The Sonoran Desert is a hot subtropical desert of about 260,000 km² (100,000 sq mi) wrapped around the northern end of the Gulf of California, covering southern Arizona and southeastern California in the United States and much of Sonora, Baja California and Baja California Sur in Mexico. It runs from near sea level along the Gulf and the lower Colorado River up to "sky island" summits such as Mica Mountain, 2,641 m (8,666 ft), east of Tucson, and its largest cities are Phoenix, Tucson, Mexicali and Hermosillo. Unlike the neighbouring Mojave, with its dry summers and cold winters, it has subtropical warmth in winter and two seasons of rainfall, a pattern that gives it more plant species than any other desert in the world, and it is the only place where the saguaro cactus grows wild. The species below, each tied to a named park, refuge or spring, show how that geography works.
1.8 Million Saguaros in Saguaro National Park
The saguaro (Carnegiea gigantea) grows wild only in the Sonoran Desert: in Arizona, in Sonora, and in a small Californian outpost in the Whipple Mountains and Imperial County, usually below about 1,400 m (4,500 ft). The densest protected stands are in Saguaro National Park, 37,116 hectares (2016 figure) split between the Tucson Mountain District about 16 km west of Tucson and the larger Rincon Mountain District about 16 km east. The National Park Service estimates the park holds about 1.8 million saguaros, alongside 24 other abundant cacti such as the fishhook barrel and teddybear cholla.
The park began as Saguaro National Monument, proclaimed by President Herbert Hoover on 1 March 1933, and became a national park in 1994. The cactus it protects is extraordinarily slow: it can take 20 to 50 years to reach 1 m, and its first arm appears at about 50 to 100 years of age, later where rain is scarce. Mature plants routinely live 150 to 200 years, and a fully hydrated adult can weigh 1,500 to 2,200 kg. The tallest ever measured was an armless 23.8 m specimen near Cave Creek, Arizona, toppled by a windstorm in 1986.
The park also shows how sharply desert gives way to forest. The Tucson Mountain District tops out at 1,429 m on Wasson Peak and holds only desert scrub and grassland; the Rincon Mountain District climbs to Mica Mountain through oak woodland and pine forest to mixed conifer forest of Douglas-fir and white fir. Inventories in 2011 and 2013 by scientists and thousands of volunteers produced the counts below.
Bats, Doves and Woodpeckers That Live Off the Saguaro
Biologists call the saguaro a keystone species because so many animals depend on it. Its white flowers, the state wildflower of Arizona, open after sunset between April and June and last less than 24 hours; in southern Arizona flowering peaks around 4 June. By night the main pollinator is the lesser long-nosed bat (Leptonycteris yerbabuenae), a 15–25 g nectar feeder that reaches Arizona between April and July and heads south in September, some individuals covering about 1,600 km a year as they follow saguaro, organ pipe cactus and agave blooms.
By day, honey bees, hummingbirds and the white-winged dove take over; for doves living in the desert, 60 per cent or more of the diet comes from the saguaro, and their breeding is timed to its flowering. Gila woodpeckers drill a fresh nest hole in the stem each season, leaving the old ones for elf owls, tyrant flycatchers and wrens. In 2020 a bald eagle was found nesting in a saguaro for the first time since 1937.
Gila Monsters and Desert Tortoises of the Arizona Upland
The Arizona Upland, the subregion that receives 100–300 mm of rain a year in a regular two-season pattern, is home to the Gila monster (Heloderma suspectum). It is the only venomous lizard native to the United States and the largest native lizard north of the Mexican border; the biggest reach 51–56 cm, and most weigh 550–800 g. Its name comes from the Gila River basin. Gila monsters spend about 90 per cent of their lives underground in burrows or rock shelters, emerging mostly in spring mornings or after summer thunderstorms, and store fat in the tail and water in the bladder.
A peptide in its venom, exendin-4, became the basis of exenatide, a type 2 diabetes drug approved in the United States in 2005 and the forerunner of today's GLP-1 receptor agonists.
The same slopes hold the Sonoran Desert tortoise (Gopherus morafkai), split from the Mojave desert tortoise by DNA work in 2011, with the Colorado River as the dividing line. Saguaro National Park alone lists 36 reptile species, among them western diamondback rattlesnakes and coral snakes, plus three amphibians; Couch's spadefoot waits in burrows and emerges to breed during the summer rains.
Pronghorn and Bighorn on the Cabeza Prieta and Kofa Refuges
In southwestern Arizona lies the Yuma Desert, a lower-elevation section of the Sonoran where annual rain is often under 90 mm. Here the Cabeza Prieta National Wildlife Refuge covers 3,480 km² along 90 km of the Mexican border, larger than Rhode Island; 93 per cent became the Cabeza Prieta Wilderness in 1990. It records more than 275 animal and nearly 400 plant species, including the two large grazers of the open desert.
