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A packrat midden is the nest of a pack rat, a small rodent found in the southwestern United States and northern Mexico. Due to a number of factors, pack rat middens may preserve the materials incorporated into it up to 40,000 years. The middens may thus be analyzed to reconstruct the environment around the midden when it was built, and comparisons between middens allow a record of vegetative and climate change to be built. Examinations and comparisons of pack rat middens have largely supplanted pollen records as a method of study in the regions where they are available.

Contents 1 Midden structure 2 Pack rat midden analysis 3 References 4 External link

Packrats are known for their characteristic searching of materials to bring back to their nests creating an ever expanding collection known as a "midden" for its messiness. In natural environments, the middens are normally built out of sticks in rock crevices or caves to protect themselves from predators. In the absence of crevices or caves, the middens are often built under trees or bushes. The packrats will also use plant fragments, animal dung and small rocks in building the nest. The vast majority of the materials will be from a radius of several dozen yards of the nest. The packrat urinates in the midden during the time it lives there; the sugar and other substances in the urine crystalize as it dries out, cementing the midden together. After a few decades, the packrat will abandon the midden and move on to start a new nest.

In 1978, paleoecologist Julio Betancourt was asked to study packrat middens. Betancourt had previously tried to imagine where the Anasazi had gotten the numerous large logs for the buildings of the treeless Chaco Canyon site in what is now northwestern New Mexico; he called midden expert Tom Van Devender and confirmed that Van Devender had found pinyon needles near the site, though none of these trees grew there in modern times. Thinking that the middens were perhaps a century old, Van Devender and Betancourt submitted the middens to radiocarbon dating and found that many of them were over 1000 years old. Research since then has found middens can last 40,000 years.

The unsuspected resilience of the middens is due to three factors. The crystallized urine dramatically slows the decay of the materials in the midden. The dry climate of the American Southwest further slows the decay, and middens that are protected from the elements under rock overhangs or in caves survive even longer.

Zoologists examine the remains of animals in middens to get a sense of the fauna in the neighborhood of the midden, while paleobotanists can reconstruct the vegetation that grew nearby. Because middens are abandoned after a short period of time, they are uncontaminated "time capsules" of several decades of natural life, centuries and millennia after they occurred.

• Betancourt, Julio L., Thomas R. Van Devender, and Paul S. Martin, eds. Packrat Middens: The Last 40,000 Years of Biotic Change, University of Arizona Press, 1990, ISBN 0-8165-1115-2.

• Pack Rat Piles: Rodent rubbish provides ice age thermometer, Science News, 24 September 2005