Rat nests as time capsules: what 50,000-year-old packrat middens tell us about climate

A packrat’s den can hold a record of a place’s plants and animals for more than 50,000 years. In the dry West, where lakes and bogs with long sediment cores are scarce, these accumulations, called middens, are among the best tools for reconstructing recent environments and climates. They are described in a 2022 Park Paleontology News article by Justin Tweet, a paleontologist working as a National Park Service partner through the American Geosciences Institute.

A well-hardened and extensive packrat midden at City of Rocks National Reserve, Idaho. NPS photo.

Why small fossils beat big ones

To study the plant and animal communities of a thousand or twenty thousand years ago, the article says, big vertebrates like mammoths are not the best fossils. Large animals are rare in an environment and not strongly restricted by it. Pollen, leaves, invertebrates and small vertebrates, much lower on the food chain, tell more, especially in a setting that gathered fossils over centuries. Lake and bog sediment cores serve that purpose, but places like Arches or Death Valley National Parks have few lakes.

What a midden is

Packrats, or woodrats, of the genus Neotoma, resemble common brown rats but have furry tails and large ears. They are solitary and territorial, foraging within about 100 to 160 feet of the den, and build large dens of plant pieces and other found objects, preferring caves and rock shelters. They also collect things that interest them, so recent dens can include human artifacts. Part of the den serves as an outhouse and dump for surplus food and building material, and that debris pile is the midden.

A bushy-tailed woodrat (Neotoma cinerea), the largest species of packrat, at Wind Cave National Park, South Dakota. NPS photo.

In dry settings, packrat urine crystallizes over time into a material called amberat, which hardens a midden into a dark, glossy mass and can preserve whatever it soaks for tens of thousands of years, as long as it does not get wet again. Plant fragments, pollen, insects, small bones, fur and droppings are all preserved this way. Eastern states have probable ancient middens too, but humidity prevents amberat from forming, so preservation is poorer.

A partially hardened midden at Wupatki National Monument, Arizona, which might hold material a few thousand years old at most. NPS photo by Justin Tweet.

What the layers show

Individual rats live only a few years, but dens in good locations are reused, so a record can build up over hundreds to thousands of years, each layer a separate midden. Studying the layers shows when particular organisms and larger communities appeared or vanished. Most studies have used plant fragments, but any fossil found in a midden can be studied the same way, from millipedes to lizard scales to packrat droppings.

In the Park Service, middens have been used to study the rate of erosion in the Grand Canyon, the extinction of ground sloths and the timing of when people left Chaco Canyon. At Death Valley researchers tracked changes in droppings’ size as a sign of climate change: a large heat-intolerant species was replaced over time by a smaller, heat-tolerant one. An extinct plant, the rabbitbrush Chrysothamnus pulchelloides, was named from material in a Chaco midden. For park management, middens give a record of past ecosystems, useful for restoring areas to pre-disturbance conditions or predicting how ecosystems will respond to climate change by how they responded before.

An unhardened midden, a pile of droppings and plant fragments, in a small rock shelter in Mojave National Preserve, California. NPS photo by Justin Tweet.

Where they are

Middens, fossil and recent, have been reported from 41 park units in western states, and probably occur in others. Some are over 50,000 years old, the practical limit of radiocarbon dating, but most are younger, from around 30,000 years old to the present. Hikers may see active or ancient middens in canyons, rock shelters and alcoves. Loose ones look like piles of plant fragments and rodent droppings; ancient hardened ones are dark and glossy with fossils embedded. Report one to park staff but leave it alone, because it is a fossil and also unsanitary. A USGS and NOAA database of North American packrat middens can be searched online.

National Park Service units with ancient or modern packrat middens, from John Day Fossil Beds in Oregon to Big Bend in Texas. NPS image.

Sources

• Justin Tweet, NPS partner paleontologist, American Geosciences Institute, “Studying the Past and Predicting the Future Using Rat Nests,” Park Paleontology News, Fall 2022, https://www.nps.gov/articles/000/studying-the-past-and-predicting-the-future-using-rat-nests.htm; rewritten in hubnx’s own words.