Sonoran pronghorn: 21 animals in 2002
The Sonoran pronghorn (Antilocapra americana sonoriensis) is an endangered subspecies found only in the lowland desert of southwestern Arizona and northwestern Sonora, and can go longer without water than other pronghorn. In 2002 only about 21 were thought to survive in Arizona; around 450 are now believed to live there, with fewer in Mexico. In 2012 reintroductions were announced for the Barry M. Goldwater Air Force Range and the Kofa National Wildlife Refuge. Parts of Cabeza Prieta close from mid-March to mid-July so that does with fawns are not disturbed.
Desert bighorn sheep and the 1939 proclamation
Cabeza Prieta was created in 1939 for a different animal. After a 1936 campaign by the Arizona Boy Scouts, prompted by Frederick Russell Burnham's warning that fewer than 150 of the sheep were left in Arizona's mountains, more than 6,000 km² were set aside at Cabeza Prieta and Kofa on 18 January 1939 for desert bighorn sheep, which live in mountains with little permanent water and can go weeks without drinking, relying on moisture from plants and from rainwater in temporary rock pools. Across North America their numbers fell to an estimated 6,700–8,100 by the 1960s; by 1993 the estimate had risen to 18,965–19,040.
Quitobaquito Springs and the Sonoyta Pupfish
Organ Pipe Cactus National Monument, 1,340 km² on the Sonora border, became a national monument in 1937 and a UNESCO Biosphere Reserve in 1976. It is one of the few places in the United States where the organ pipe cactus (Stenocereus thurberi) grows wild. Its most surprising habitat is water: Quitobaquito Springs, at 351 m (1,152 ft) beside the border, feeds a pond built in the 1860s, and people have lived around it for at least 16,000 years.
The pond holds the only US population of the endangered Sonoyta pupfish (Cyprinodon eremus), as well as the only US populations of the Sonoyta mud turtle and Howarth's white butterfly. The pupfish's whole range covers less than 11 km², here and along two short stretches of the Sonoyta River in Sonora, and it tolerates water three times saltier than the sea. In mid-2020 the pond dropped sharply; suggested causes include groundwater pumping around Sonoyta, summer heat and nearby border-wall construction.
El Jefe and the Jaguars of the Santa Rita Mountains
On its eastern edge the desert climbs into the Madrean Sky Islands, roughly 27 isolated ranges in the United States and 15 in northern Mexico, whose pine–oak woodlands are cut off from the Sierra Madre Occidental by lowland desert. The Rincons are one of them, and their forests shelter the threatened Mexican spotted owl. The desert southeast of Tucson, near the border, is also vital habitat for the only jaguars known to live in the United States.
The best-documented is a male named El Jefe ("the boss"). Cougar hunter and guide Donnie Fenn first photographed him in the Whetstone Mountains on 19 November 2011, and University of Arizona camera traps later identified him by his spots in the Santa Rita Mountains in November 2012, and he was photographed there over the following years. He vanished from Arizona after September 2015 and was photographed in central Sonora on 27 November 2021. In 2012 the US Fish and Wildlife Service set aside 3,392 km² of Arizona and New Mexico for the protection of the jaguar.
Dunes and Craters of El Pinacate and Gran Desierto de Altar
Across the border in Sonora lies the Gran Desierto de Altar, about 5,700 km² of sand sheets and dunes forming the only active erg, or sand sea, in North America, its sand delivered by the Colorado River during the Pleistocene. On its eastern side rises the volcanic Sierra Pinacate, topped by Cerro del Pinacate at about 1,200 m, whose eruptions, the last about 11,000 years ago, left maar craters such as El Elegante. NASA trained astronauts here between 1965 and 1970.
Dunes and volcanoes form El Pinacate y Gran Desierto de Altar Biosphere Reserve, 2,695 km² within a UNESCO World Heritage Site of 7,146 km². It records over 540 plant species, 40 mammals and 200 birds, including the same pronghorn, bighorn sheep and Gila monster found across the border. Pinacate comes from the Nahuatl pinacatl, a stink beetle of the Sonoran Desert.
The desert's western edge is defined by water that no longer arrives. Dams and diversions upstream have shrunk the wetlands of the Colorado River Delta to about 5 per cent of their former extent, shrinking what was once an ecological hotspot where the desert meets the Gulf of California. For the cooler, higher desert that lies east of the sky islands, with its own cacti and grasslands, read our guide to the Chihuahuan Desert, or see how the Sonoran fits among Arizona's other dry lands in the deserts of Arizona.