968; Hulse, 1978; Stebbins, 1985; Rosen and Lowe, 1996; Sullivan et al., 1996b). Sonoran green toads are found at elevations from 150–900 m (Bogert, 1962; Stebbins, 1985; Sullivan et al., 1996b).
The range of Sonoran green toads is thought to be limited to semi-arid habitats and may be expanding due to irrigation associated with increasing agricultural activity (Bogert, 1962). Hulse (1978) suggests that if this trend continues, Sonoran green toads will expand northward into the irrigated lands of Santa Rosa and Gila Valleys in Arizona. Sullivan et al. (1996b) did not find evidence to support this trend, but recent sightings near Mobile, Arizona, possibly support the northward expansion of Sonoran green toads (Sullivan, 2002; B.K. Sullivan, personal communication).
2. Historical versus Current Abundance. No estimates of abundance or censuses have been published. Large breeding aggregations (e.g., 30–200 individuals) have been reported by Bogert (1962) and Sullivan et al. (1996b, 2000), but the current status of the species remains unknown (Bury et al., 1980; U.S.F.W.S., 1989).
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Sonoran green toads are explosive breeders (Sullivan et al., 1996b). Breeding occurs opportunistically in July–August with the onset of the summer rains (Savage, 1954; Bogert, 1962; Sullivan et al., 1996b; Behler and King, 1998). Males arrive at temporary pools 1–2 d after rains begin (Bogert, 1962), and it is speculated that Sonoran green toads may delay breeding at temporary pools until water levels have stabilized (Bogert, 1962). Breeding has been recorded at air temperatures from 22.5–32.6 ˚C (Bogert, 1962; Ferguson and Lowe, 1969; Sullivan et al., 2000).
In low-density breeding aggregations, all males call actively (Sullivan et al., 1996b). Alternative mating approaches (e.g., satellite males and actively searching males) have been observed at large breeding aggregations (Sullivan et al., 1996b). Their call (Bogert, 1998) sounds to the human ear like “the buzzer on an electric alarm clock” (Savage, 1954) or “a rapid cricket-like trill, on one pitch…a mixture of a buzz and a whistle” (Stebbins, 1954a). Pulse rate and dominant frequency of the call varies with temperature and are negatively correlated with male size (Sullivan et al., 2000).
ii. Breeding habitat. Breeding has been observed in temporary pools formed in roadside ditches, rainwater sumps, cattle tanks, and wash bottoms (Hulse, 1978; Stebbins, 1985; Sullivan et al., 1996b). Males usually call from clumps of vegetation (e.g., grasses or shrubs) on dry, damp, or wet ground within 1–5 m of water (Ferguson and Lowe, 1969; Sullivan et al., 1996b). Males have been observed calling up to 18 m from water (Ferguson and Lowe, 1969). Females approach a calling male on land, and the male continues to call until the female elicits amplexus by touching the male (Bogert, 1962; Sullivan et al., 1996b). The amplexing pair moves to the water to oviposit (Bogert, 1962; Sullivan et al., 1996b).
i. Egg deposition sites. Unlike most other true toads, Zweifel (1970) suggests eggs are not laid in strands, but rather individually or in small clumps (see also Ferguson and Lowe, 1969; Hulse, 1978). Eggs are about 1.2 mm in diameter and hatch at a more advanced stage (i.e., Limbaugh and Volpe stage 19) than other true toads (i.e., Limbaugh and Volpe stage 16 or 17; Limbaugh and Volpe, 1957; Zweifel, 1970; Hulse, 1978).
ii. Clutch size. Clutch size is unknown in the wild. Ferguson and Lowe (1969) artificially extracted 50–200 eggs from Sonoran green toad females by hormone injection and stripping.
i. Length of larval stage. Duration of larval stage in the wild has not been reported, but is expected to be short (e.g., 2–3 wk) based on the duration of temporary pools. Savage (1954) reports collecting a recently metamorphosed individual 13 d after observing breeding in the area. Tadpoles hatch after 2–3 d at 3.1–3.4 mm TL, a size similar to other members of the green toad group (Zweifel, 1970).
ii. Larval requirements.
a. Food. Feeding habits of larvae have not been observed in the wild. Larvae have been raised with limited success in the laboratory on boiled lettuce (Ferguson and Lowe, 1969). Unlike most other true toads, larvae in the green toad group have only two rows of labial teeth posterior to the beak (Zweifel, 1970).
b. Cover. Unknown.
iii. Larval polymorphisms (carnivorous/cannibal morphs). Unknown.
iv. Features of metamorphosis. Knowledge of newly metamorphosed animals is limited. Savage (1954) reports collecting one recently metamorphosed individual (20 mm SVL) moving diurnally across hot sand at temperatures of 38 ˚C.
v. Post-metamorphic migrations. See "Features of metamorphosis" above.
D. Juvenile Habitat. Little is known of juvenile habitat characteristics. Juveniles have been collected in localities with adults and are easily confused with adults of other species in the green toad group (Savage, 1954; Riemer, 1955; Bogert, 1962; Jones et al., 1983; Sullivan et al., 1996b).
E. Adult Habitat. Adult Sonoran green toads have been observed in creosote flats, upland saguaro-palo verde associations, mesquite-grasslands, and arid and semiarid grasslands; they also extend into the Pacific Coastal Plain near Hermosillo, Mexico (Bogert, 1962; Hulse, 1978; Stebbins, 1985; Sullivan et al., 1996b; Behler and King, 1998). Adults are nocturnal and rarely seen except at breeding aggregations (Stebbins, 1985; Behler and King, 1998). The habits of Sonoran green toads away from breeding aggregations have not been studied.
