As an image on keychains or a prop in cowboy westerns, the saguaro (Carnegiea gigantea) symbolizes the Southwest more than any other species. Keystones of the desert ecosystem, saguaro cacti are unique to the Sonoran, their range further confined to elevations between sea level and 4,000 feet because they are intolerant of freezing temperatures.
A young saguaro develops under the indifferent but indispensable care of a nurse tree, most often a palo verde (Cercidium spp.), desert ironwood (Olneya tesota), or mesquite (Prosopis spp.). The nurse tree shelters the saguaro from extreme heat and frost, and from foraging animals. It takes 25 years on average for the cactus to grow a foot high. As it matures and develops its first arms at between 50 to 100 years of age, the saguaro may sap enough water and nutrients from the surrounding soil to kill off its aging nurse.
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| Saguaro blossoms. (photos.com) |
Saguaros grow two to 50 arms, which can twist, due to frost damage, into bizarre contortions. The roots of this succulent plant penetrate no more than four inches into the soil, yet they spread as wide as the plant is tall. Fully grown saguaros may tower 50 feet and weigh six tons—of which 90 percent is water.
These prickly giants lord generously over the ecosystem around them. Hawks and owls use saguaro column tops as sentry points to peer for prey across the wide expanse of the desert. Red-tailed hawks (Buteo jamaicensis) also build nests in the crooks of the saguaros' arms. Gilded flickers (Colaptes chrysoides) and Gila woodpeckers (Melanerpes uropygialis) hammer nesting cavities right into the sides of saguaro, causing the cacti's walls to seal with scar tissue. When the cacti die and disintegrate, the hardened scar tissue remains in a hollow shape called a "saguaro boot." (Seri Indians from the arid coast of Sonora, Mexico, use these boots to carry and store water.) Flickers and woodpeckers abandon saguaro cavities after a year, allowing elf owls (Micrathene whitneyi), purple martins (Progne subis), and other birds to move in. The cavities remain relatively cool during the day and warm at night, offering relief from the desert's temperature extremes.
Creamy-white flowers blossom on saguaros in late spring. Individual flowers begin opening after sunset and are fully unfurled by midnight, then close by the following afternoon, never to open again. Lesser long-nosed bats and Mexican long-tongued bats (Choeronycteris mexicana) use their elongated muzzles to reach deep into saguaro blossoms for nectar. During the day, the saguaro flowers are visited by wasps, bees, butterflies, and birds.
Once the saguaro fruit ripens in June, lesser long-nosed bats, white-winged doves (Zenaida asiatica), Gila woodpeckers, and other birds consume the fleshy red pulp and thereby disperse the seeds, which pass through their guts intact. A host of mammals, including Harris antelope squirrels (Ammospermophilus harrisii), cactus mice (Peromyscus eremicus), skunks (family Mustelidae), and coyotes (Canis latrans), gobble up saguaro fruits that fall to the desert floor. For centuries, the Tohono O'odham people have used long poles made from the ribs of the cactus to harvest saguaro fruit, which they make into syrups, jams, and ceremonial wines.
Saguaros seem to be perfectly evolved for pollination by bats, according to Mark Dimmitt of the Arizona-Sonora Desert Museum. Their flowers bloom high in the air, open at night, produce copious nectar and pollen, and smell like an overripe melon. Yet research conducted at Kino Bay, along the coast of Sonora, Mexico, suggests that lesser long-nosed bats prefer organ-pipe and cardón cacti to saguaros, although in southern Arizona, a region where few organ-pipe and no cardón cacti grow, lesser long-nosed bats depend much more heavily on saguaros for sustenance. Most saguaros are likely pollinated by bees and white-winged doves, species that feed during the day. Scientists are puzzled by this pollination paradox. "Have we encountered the early stage of an evolutionary shift?" writes Mark Dimmitt in A Natural History of the Sonoran Desert. "In 100,000 years will saguaros have diurnal yellow flowers [to better attract bees and birds]? Or do the bats still have sufficient, yet-unknown influence on saguaro evolution to maintain the status quo?" Perhaps the saguaros know, but they aren't talking.
—Alex Hawes
ZooGoer 34(1)
2005. Copyright 2005 Friends of the National Zoo.
All rights reserved.
