Nature’s Historian- the Packrat

Packrat midden in Utah. Photo by Larry Coats

Everybody knows the type—someone who cannot throw anything away without having a mild anxiety attack, who saves everything because it might be useful/valuable/fashionable again…someday.  It turns out that this quirk is not unique to humans, some wild animals exhibit this behavior as well. In fact, one Grand Valley species is so renowned for this behavior its name has become synonymous with it- the packrat.


“Packrat” is a common nickname for a woodrat.  There are 22 species of woodrats in North America, six of which occur in Colorado.  One of the best known species, the bushy-tailed woodrat, is a Grand Valley native. It is a far more appealing creature than its name might suggest.  Measuring six to nine inches from nose to base of tail, the bushy-tailed woodrat is one of the largest packrats, distinguished by its long tail, large eyes and ears and soft grey-brown coat.  But you are unlikely to get a look at one unless you frequent canyon country at night—packrats are nocturnal.  What you are more likely to notice are their dens.

Bushy-tailed packrat. Photo by Larry Coats

 Life is dangerous for a tasty morsel like a packrat so they fashion themselves a bunker-like den in a cliff crevice or rock outcropping. Dens have multiple entrances, a nest area for sleeping and staying warm, food caches and a combination storage-toilet area called a “midden”.  A good den is valuable property.  Like a historic home, if a packrat dies, another will quickly move in and thus dens are used by generations.  The midden is perhaps the most interesting feature of the den. For reasons that are not totally clear to scientists, packrats like to collect stuff.  Like a yard sale junkie, they love to score treasures like small bones, plant parts, coyote feces and small shiny objects. They stash these items in their middens and then they pee on them.


Packrat pee is a peculiar thing.  Many of the desert plants that packrats eat protect themselves by including defensive chemicals in their tasty bits. The packrat has adapted to this strategy by eating the plants anyway and excreting the nasty chemicals in its urine.  As the urine dries, these chemicals crystalize into a hard sticky substance called “amberat” that vaguely resembles amber (fossilized tree sap) or brownish asphalt.  Middens and their amberat deposits are commonly seen in the cliffs and rock outcroppings of the Colorado National Monument and McInnis Canyons National Conservation Area. 


Though scientists aren’t sure why packrats create middens they are awfully glad that they do. Amberat cements and protects items in the midden, and because they are continually added to and generally placed in sheltered dry areas, middens act like a time capsule.  Scientists have found middens that are up to 50,000 years old containing well-preserved plant and bone fossils.  They can carbon date these materials and use them to understand what plants and animals were present in the area tens of thousands of years ago.  According to the biologist Robert B. Finley Jr., “were it not for the general habits of the packrat much of the late Quaternary biology of the western United States would remain a mystery.”


When we call someone a “packrat” we usually don’t mean it as a compliment, but perhaps we need to rethink this.  The packrat is a resourceful survivor whose hoarding habits have provided unique and valuable scientific information. Maybe the human packrats in our lives deserve a little more appreciation.  Their accumulations of reusable gift bags and outdated college textbooks might have scientific value someday.  Let’s just be thankful that they don’t pee on them.

An edited version of this story originally appeared in the Grand Junction Daily Sentinel on November 13, 2013.

Nature’s Historian- the Packrat

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