The Sonoran Green Toad
Anaxyrus retiformis (Sanders and Smith, 1951)
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.