F. Home Range Size. Unknown.
G. Territories. Unknown.
H. Aestivation/Avoiding Dessication. Based on the morphology of the head and palmar and metatarsal tubercles, Sonoran green toads are speculated to be fossorial for most of the year, emerging only to breed during summer rains (Savage, 1954). However, whether Sonoran green toads enter a physiological state such as aestivation or torpor has not been studied.
I. Seasonal Migrations. Adult frogs migrate to breeding pools opportunistically at the onset of rains in July–August (Savage, 1954; Bogert, 1962; Sullivan et al., 1996b; Behler and King, 1998). Movements during the nonbreeding season are unknown.
J. Torpor (Hibernation). Unknown. See "Aestivation/Avoiding Dessication" above.
K. Interspecific Associations/Exclusions. Sonoran green toads will hybridize with little Mexican toads (Bufo kelloggi; Riemer, 1955; Smith and Chrapliwy, 1958; Ferguson and Lowe, 1969; but see Bogert, 1962), green toads (B. debilis; Ferguson and Lowe, 1969; Hulse, 1978), and red-spotted toads (B. punctatus; Ferguson and Lowe, 1969; Bowker and Sullivan, 1991; Sullivan et al., 1996b; for a discussion of hybridization see Blair, 1972b).
Sonoran green toads have been observed at breeding pools with lowland burrowing treefrogs (Pternohyla fodiens), western narrow-mouthed toads (Gastrophyne olivacea), red-spotted toads, little Mexican toads, Great Plains toads (Bufo cognatus), Colorado River toads (B. alvarius), Couch’s spadefoot toads (Scaphiopus couchii), and southern spadefoot toads (Spea multiplicata; Bogert, 1962; Sullivan et al., 1996b).
Bufo debilis, B. kelloggi, and B. retiformis form a species group referred to as the B. debilis group based on morphology and behavior (Bogert, 1962). While the B. debilis group was placed in the B. punctatus species group (Ferguson and Lowe, 1969), recent analysis of morphology, mtDNA sequence divergence, and behavior indicates the B. punctatus group is not monophyletic, and B. debilis and B. retiformis are closely related (Graybeal, 1997; Sullivan et al., 2000).
L. Age/Size at Reproductive Maturity. Based on 12 specimens from across the range, breeding males are 40–47 mm SVL and breeding females are 45–49 mm (Savage, 1954; Hulse, 1978). Bogert (1962) reports size range of 39–47 mm SVL for males and 46–57 mm SVL for females based on 42 specimens from Sonora, Mexico. Stebbins (1985) reports a size range of 28–56 mm SVL. Behler and King (1998) report a size range of 38–57 mm. The largest individual recorded is a 60 mm SVL female collected near Why, Arizona (Boundy and Balgooyen, 1988).
M. Longevity. The longevity of Sonoran green toads is unknown in the wild. A wild-caught adult lived 3 yr, 4 mo in captivity (Bowler, 1977). As of 29 March 2001, the Arizona Sonora Desert Museum housed a living individual that was collected 17 November 1983, and the museum has records of individuals that lived 15 yr, 3 mo and 14 yr, 7 mo in captivity (C.S. Ivanyi, personal communication).
N. Feeding Behavior. Unknown.
O. Predators. Little is known about predators of Sonoran green toads. American bullfrogs (Rana catesbeiana) have been reported to prey on adults toads of the B. debilis group (Stuart, 1995).
P. Anti-Predator Mechanisms. Sonoran green toads have large parotoid glands that secrete toxins that are harmful if ingested by a predator (Sanders and Smith, 1951; Lutz, 1971; Hulse, 1978). Release calls are easily elicited by males when handled (Sullivan et al., 1996b). While many authors have commented on the striking green coloration of Sonoran green toads (e.g., Stebbins, 1985; Behler and King, 1998), any function of this coloration is unknown.
Q. Diseases. Unknown.
R. Parasites. The helminths Distoichometra bufonis (Cestoda), Aplectana incerta (Nematoda), Aplectana itzocanensis (Nematoda), Oswaldocruzia pipiens (Nematoda), Psyaloptera sp. (Nematoda), and Rhabdias americanus (Nematoda) have been found infecting Sonoran green toads (Goldberg et al., 1996a).
4. Conservation. The status of Sonoran green toad populations is unknown, and there is no monitoring program for this species. Scientists currently working in the Sonoran Desert in Arizona observe large breeding aggregations of toads during and after summer monsoon storms (S.M.B., C.R. Schwalbe, B.K. Sullivan, personal observations).
Sonoran green toads were removed from C.I.T.E.S. Appendix II because the species is thought to have no major international trade or human threats and is protected by state and federal laws (C.I.T.E.S., 2000; U.S.F.W.S., 2001b). Sonoran green toads are protected as Sujeta a Protección Especial (Determined Subject to Special Protection) in Mexico, meaning utilization is limited due to reduced populations, restricted distribution, or to favor recovery and conservation of the taxon or associated taxa. Sonoran green toads are ranked at G3G4 and S4 by the State of Arizona, meaning the species is apparently secure but uncommon in parts of its range. Collection of Sonoran green toads in Arizona is limited to ten toads per year with a fishing license.
1Sean M. Blomquist
Arizona Game and Fish Department
2221 West Greenway Road
Phoenix, Arizona 85023-4312
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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Citation: AmphibiaWeb. 2018. <http://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 10 Dec 2018.
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