Sonoran Desert Toad, Incilius alvarius
The short account. It occurs from Arizona and New Mexico southward to Central Arizona; only recently did it invade southeastern California, but it may have extirpated – it has not been seen there since the 1970s. This toad is common in the Sonoran Desert. It occurs in various habitats, including creosote bush desertscrub, grasslands up into oak-pine woodlands, thornscrub, and tropical deciduous forests in Mexico. Like other toads it is an opportunistic predator and feeds on various insects and small vertebrates. Sonoran Desert toads may be active from April to September, though they become more visible in the summer monsoon. They are nocturnal during the hot summer months. The male’s call is weak, sounding somewhat like a ferryboat whistle. Eggs are laid in temporary rain pools and permanent ponds. Larvae metamorphose after 6 to 10 weeks. This species is long-lived, and may live for several decades
Like other toads, the Sonoran Desert toads have incredibly potent, defensive toxins released from several glands (primarily the paratoids) in the skin. Predators that harass this species generally are poisoned through the mouth, nose, or eyes. Dogs have the habit of sticking their face in vegetation, and if they find a toad they are dangerously close to becoming poisoned. The toxins are strong enough to kill full-grown dogs that pick up or mouth the toads. Dogs under the effect of the toxins have excessive salivation, irregular heartbeat and gait, and pawing at the mouth. If a dog displays any of these symptoms, use a garden hose to rinse its mouth from back to front and consult a veterinarian.
The long account.
Adults may be as large as 190 mm (Heringhi 1969, in Fouquette 1970; Degenhardt et al., 1996). Males reach 156 mm, and females attain 190 mm (Wright and Wright 1949). Sullivan and Fernandez (1999) studied populations in which adult males were mainly two to four years old.
The Sonoran Desert Toad has large parotoid glands, large glands on the legs, and a large white wart at the angle of the jaw. All these traits combine to distinguish this toad from other Arizona anurans. Newly metamorphosed alvarius may be mistaken for Red-Spotted Toads. However, the Red-Spotted Toad tends to be tan instead of olive green and lacks the white swelling or raised area at the corner of the mouth that will eventually become a tubercle (Lazaroff et al. 2006).
The larvae are mottled gold to brass brown and may reach a total length of 56 mm. The body is slightly depressed, the eyes are dorsal, the anal is medial, the oral disk is emarginate, and the tail fin ends near the tail-body junction. The body is oval in dorsal view, and the snout is broadly rounded. Both fins are lower than the height of the tail musculature height. The tail musculature does not extend to the tip of the tail. The nostrils are closer to the eye than the snout, and the eyes are medium and situated closer to the snout than the spiracle (Altig 1971).
Voice. The call is a low-pitched whistle that lasts a half-second to one second. Dickerson (1906), writing about the Sonoran Desert Toad, said, “When held in hand, this toad jerks spasmodically, and vibrates the whole body, as if about to explode with wrath. The only sound produced in protest is a gentle chirping note…” This is likely a release call.
Distribution and Habitat. The species ranges from southwestern New Mexico westward across southern Arizona to southeastern California, southward into northern Baja, and as far south as Guirocoba, Sonora, Mexico, and is restricted to the Sonoran Biotic Province. It is absent from the lower Colorado River area. The elevational range is from sea level to 1,600 meters. It is primarily in deserts, but it ranges from mesquite-creosote bush lowlands, arid grasslands, and rocky riparian zones with sycamore and cottonwoods to oak-walnut woodlands in mountain canyons (Schmidt, 1953; Fouquette, 1970; Stebbins, 1985; Holycross et al., 1999). However, it will also use urbanized habitats and take advantage of storm drains, golf course ponds, and other man-made water features, such as swimming pools. Stebbins (1962) considers this a semi-aquatic toad, noting that it is more aquatic than other toads. We have seen this toad sitting at the bottom of a tank more than 1.5 m deep and using shallow pools. Santos-Berra et al. (2008) found this species associated with prairie dog grasslands. It occurs in Sonoran Desertscrub (Lower Colorado River), Great Basin Desert Scrub, and Semidesert Grasslands (Brennan and Holycross 2006).
Diet. Sonoran Desert Toads actively forage for food but also hunt from ambush. Prey includes beetles, grasshoppers, wasps, centipedes, millipedes, ants, termites, solpugids, spiders, snails, scorpions, other frogs and toads, small lizards, mice, and probably any animals they can overpower.
Movement. Beck (in Fouquet et al. 2005) radio-tracked an adult Colorado River toad for 390 days in the Tucson Mountains, Pima County, Arizona. During that time, activity centered around the release site, although movements greater than 400 m were noted in a single day. He also followed an adult Colorado River toad that remained in the same burrow under a railroad tie from 26 September to 17 June (about nine months). The toad’s body temperature ranged from 11.7–29.7 ˚C. It is possible that during part of that period below the surface, the toad may have been dormant.
During most of the year, the Sonoran Desert Toad shelters in rodent burrows (Lowe, 1964), and at least some individuals become active before the monsoon. Our observations suggest activity may start before the onset of the monsoons. Individuals have been observed at night in desert grasslands feeding in June.
