I have had a running battle with packrats at my house. They are prodigious burrowers, and I am afraid they will eventually undermine the walls. I have tried everything before resorting to poison, which I hate having on the property. This morning a dead packrat was sprawled in the middle of the power house when I turned on the inverter. The packrats keep eating the bait, but they keep burrowing, too.
I guess there are always new generations of workers to keep up the assault.
Packrats, also known as woodrats and sometimes trade rats, belong to the genus Neotoma. The packrats in this immediate neighborhood are gray and, as the name suggests, rat-like in appearance. They have prominent eyes and large ears. They look like rats to me, but the only thing that keeps me from thinking I live on a rat-infested property is their name.
Packrats get their common name from the habit of collecting—whether it be bits of bone, plant material, or small objects and have hence made it into our slang for “hoarder” or “collector.” Their less-common name “trade rat” comes from their habit of dropping what they are carrying if they find something they like better (such as shiny things).
This collecting habit may be an annoyance to those of us who live with them, but it’s proven to be a boon for paleo-ecologists who use packrat middens or dens to learn about the past.
Unlike other desert rodents, packrats void copious amounts of urine and thus must rely on succulent plant material and protection from the sun to maintain their water balance. They seek shelter in caves and rock fissures, under mesquite trees, and in my house—and then improve these shelters with a loose mound of sticks, plant material, bones and mammal dung. Often these dens, or middens, are armored with a thick layer of prickly pear or cholla cactus, making them unappealing to predators that may have a packrat snack in mind.
Fossil middens (resembling blocks of asphalt with the consistency and mass of an unfired adobe brick), represent only part of an active den. Packrats will typically use a portion of their den as an outhouse, and unused and discarded plant fragments which accumulate in these areas become saturated in rat urine.
In dry climates such as ours, the urine crystallizes into a substance called “amberat” which has a number of useful properties. Most obviously, it binds the midden materials. It is self-sealing to an extent: it rehydrates under humid conditions and becomes sticky, so dust and dirt become trapped on the outside surfaces and prevent moisture from penetrating more deeply. Saturating plant material with amberat is comparable to packing it in salt, protecting it from decay.
Packrats are solitary animals, and individual middens are thought to represent a few years of activity. However, choice settings are reused over and over again, so large fossil middens spanning thousands of years can be produced. The foraging range of a packrat has a radius of about 100 to 160 ft (30 to 50 m), so their middens are local records.
Because packrat middens are often built in caves or rock fissures where they are protected from the weather, they can survive intact for tens of thousands of years. This is where a long-term midden deposit is quite useful: as conditions change and different plants become more or less abundant, the changes will be recorded by generations of rats.
Packrat middens are excellent sources of pollen, plant fragments, body fossils of insects, spiders, millipedes, small vertebrates, and fossil rodent droppings. They span at least 40,000 to 50,000 years and are particularly well-represented in the deserts from western Texas to eastern California, areas that are lacking in other sources of paleoecological fossils like lakes and marshes. By far the most popular aspect of fossil middens has been their plant fossils. Researchers reconstruct changes in plant assemblages over time, which can be used as a proxy for climate.
The master at packrat midden story interpretation for the Chihuahuan Desert region is Dr. Thomas Van Devender, a research scientist with the Arizona-Sonora Desert Museum. He and his colleagues have analyzed hundreds of packrat middens throughout the Chihuahuan Desert.
Van Devender and his colleagues have looked at 14 middens in the Maravillas Canyon in the Black Gap Wildlife Management Area, on the eastern side of Big Bend National Park from here. The oldest samples indicate that 28,000 years ago the canyon slopes were covered with a woodland of pinyon pine, shrub oak and junipers.
Two species of junipers were found in the samples: red-berry juniper and ash juniper. This is interesting because—while red-berry juniper is still the most common juniper in the Trans-Pecos—the ash juniper is fairly rare in this region now. With the climate change that marked the end of the Ice Age, ash junipers retreated eastward and are now most commonly found on the Edwards Plateau.
The presence of ash juniper is believed to be an indication that the climate was wetter during the distant past. Other indicators of nearby water include the remains of over 30 species of amphibians found in the Maravillas Canyon middens.
Other plants include Hinckley oaks—a small tree that rarely grows over three feet tall and is now considered an endangered species. But 24,000 years ago, the Hinckley oak was the dominant oak in a woodland scattered with lechuguilla, althorn and sotol—a combination of plants you’d be hard pressed to find today.
The Maravillas Canyon middens also revealed another surprise—the remains of a California Condor. These bones, and others found near Mule Ears Peak in Big Bend National Park, indicate that these majestic birds—having a wing span of more than nine feet—soared through the West Texas skies over 10,000 years ago.
Not bad for heaps of plant matter and other things cemented by rat pee. But what will happen over time to my house? I’d lay bets that the packrats will win.
Many thanks to Cathryn Hoyt of the Chihuahuan Desert Nature Center for much of the content of this post.
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