Pleistocene Pack Rat Middens

The Neotoma genus includes 22 species of rodents known as packrats in the west and woodrats in the east.  The eastern woodrat (Neotoma floridana) looks like an overgrown field mouse with a brown back and white belly.  They’re entirely vegetarian, feeding on acorns, nuts, and such common woodland plants as Virginia creeper and greenbrier–foods so abundant in the average woodlot that they don’t have to forage far from the safety of their bulky nests.  Unlike the invasive Norway rat (Rattus norvegicus) and the house mouse (Mus musculus), both of which were accidental stowaways on colonial ships, woodrats cause little damage to agriculture.

Eastern woodrat.  Reportedly, they make good pets and do almost no damage to agriculture.  I’ve never seen one.  For some unknown reason they’re absent from the Augusta, Georgia region but live in most the rest of the state.

In Georgia woodrat bones have been found in just about every Pleistocene fossil site where small mammals accumulated, proving they’ve been common residents of eastern deciduous forests here for millenia.  It’s unfortunate, however, that none of the nests they built during the Pleistocene have been preserved in the east.  All species in the Neotoma genus build very large nests out of sticks upon which they urinate.  The sugar in their urine is sticky, and it acts like cement glue that holds the sticks together.  It’s also a preservative that turns the wood into a substance known as amberat.  Amberat preserves the wood for as long as 50,000 years.  Eastern woodrats build their nests in forests under tree stumps or even in trees.  Rain and moist soil eventually destroy these nests.  But western packrats often build their nests in caves and rock shelters where they remain intact for tens of thousands of years.  These ancient nests provide a treasure chest of data for paleoecologists.

Bushy-tailed  packrat (Neotoma cinerea) nest.  The next is so bulky, large predators probably don’t want to waste the energy trying to tear them apart to get such a small meal.  Rats can escape through a back entrance when fleeing smaller predators.  The sticks are cemented together with sugary rat urine.

Fossil packrat nest in a cave.  The rat urine preserves them for tens of thousands of years as long as they are unexposed to rain.

Scientists have analyzed over 2,000 western packrat nests from caves found from Mexico to British Columbia and from west Texas to east California.  By carbon-dating the wood and identifying the species of tree from which it originated, scientists can determine the forest composition at the time the packrat built the nest.  Moreover, packrats have a curious habit of collecting odd objects, and they often carry fossils, feces, and insect remains back to the nest with them.

Diagram showing changes in forest composition over a 25,000 year period based on evidence from packrat middens.  During the Ice Age some associations of tree species were distributed 700 meters lower in elevation than they are today, and many species compositions have no modern analogue.

The evidence from fossil packrat middens suggests certain associations of woody plants were distributed about 700 meters lower in elevation during the Last Glacial Maximum (~28,000 BP-~15,000 BP) than they are today.  Fir, spruce, and limber pine grew together in vast forests below subalpine meadows which in turn were also lower in elevation than they are today.  Juniper, pinyon pine, and oak grew at elevations between 300 Meters -1700 Meters where highland desert prevails today.  Desert vegetation was nearly absent then and so was ponderosa pine which today comprises the largest zone of forest in the Rocky Mountains.  Evidence from packrat middens suggests this commercially important species didn’t dominate western forests until just 500 years ago.  Much of the composition of Ice Age Rocky Mountain environments have no modern analogue.  The association of plant species everywhere is truly random and transient.

On a humorous sidenote I came across this historical anecdote:  In the 19th century starving miners discovered a packrat midden in a cave.  Some of them mistakenly thought it was a kind of candy they considered manna from heaven.  Packrat middens glisten and have a sweetish resiny odor, explaining the delusion.  A few of them dared to eat it.  Needless to say, they became “nauseated” after eating the 10,000 year old wood cemented together with rat urine.


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7 Responses to “Pleistocene Pack Rat Middens”

  1. Mark LaRoux Says:

    The Allegheny woodrat (N. magister) leaves a midden in karst areas too. I search for the middens when I walk outcroppings in eastern Huntsville. They look like an Eastern woodrat with a furry tail and a bit different coloring. The ones I’ve found on Monte Sano (Huntsville area) had not been identified since the 1950’s, so it was great to finally catch one, photograph and weigh her, and release her. There’s a metapopulation that I thought was isolated, but apparently migrate and go very deep into mines and caves in hot and cold weather.

  2. markgelbart Says:

    Maybe you can find one of their middens dating back to the Pleistocene. A first for the east.

  3. James Robert Smith Says:

    Wow. That’s one source of information I’d have never suspected! Fossilized pack rat nests!

    I’ve seen a fair number of rodents in my travels around the south. I’ve sen the cotton rat a few times. I’ve also seen a couple of very rare vole species–one on the summit of Charlie’s Bunion in the Great Smoky Mountains National Park (5,300 feet above sea level), and one at Wind Rock in West Virginia (around 4,200 feet above sea level).

    Genuinely wild rodents are hard to spot when you’re in the back country. However, I always hang my food when backpacking in wilderness–for I’m much more likely to have my food stash raided by persistent rodents than marauding bears.

  4. markgelbart Says:

    I’m glad I wrote about it then.

    I thought fossil packrat middens were common knowledge. I guess it’s only common knowledge for paleogeeks like me.

    I’ve seen cotton rats too. I’ll have something about them in a future essay.

  5. Mark LaRoux Says:

    That’s exactly what I’ve been trying to find, though I haven’t found an institute to partner with for the middens I’ve already got that need testing. It’s actually better to leave them where they are and wait for better collecting methods than expose them to the elements by removing them (as I learned in summertime). Ideally, I’d like to get with the NSS ( and look through their old ‘cave cards’ that listed exact contents of each cave entered back in the 60’s and 70’s. Meanwhile, l get busy with other stuff…you know how it is. N. magister is really the only animal that brings hard mast deep into caves, so they do a good job of distributing leafy matter deep into caves. I’m not a pollen expert, but I know there HAS to be pollen stored in the middens in karst areas in the southeast. Hate to see a resource wasted…

  6. markgelbart Says:

    Are the middens you’ve seen fossilized?

    I can try to find someone in academia who might be interested.

  7. Chinchilla Rat (Abrocoma sp.) Middens | GeorgiaBeforePeople Says:

    […] rat middens provide a 50,000 year record of environmental changes in the Rocky Mountains. (See:  Chinchilla rats (Abrocoma sp.) offer the same opportunity for South American paleoecologists.  […]

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