Geoffrey Spaulding and Kenneth L. Cole lifted off from a high plateau in the Grand Canyon, their helicopter laden with so many packrat nests that it could barely climb.

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.”

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