Sonoran Desert Toad

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Sonoran Desert Toad

Incilius alvarius Girard (in Baird 1859)

Willis King (1932) described this toad in Tucson in the early 20th century.

This large toad was common around the barns and gardens at the University Farm. No call was heard, or breeding activities observed during July and August. The species is of considerable economic importance because of the great quantity of June and fruit beetles it catches, as well as spiders and grasshoppers. It also feeds on small lizards. I have handled numbers of them and have seen others do likewise with no ill effects.  The body color varies from a bluish gray to green or brown. The smaller warts of the back are often of a rusty color, as are the tips of the digits. The belly is white. The animals are often found in open pits, holes or washouts where they have fallen. One specimen I have just fills a quart jar.

Adults may be as large as 190 mm (Heringhi 1969, in Fouquette 1970; Degenhardt et al., 1996). Males reach 156 mm females attain 190 mm (Wright and Wright 1949). Sullivan and Fernandez (1999) studied populations in which adult males were mostly 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 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 a 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. In dorsal view the body is oval, 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 the eye than the snout, 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 the hand, this toad jerks spasmodically, and vibrates the whole body, as if about to explode with wrath.  The only sound, however, 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 apparently absent from the lower Colorado River area.  Elevational range is from  sea level to 1,600 meters, and is primarily in deserts, but it ranges from mesquite-creosote bush lowlands, arid grasslands, 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 that was more than 1.5 m deep as well as 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 (Holycross and Brennan 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 a period of 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 during that time ranged from 11.7–29.7 ˚C. It is possible that during part of that period below the surface the toad may have been in a state of dormancy.

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 prior to the onset of the monsoons. Individuals have been observed at night in desert grasslands feeding in June.

Reproduction. Goldberg (2018) examined 43 specimens from Pima County, Arizona and found the smallest mature male measured 108 mm SUL and was 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 to be 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) contained early diplotene oocytes in its ovary and was considered a juvenile. Stebbins (2003) reported I. alvarius was not dependent on rainfall for breeding. and 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 on only one night, two or three days after heavy rains (Sullivan and Malmos 1994).  Breeding sites include irrigation ditches and stock tanks, as well as ephemeral and permanent ponds.  Thus, the Sonoran Desert Toad may not depend on rainfall to stimulate reproduction if permanent water is available.  They use the same breeding sites as Couch’s Spadefoots, Mexican Spadefoots, Great Plains Toads, 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). Time to metamorphosis is less than four weeks.

Predatory Defense. The large skin glands produce a toxic cocktail of molecules implicated in the poisoning and death of dogs and humans.  Perhaps the earliest reports of the highly toxic nature of this species is 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 in a very un-froglike manner.

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 for him 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 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. In parotoid and coxal glands they found five to fifteen per cent of the dry weight is made up by this compound.  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. 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 designated 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) placed it in the genus Cranopsis, using 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, using the combination Incilius alvarius by implication.