The Sonoran Green Toad
Anaxyrus retiformis (Sanders and Smith, 1951)
Adult males are typically 40 to 47 mm SUL and adult females 45 to 49 mm SUL, but some individuals may attain 60 mm. The Sonoran Green Toad is an attractive anuran with a distinctive pattern and coloration. This species has a flattened head and body and a distinctive black reticulated pattern on its dorsum that surrounds patches of vivid yellow. The dorsal pattern distinguishes this species from the Chihuahuan Green Toad, which lacks the reticulated pattern. Other traits useful for identification: warts on parotoid glands greatly reduced, each tipped with a distinct black point; cranial crests reduced or absent; supraorbital and preorbital crests represented by a series of discontinuous, small black-tipped warts, the postorbital, suborbital and labial crests consisting of a few scattered, dark-tipped warts; suborbital crest almost completely absent except for a few disjunct warts; warts on upper eyelid pointed with black; thenar tubercle (base of thumb) much larger than subarticular tubercles (ventral side of each digit); dorsal color pattern appears as a series of large green (whitish in preservative), ovate blotches surrounded by black (Savage 1954). Zweifel (1970) describe the changes in coloration of the tadpoles noting that pigmentation in this form is much the same as in A. debilis. The hatchling (stage 19) is yellow with some scattered melanic pigment. Melanophores gradually appear over the body and tail musculature, and some golden cells are present early in stage 25; but the basic color is pale yellow. The largest tadpoles (stage 28) have a heavy stippling of melanophores on the body (except for a free area where the tail musculature meets the abdomen), and these cells are especially dense on the abdomen. On the tail musculature, melanophores are more abundant on the dorsal half. Golden cells are present everywhere among the melanophores, but are not conspicuous, except on the lateroventral part of the abdomen where they exist in the superficial layers in the absence of melanin. The upper tail fin shows some dendritic accumulations of melanophores, but the lower fin is immaculate.
Voice. The advertisement call has been compared to the buzzer on an electric alarm clock, a rapid cricket-like trill and a one pitch mixture of a buzz and a whistle (Bogert, 1998; Savage, 1954; Stebbins, 1954a). Sullivan et al. (1996) noted that retiformis possesses an unusually high-pitched, short-duration advertisement call like Gastrophryne matazalanensis, and identification based on calls can only be confidently determined with analysis of signals in the laboratory. On average, A. retiformis calls are longer and lower in frequency than the calls of Gastrophryne. The pulse rate and dominant frequency of the call varies with temperature and are negatively correlated with male size (Sullivan et al. 2000).
Distribution and Habitat. The Sonoran Green Toad is known only from Pima and Pinal counties in south-central Arizona and ranges southward into Sonora just north of Guaymas, Mexico (Stebbins 1985). The range of Sonoran Green Toad 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). They are found at elevations from 150–900 m (Bogert 1962; Stebbins 1985; Sullivan et al. 1996b).Its distribution seems restricted to semi-arid habitats, but they may be able to expand with the irrigation of arid landscapes (Bogert 1962). Hulse (1978) followed this suggestion but Sullivan et al. (1996b) found no evidence to support it. Observations of the toad near Mobile, Arizona suggested the expansion may be occurring (Sullivan 2002). Sonoran green toads have been observed in creosote flats, upland saguaro-palo verde associations, mesquite-grasslands, and arid and semiarid grasslands (Bogert 1962; Hulse 1978; Stebbins 1985; Sullivan et al. 1996b). Adults are nocturnal and the species is rarely encountered except when breeding (Stebbins 1985). It is endemic to the Sonoran Desert. In Arizona, the Sonoran Green Toad is found mostly within tribal lands of the Tohono O’odham Nation, so it is mostly protected by tribal regulations. It occurs in Sonoran Desertscrub (Holycross and Brennan 2006).
Reproduction. Like the Western Chihuahuan Green Toad, it is an explosive breeder (Sullivan et al., 1996b). Breeding coincides with the summer monsoon in July–August (Savage 1954; Bogert 1962; Sullivan et al. 1996b). Males arrive at temporary pools one or two days after rains begin (Bogert 1962), and Bogert speculated that the toad may delay breeding until water levels have stabilized. Reproductive behavior occurs at air temperatures from 22.5–32.6 ˚C (Bogert 1962; Ferguson and Lowe 1969; Sullivan et al. 2000). Bogert (1962) and Sullivan et al. (1996b, 2000) reported large breeding aggregations (30–200 individuals).In low-density breeding aggregations, all males call, while in high density aggregations some males adopt alternate strategies some are satellite males and others are actively searching for females (Sullivan et al. 1996b). Temporary pools used for reproduction include roadside ditches, cattle tanks, and washes (Hulse 1978; Stebbins 1985; Sullivan et al. 1996b). Males often call from clumps of vegetation that may be in the water or with a few meters of the water (Ferguson and Lowe 1969; Sullivan et al. 1996b); although males have been observed calling as far as 18 m from water (Ferguson and Lowe 1969). Females approach a calling male on land, the male continues to call until the female touches the male, this acceptance elicits amplexus (Bogert 1962; Sullivan et al. 1996b) and the pair moves to the water. Tiny eggs (about 1.2 mm) are laid individually or in small clumps. Eggs hatch at an advanced stage after 48 to 72 hours, when the tadpoles are about 3 mm long. Time to metamorphosis is temperature-dependent, but usually takes two or three weeks. Males are sexually mature at a smaller body size than are females. The color pattern may be aposematic (a warning coloration) and remind predators to avoid the toxins in the future.
Climate Change. Rinnan (2015) assessed the impact of climate change on the Sonoran Green Toad and found it had an average index of exposure (AIE) of 119.9 (placing it in the top four amphibians most susceptible to climate change). It inhabits an area of 71,000 km2. The IUCN status is a species of Least Concern. However, like many other desert amphibians it estivates most of the year, emerges only during a brief period of heavy rainfall and thus has a low climate breadth and its high AIE suggests that it may face substantial challenges in the future.
