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The Neotoma genus packrats have lent their name to humans who won’t throw anything anyway. These rodents, also known as wood rats, were aptly chosen as a comparison. They make stick nests that they jumble up with food, waste, and other debris, as well as the things they collect. When they live near humans, their collections can include silverware, shoes, bits of clothing, newspapers, even pieces of the traps set out for them. They’ve also been called trade rats because they “trade” an object on the way home for something they like better. In PrairyErth, William Least Heat-Moon wrote “campers have awakened in the morning to find a pocketknife or compass traded for a pinecone or deer turd.”

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These treasures can remain in their nests for a rather long time. Biologists have been studying the last 50,000 years of plant, animal, and climate interactions in the American Southwest by studying the remains (middens) of these packrat nests. These middens can provide a record of climatic changes, vegetation profiles, and animal life during this period.

Here’s the thing: packrats sometimes pee in their nests. The urine crystalizes as it dries out, helping to solidify and preserve the materials of the nest in the arid terrain of the rats’ habitat. Fossilized packrat middens thus become inadvertent hoards of pollen and other plant remains, as well as fragments of insects, reptiles, and even other mammals that may have wandered in. (DNA can now be used to identify such fragments down to the genus if not species level.) Since packrats tend to collect material from a limited range around their nests, their middens can be read as localized climate change indicators.

Steven D. Emslie, Mark Stiger, and Ellen Wambach write that the urine-solidified middens are sampled with rock hammers. These are then rinsed and soaked in water to remove the urine; the cleaned and then dried fragments of seeds, bark, twigs, leaves, and solid feces (used for radiocarbon dating) are sorted and identified.

In their study, Emslie, et al. took microbotanical remains from seventeen bushy-tailed packrat nests in the Upper Gunnison Basin in Colorado to reconstruct environmental changes during the last three thousand years. They chart a rising and falling of the timberline and the appearance and disappearance of various pine species (some like ponderosa and lodgepole no longer found locally). Bristlecone pine moved into cooler, dryer lower elevations during cold periods, for instance between 1500-1850 (sometimes called the “Little Ice Age”).

By studying the response of plants and animals to differing temperature and rainfall regimes, paleobiologists can offer models for the rapid disruptions we’re seeing now because of the earth’s rising temperature. Pollen and dendrochronology have become standard markers for such re-creations of historical environments. Packrat middens offer another source for tracing the varied careers of plants and animals through time.


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The Southwestern Naturalist, Vol. 50, No. 2 (Jun., 2005), pp. 209-215
Southwestern Association of Naturalists