Woodrats: How the Desert's Smallest Librarians Contributed to Scientific Discovery

Neotoma lepida, the desert woodrat | Creative Commons photo by randomtruth

When Philip Wells and Carl Jorgenson set out to hike on Aysees Peak in Nevada's Mojave Desert one day a half century ago, they didn't expect to find one of the most important libraries in the world. They especially didn't expect that the archivist would be one of the desert's most common rodents.

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Wells, a botanist, and Jorgenson, a mammalogist, were doing ecological survey work on land now part of the Nevada Test Site for the Atomic Energy Commission. On that day in 1961 the two had been looking for juniper trees on Aysees Peak, without success. The nearest wooded mountains were almost 20 miles away.

As the sun sank toward the west and the pair headed downhill, Jorgenson spotted an odd outcrop beneath a rock overhang. The "outcrop" was made up of a resinous substance like dirty amber or burnt peanut brittle. Jorgenson broke open a piece of the resin. Inside were juniper twigs, 30 kilometers from the nearest living juniper.

The pair sent samples to UCLA for Carbon-14 dating. The amberized twigs turned out to be somewhere in the neighborhood of 9,300 years old. The juniper twigs had been collected by desert woodrats, members of the species Neotoma lepida, somewhere around 7300 BC.

Desert woodrats are one of the most common mammals in the California desert. Their range extends from the southern tip of Baja California into the sagebrush deserts of Oregon and Idaho, and eastward across the Canyonlands District into westernmost Colorado as well. They're large for desert rats, nine to fifteen inches long at maturity including their tails, and weighing as much as a third of a pound. They eat succulent plants, which is where they get most of their water. Unlike their neighbors the kangaroo rats, desert woodrats aren't very good at extracting metabolic water from dry food.

Woodrats are popularly called "packrats," and with good reason. They're forever collecting little pieces of plant material, wood, brightly colored bits of stone, small bones, anything that strikes their visual fancy. They bring their baubles to their nests, referred to as "middens" for their trash-heap appearance. Much of what they gather they eat: the rest they incorporate into the midden's architecture.

Packrat middens serve a few important purposes. Of most immediate importance to the packrat is armor: most packrat middens in the desert have a lot of cholla and other cactus parts added to them and serve as a formidable deterrent to hungry coyotes or others intent on eating a packrat for lunch. A good-sized midden, with its interior passageways and chambers, also offers climate control to the packrat. The temperature inside will be cooler than the surrounding desert, and a chamber inside a large midden may be significantly more humid, reducing the rat's water loss.

Packrats build their middens wherever nature can give them a headstart: in the center of yucca clumps or cholla patches, under fallen Joshua tree trunks, or in rock shelters or caves. Once built a midden can last for centuries, with successive generations moving in once the previous tenant dies in much the same way that rent controlled apartments in Manhattan pass down from one tenant to the next. Each occupant spends much of its time adding new material to the midden -- stray cholla stems, 50-year-old beer can pull tabs, the car keys of unwary campers.

Being rats, desert woodrats excrete a lot. The center and lower parts of packrat middens essentially consist of tidbits brought in from the surrounding desert mixed liberally with rat feces excreted at the rat's indoor leisure. And since woodrats aren't quite as water-efficient as other desert rodents, all of the detritus beneath the floor of each chamber in the midden becomes saturated with copious amounts of urine.

Packrat kidneys are reasonably efficient at separating waste salts from liquid, which means that packrat pee is rather concentrated. As the salts build up in the layers of plant and other material in the midden, the whole mass congeals into a resinous, sort of ambery substance which preserved the perishable material inside. If the midden is out in the open, each really good rain will dissolve some of the accumulated salt and flush it away. But where desert woodrats have built their middens beneath rock overhangs and in caves, where no rain can reach, the pile just lasts. And lasts.

On their way to getting stuck in Death Valley in 1849, the ill-fated (but ultimately quite lucky) Manly Party stopped near Papoose Dry Lake, only about 30 miles from where Wells and Jorgenson made their discovery. Hungry and desperate for food, the pioneers found interesting niches in a high cliff. William Manly later wrote in his memoirs that in this canyon,

"...we found balls of a glistening substance looking like pieces of variegated candy stuck together ... it was evidently food of some sort, and we found it sweet but sickish, and those who were hungry...making a good meal of it, were a little troubled with nausea afterwards."

Probably not nearly as much nausea as they'd have felt had they known they'd been eating centuries' worth of rat leavings.

Fortunately, 112 years later, Wells and Jorgenson -- who had presumably brought a better lunch anyway -- recognized their own packrat midden for what it was. Over the next few seasons they examined another eight middens, found material in them dating from 7000-12000 years old, and in 1964 published an epochal paper in Science that changed the face of Pleistocene paleontology.

Since then, thousands of packrat middens across the southwest have been examined, a process that involves tedious washing of urine-embalmed solids, most of which are ancient fecal pellets, and then examining any plant material and the occasional bone fragment to identify their origin, usually under a microscope. Middens have been dated at anywhere from 5,000 to 40,000 years old, with some very likely even older but beyond the reach of accurate carbon dating.

Desert woodrat midden beneath a rock overhang | Creative Commons photo by randomtruth

So there's some very old stuff at the bottom of ancient packrat litterboxes. Why go to all the trouble and distasteful labor involved in cataloguing it? Here's why: Everything in a packrat midden was collected and put there by a resident packrat. Packrats don't travel widely: generally, they don't move more than about 100 feet from their midden, often less than that.

Which means that if you have a layer in a packrat midden that has been dated at 15,000 years old, every piece of plant material in that layer almost certainly came from a plant that grew within a 100-foot radius of the midden, barring the occasional wind-driven leaf. And as packrats are rather catholic in their gathering habits, each layer can provide a reasonably complete inventory of what grew in the neighborhood at the time.

In other words, Wells and Jorgenson knew that the packrat responsible for the midden they found heading down Aysees Peak hadn't trekked over from the wooded mountains 20 miles away to bring those juniper twigs to the midden. The fact that juniper twigs were in the midden meant that junipers had once grown right there, within a stone's throw of where they stood.

With each midden surveyed, paleontologists get another chunk of extremely detailed data on the ecological history of the surrounding fifth of an acre. Before 1961, paleontologists studying the last 40,000 years were limited to the record offered by fossils preserved under unusual circumstances, leading to an image of the past that was more gap than puzzle piece. Even the wonderfully complete picture offered by the deposits at the La Brea Tar Pits only really described the conditions in the Pleistocene Miracle Mile district.

But with the advent of packrat midden science, an astonishingly detailed and diverse record emerged, materials deliberately collected and preserved in "amber," across a wide swath of the arid southwest. Paleontologists were able not just to speculate at general conditions 15,000 years ago in the Southwest, but to state with authority that forests disappeared from this mountain 5,000 years before they disappeared from that one over there. They could chart changes and cycles in the vegetative communities, and the animals that lived in them, with an astonishing degree of precision over the last 40,000 years.

A major advance in human knowledge that took place that day in 1961, and we owe it all to thousands of generations of desert librarians no more than 15 inches long. Including the tail.

Chris Clarke is an environmental writer of two decades standing. Author of Walking With Zeke, he writes from Palm Springs regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.

About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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