My (JCM) neighbors have one of these toads that has lived in their yard for several years. During monsoon evenings, it will move about a hundred meters from their yard to my driveway to sit under my lights and feed on insects. It returns to my neighbor’s yard when I turn off the lights.
Reproduction. Goldberg (2018) examined 43 specimens from Pima County, Arizona, and found that the smallest mature male measured 108 mm SUL from September (late in the reproductive season), and contained mainly spermatogonia. Wright and Wright (1949) previously reported males mature at 80 mm SUL and found an 87 mm female mature. Goldberg’s smallest mature female measured 88 mm SUL and was from September and contained only early diplotene oocytes. It presumably completed spawning earlier in the year. A smaller female (SUL = 74 mm) had early diplotene oocytes in its ovary and was considered a juvenile. Stebbins (2003) reported I. alvarius was not dependent on rainfall for breeding. There are reports of breeding before the onset of summer rainfall in June in New Mexico and tadpoles in October, indicating a prolonged breeding season may, in some cases, occur (Degenhardt et al.1996).
Breeding may occur only one night, two or three days after heavy rains (Sullivan and Malmos 1994). Breeding sites include irrigation ditches, stock tanks, and temporary and permanent ponds. Thus, if permanent water is available, the Sonoran Desert Toad may not depend on rainfall to stimulate reproduction. They use the same breeding sites as Couch’s, Mexican Spadefoots, Great Plains, Red-Spotted Toads, and Woodhouse’s Toads.
Males have at least two strategies for finding a mate. Advertisement calls are used when a few other males are present, but males may rely more on actively searching for a mate when the competition from other males is great.
Clutches contain up to 8,000 eggs, with the eggs enveloped in tubes of colorless jelly. Eggs average 1.6 mm in diameter and are packed between about 5–7/cm (Wright and Wright 1949, Savage and Schuierer 1961). The time to metamorphosis is less than four weeks.
Predators and Defense. The large skin glands produce a toxic cocktail of molecules implicated in the poisoning and death of dogs and humans. The earliest reports of the highly toxic nature of this species are Musgrave and Cochran (1929). The defensive behavior of the Sonoran Desert Toad was described by Hanson and Vial (1956). They used a skunk as a potential predator, and when the toad was face-to-face with the skunk, it would tilt its body toward the mammal or lower its head and raise its posterior body to present its anatomy and toxins. When disturbed, they will stand on all four limbs and run very un-froglike behavior.
Weil and Davis (1994) chronicle a case of a dog owner who removed a Sonoran Desert Toad from his dog’s mouth within ten seconds. Despite the quick action, within 30 minutes, the dog began to salivate profusely, went into convulsions, and died from apparent respiratory failure. In 1986, a five-year-old boy with profuse salivation and continuous seizures was admitted to the University of Arizona Medical Center. Seizure activity started within 15 minutes of his licking a toad. The child survived, but it took a full week to return to normal. The Sonoran Desert Toad produces large quantities of the potent and toxic hallucinogen, S-methoxyN, N-dimethyltryptamine (5MeO-DMT), a molecule that can cross the blood-brain barrier.
Erspamer et al. (1967) noted that the skin contains several indolealkylamines and their metabolites The most abundant representative of 5-hydroxyindolealkylamines is, as in numerous other toads, bufotenine, with up to 3 mg per gram of dry skin, the most abundant representative of 5-methoxyindolealkylamines, O -methylbufotenine. This compound makes up five to fifteen percent of the dry weight in parotoid and coxal glands. The authors also found new compounds and enzymes involved in the chemical defenses of this toad.
At least two predators, ravens, and raccoons, have learned to remove the toads’ highly toxic skin before eating them. Trap cameras set up at the Northern Jaguar Reserve in Sonora captured images of bobcats, coyotes, and badgers interacting with Sonoran Desert Toads. The Bobcat and the Coyote ignored the toads. The Badger, however, was walking off with the toad in its mouth (Gutierrez-Gonzalez et al. 2016). The invasive Northern Crayfish, Orconectes virilis, may also be an important predator on Sonoran Desert Toad tadpoles at some localities.
Taxonomy and Systematics. Girard (in Baird 1859, 2:26) described Bufo alvarius based upon two syntypes (USNM 2571-72). Cope (1889:267) designated USNM 2572 the lectotype and Fouquette (1968:71) formally established USNM 2572 the lectotype with a type locality of the Valley of the Gila and Colorado [Rivers]; corrected to Fort Yuma, California. USA by Cope (1889:267), restricted to “Yuma”, Yuma Co., Arizona by Smith and Taylor (1950:355); restricted to Colorado River bottomlands below Yuma, Arizona by Schmidt (1953:61), corrected to [old] Fort Yuma, Imperial County, California (on the north bank of the Colorado River, opposite its junction with the Gila River by Fouquette 1968:70-72). Cope (1862:358) placed it in the genus Phrynoidis, using the combination Phrynoidis alvarius. Frost et al. (2006:364) put it in the genus Cranopsis, utilizing the combination Cranopsis alvaria. Frost et al. (2006b:558) moved it to the genus Ollotis, using the combination Ollotis alvaria, and Frost et al. (2009:418) moved it to the genus Incilius, utilizing the combination Incilius alvarius by implication.
