oxalic acid

Pack Rats are Desert Archaeologists

The Pack Rat (Neotoma albigula), also known as the White-throated wood rat, has been in this region for at least 50,000 years. And it has been collecting things.

Pat rats are troublesome around one’s home because they will eat insulation from automobile wires and get into the walls of a house. However, their habits have made them good desert archaeologists.


The pack rat measures 8 inches long, head and body, and the tail is another 5- to 7 inches. The tail is covered by loose skin which can be shed to escape a predator. Pack rats are usually solitary.

A pack rat nest is a loose conglomeration of twigs and grass, fortified against predators with cactus parts, especially cholla. The nest consists of tunnels and chambers with separate areas for food storage, sleeping, nursery, and waste products. There are often many insects in the nest feeding on collected plant material. Kissing bugs also inhabit the nest and feed on pack rat blood.

Currently, the pat rat obtains nourishment and water mainly from feeding on prickly pear cactus. Its kidneys have evolved to deal with and excrete oxalic acid derived from the cactus. It is one of the few animals that can do this. (See my blog: Can you get potable water from a cactus for an explanation of oxalic acid.) That ability and its habit of collecting things is what makes the pack rat a good desert archaeologist.

The pack rat places his collectibles in the waste chamber and urinates on them (instead of going outdoors where it would be subject to predation from Great Horned owls, snakes, and coyotes). Because of the high calcium oxalate content, the urine solidifies and preserves the collection in a hard resinous or vitreous-looking material called a midden. The midden looks much like peanut brittle, or a chocolate bar filled with nuts, but that’s another story.

The midden is a layered material preserving leaves, sticks, seeds, and other plant parts, bones, stones, feathers, and everything else the pack rat collects, usually from within a 100-foot radius of the nest. At the Fort Bowie National Historic site, at set of false teeth found in a pack rat nest is on display. One can excavate and date the layers (usually by the C14 method) to see when certain plants first appeared in the region. The urine acts as a bactericide, so the organic material is preserved. Some midden material from western Arizona has been dated as old as 40,000 years.

There are some nests in the Tucson Mountains that have been continuously inhabited for the last 5,000 years. One nest near Contzen Pass off Picture Rocks Road has been dated at 21,000 years old. At that time, the area that now has saguaros and palo verde trees, had junipers and oak trees.

Study of middens near the Waterman Mountains (the range on the west side of Avra Valley), shows that 11,000 years ago the area was a woodland forest with pines and oaks. Study of fossils from middens show that the desert has come and gone many times. Since the last glacial epoch, as temperate forests retreated to higher elevations, plants from much farther south migrated northward. Analysis of midden material indicates that mesquite trees arrived in southern Arizona about 15,000 years ago; saguaros about 9,000 years ago, and Foothills palo verde trees about 4,000 years ago. (See the Natural History category of this blog for more stories on desert plants and animals.)

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That other story, paraphrased from Sonorensis, Fall 1983, about a first encounter with midden material:

In the 1849 gold rush a group of emigrants crossed the inhospitable Mojave desert from Utah to California on a journey that nearly ended in disaster in Death Valley. When the party became desperate for food and water near Papoose Lake in southeastern Nevada a search party split from the main group. In a nearby canyon they came to a high cliff and in its face were niches or cavities as large as a barrel and in some of them found balls of a glistening substance looking like pieces of variegated candy stuck together. It was evidently food of some sort, and they found it to be sweet but sickish. Some made a good meal of it but were a little nauseated afterwards. Yum!

Can You Get Potable Water From a Cactus?

Generally NO. Moisture within the pulp of a cactus is very acidic and many cacti contain toxic alkaloids. You can, however, eat the fruit.

The moisture is acidic because of the way many succulents, including cacti, carry on photosynthesis, the process by which carbon dioxide and water are turned into carbohydrates.

Most plants have their pores (stomates) open during the day to take in carbon dioxide, and use sunlight as a catalyst for the reaction: Carbon dioxide + water becomes sugar + oxygen. But in the desert, plants with pores open during the hot days, lose much water through evapotranspiration.

So, succulents use a modified version of photosynthesis called CAM (Crassulacean Acid Metabolism). CAM plants open their stomates only at night when it is cooler so there is less evapotranspiration. Because there is no sunlight to act as a catalyst, carbon dioxide is stored as an organic acid, principally Malic Acid (C4H6O5). Carbon dioxide is gradually released from the acid during the next day. CAM plants use about one-tenth the water to produce each unit of carbohydrate compared to standard photosynthesis. The price: a much slower growth rate.

Many plants contain malic acid, but usually in lesser quantities than found in cacti. Also cooking generally destroys the acid.

Besides malic acid, succulents produce Oxalic Acid (C2H2O4), which is toxic, as another product of photosynthesis. “Its chief function seems to be sequestering metals, principally calcium. Calcium oxalates often occur as crystalline minerals within the cactus pulp. Their function seems to be aiding structural integrity and enzymatic processes. In fact two crystalline calcium oxalate minerals have been identified in all cacti tested: CaC2O4.2H2O (weddellite) and CaC2O4.H2O (whewellite).” [Source: Plant Physiology, February 2002, Vol. 128, pp. 707-713.] Oxalates are also formed with heavy metals such as copper, perhaps to reduce toxicity to the plant.

Oxalic acid is toxic to humans because it combines with calcium in our bodies to produce calcium oxalates which clog up our kidneys.

So, what about the barrel cactus. Can’t we get water from those? Did you bring along a machete and solar still?

The Seri Indians sometimes used the Fishhook barrel (Ferocactus wislizeni) for emergency water. However, drinking the juice on an empty stomach often caused diarrhea, and some Seri report pain in their bones if they walk a long distance after drinking the juice. The Seri called the Coville barrel (Ferocactus emoryi), “barrel that kills” because eating the flesh of the cactus causes nausea, diarrhea, and temporary paralysis. Think you can tell the two apart? (See: Edible Desert Plants – Barrel Cactus Fruit).

What about Prickly Pear pads we sometimes see in grocery stories or on the menu of Mexican restaurants? What you see are generally young spring pads which naturally contain less oxalic acid. Cooking leaches out the acid. In an emergency you can eat the young pads raw. And there are some spineless cultivars that naturally contain little oxalic acid which can also be eaten raw. These were developed mainly as cattle feed.

The bottom line is you really cannot get a drink from a cactus in spite of what you may have seen in old cowboy movies.