Just as modern North American packrats—small, bushy-tailed rodents, also 

known as woodrats—like to hoard things, 

their prehistoric ancestors also collected 

objects from their surroundings, including 

parts of plants and animals, in garbage piles 

or dunghills. From these “middens,” U.S. 

Geological Survey (USGS) and Northern 

Arizona University scientists are reconstructing ancient plant communities and 

natural systems of the Southwestern United 

States from as long ago as 50,000 years.

How does a garbage pile survive tens of 

thousands of years? The secret is packrat 

urine in combination with a desert climate 

and the protection of the midden from the 

elements. To conserve water in an arid environment, the packrat produces very viscous, concentrated urine and often urinates 

on its garbage pile, which is usually near 

its nest. The urine then dries and crystallizes, acting as a glue to hold the midden 

together. The midden becomes a solid 

barrier protecting the nest from predators. 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2008–3053

2008

Prehistoric Packrats Piled Up Clues to Climate Change

In cooperation with Northern Arizona University

cientists from the U.S. Geological SSurvey and Northern Arizona University studying climate change in the 

Southwestern United States are getting a helping hand—or would that be 

paw?—from prehistoric packrats. By 

hoarding parts of animals and plants, 

including seeds and leaves, in garbage 

piles or “middens,” these bushy-tailed 

rodents preserved crucial ecological 

and environmental information about 

the past. From these middens, scientists are able to reconstruct plant communities and natural systems from as 

long ago as 50,000 years. The contents 

of middens allow scientists to understand how ecosystems responded to 

rapid, large-scale climate changes of 

the past. The insights gained from midden research could offer clues to future 

changes driven by rapid climate shifts .

U.S. Geological Survey scientists are studying the contents of ancient packrat garbage piles or “middens” 

in the Southwestern United States that are as much as 50,000 years old to help understand how ecosystems 

respond to large-scale, rapid climate change. In this photograph, a scientist is using a special sampling pole to 

collect a 12,500-year-old midden deposit tucked into a canyon alcove.
Over time, the plant and animal parts encased within this crystallized urine become 

mummified and invulnerable to decay. As 

long as the midden is protected from water, 

say under a rock ledge or in a dry cave, it 

remains intact.

Middens contain mostly twigs, leaves, 

rocks, and packrat fecal pellets, but they 

also contain seeds, fruits, the bones of small 

animals, pollen, lizard scales, and insects. 

Packrats collect just about anything; however, they can only carry their haul a short 

distance, about 65–160 feet (20–50 m). As 

a result, middens allow scientists to reconstruct past environments at very specific 

times and places. 

To unravel the exact nature of the materials contained in an ancient midden, scientists wash small sections of the midden 

in water and extract the plant and animal 

fossils from the matrix of crystallized 

urine. The fossilized bits are then identified by comparing them to reference collections of plants, pollen, bones, and other 

materials. The comparative collection at 

the USGS-Northern Arizona University 

Macrobotanical Laboratory contains more 

than 2,500 identified specimens of seeds, 

leaves, fruits, flowers, and twigs. Some 

specimens can be viewed at the online 

Macrobotanical Digital Library at http://

sbsc.wr.usgs.gov/cprs/research/projects/

global_change/MacroDigitalLibrary.asp.

As the result of midden research, USGS 

scientists have been able to reconstruct 

the vegetation of Grand Canyon as it 

existed during the last ice age. During 

this period, plants escaped the colder climates by growing at lower elevations than 

they do today or by growing in warmer, 

more southerly latitudes. As the ice age 

ended 11,600 years ago, climates rapidly 

warmed, increasing by more than 7.2°F 

(4°C) in less than 200 years. This rate of 

warming is similar to the 0.9°F (0.5°C) 

increase that has been measured globally 

over the past 25 years. Even at the lowest rates of warming predicted by climate 

models, the total increase over the next 

200 years would equal the 7.2°F (4°C) increase that occurred after the last ice age.

Following the last ice age, Grand Canyon plant species—many of the same 

species found today—responded at different rates to the warming climate. Species 

adapted for rapid dispersal, like those with 

small windborne seeds, quickly became 

established at higher elevations and farther north, escaping the increasing heat. 

Plant species whose seeds are dispersed by 

animals also quickly migrated vertically 

to more suitable local habitat; however, 

reaching the new northern limits of their 

habitat ranges, some of which were more 

than a 1,000 miles away, took 4,000 to 

8,000 years. Other species, including those 

that reproduce by shoots or spreading 

roots, migrated even more slowly. Because 

individual species migrated at different 

rates, local ice-age plant communities were 

replaced by new communities with different species compositions.

Similar changes appear to be beginning 

today, as some older, established tree populations are dying, while rapidly dispersing 

weeds and grasses with windblown seeds 

are becoming more abundant. Trees and 

shrubs could expand into nearby higher elevation habitats within only a few hundred 

years, as they did in the past; however, 

northward migrations across long distances 

will be very slow. National parks and other 

natural areas may preserve rare species, if 

higher or wetter habitats in these places are 

close enough to be rapidly invaded by plant 

species responding to warming. Species 

populations that are already restricted to 

the highest elevations or wettest areas have 

nowhere to go and may not survive.

Thanks to the hoarding behavior of prehistoric packrats, USGS and other scientists are gaining a better understanding of 

how ecosystems responded to rapid, largescale climate changes of the past. This 

invaluable information may help scientists 

answer crucial questions about future 

ecological and other changes that may be 

caused by rapid climate shifts. 

This 12,500-yearold snakeweed (Gutierrezia 

sarothrae) flower was preserved in 

a packrat midden built near the end of 

the last ice age in what is now Glen Canyon 

National Recreation Area, Utah. It still has tiny 

pink petals, which are much smaller than the tip of 

a ballpoint pen. Information provided by fossils like 

this one found in prehistoric packrat middens may 

provide invaluable information that scientists can use 

to understand changes caused by future rapid climate 

change. (Two millimeters is about a tenth of an inch.)

Printed on recycled paper

For more information contact:

USGS Southwest Biological

Science Center

Colorado Plateau Research Station

Northern Arizona University

P.O. Box 5614

Flagstaff, AZ 86011

928-523-2029 or ken_cole@usgs.gov

Online Resources:

http://sbsc.wr.usgs.gov/cprs/research/projects/

global_change/cprsmain.asp

http://www.nau.edu/~qsp/macrobotanical_lab.html

This Fact Sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2008/3053/

Kenneth L. Cole

Edited by James W. Hendley II

Graphic design by Susan Mayfield

The packrat (Neotoma albigula), is a small rodent that 

collects just about anything—seeds, twigs, fruits, the 

bones of small animals, lizard scales, and insects—

in fossilized middens preserved by the dry climates 

of the Southwest. (Photograph courtesy of www.

saguaro-juniper.com.)