To Dr Spaulding, a geologist with the engineering company CH2M Hill, and Dr Cole, an ecologist for the United States Geological Survey, the nests were precious cargo. Packrats, which look like brown squirrels with Dumbo ears, are skilled home builders, and their massive nests, known as middens, can last 10,000 to 20,000 years.

For that reason, the middens serve as time capsules of desert ecology. By analysing preserved ancient plants and scat from a variety of middens dating back 12,000 years, Cole recently proved that a miniature ice age known as the Younger Dryas, long thought to have been confined to the North Atlantic, was also felt in the American Southwest.

Through carbon dating of specimens from packrat nests, scientists can put together a portrait of the prevalence of different plant species in an area over time, similar to the long view obtained by drilling into Arctic ice cores for plant samples trapped there.

“There are twigs and seeds and things in the middens that are easy to work with,” Cole said. “You don’t have to look at some microscopic piece of leaf. The rats collect some of everything. The nest is like a snapshot of that particular spot at some time in the past.”

The packrats’ middens, hulking structures up to 10 ft wide, serve as trash heaps, climate-controlled homes and defensive ramparts. “The rats leave piles of debris in front of the entrance to preserve moisture,” Cole said, “and the debris helps seal up the crevice so larger animals can’t get in.”

To turn grasses and leaf bits into sturdy walls, the rats use the only natural glue available: their own urine. “Packrats don’t drink water, so their urine is very viscous,” Cole said. “It crystallises and becomes solid.” Like hardened amber, the solidified waste is an ideal matrix to keep plant fragments intact for thousands of years.

When researchers find a new nest in a cave or an outcropping, they pick it apart like prospectors—an extremely smelly process—and analyse the plants they find for carbon-isotope ratios that identify them as members of a particular species.

If a climate shift occurs, changes in these norms are recorded in the contents of middens at different altitudes. During cooler periods, for instance, most vegetation migrates downhill. As warming occurs, the plants move back uphill; some even disappear from the middens altogether.

Because no layers of sea or ice sediments can be found in most dry inland regions, plants from the middens have been scientists’ only way of piecing together climactic records for much of the Western US.

Middens contain markers of other types of environmental change as well. After dissolving urine from 10,000-year-old nests near Yucca Mountain, Nev., Mitch Plummer, a hydrogeologist at the Idaho National Laboratory, found that the samples had surprisingly high levels of chlorine 36 isotopes.

New York Times / ELIZABETH SVOBODA