Packrat time machines

By Dr. CATHRYN HOYT

Have you ever been camping in the desert and lost your pocketknife, compass, or shiny new fork? You might have been the victim of a packrat.

Packrats are compact, long-tailed rodents with prominent eyes and large ears. Their name comes from their habit of collecting the desert’s flotsam and jetsam – bits of bone, plant parts, even pocketknives. This habit may be annoying to campers, but it’s a boon for paleo-ecologists, who use packrat dens to learn about the past.

Packrats seek protection from the sun in caves, rock fissures, or under mesquite trees, where they build loose mounds of sticks, plant material, bones and mammal dung. Often these dens – called middens – are armored with prickly pear or cholla cactus to deter predators looking for a snack. Middens can remain intact for tens of thousands of years, giving paleo-ecologists a time machine into the past.

Ancient middens are rock-hard and usually coated with amberat, or fossilized rat urine. When “melted” in warm water, the midden falls apart and the various pieces of plant material, bones, insect casings and pollen can be picked out and analyzed. As these bits are identified, paleo-ecologists can develop a picture of what lived within 50 meters of the packrat’s shelter. And by analyzing numerous middens from the same area, paleo-ecologists can interpret how the environment has changed over time.

Dr. Thomas Van Devender, a researcher scientist at the Arizona-Sonora Desert Museum, and his colleagues, have analyzed hundreds of middens in the Chihuahuan Desert. Fourteen of these came from Maravillas Canyon in Black Gap Wildlife Management Area.

In the Maravillas middens, Devender and his colleagues found two species of juniper: red berry and ash juniper. While red-berry juniper is still the most common juniper in the Trans-Pecos, the ash juniper is fairly rare here now. Ash juniper’s presence in the midden is believed to indicate that the climate was wetter during the distant past. Other evidence of a wetter past environment included the remains of over 30 species of amphibians found in the middens.

The Maravillas Canyon middens 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 – with a wingspan of more than nine feet – soared our skies more than 10,000 years ago.

If you go to Rio Grande Village in Big Bend National Park, stop a moment before the tunnel and consider the landscape. Middens have told us that 24,000 years ago in this area there were motts of Hinckley oaks—a small tree that rarely grows over three feet tall and is now considered an endangered species. But 24,000 years ago, this was the dominant oak in a woodland area scattered with lechuguilla, althorn and sotol – a plant combination you’d be hard pressed to find today.

Packrat middens hold many stories. So the next time you lose your pocketknife to a packrat, just imagine what stories will be told when a paleo-ecologist finds it 20,000 years from now.

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Nature Notes is produced by the Chihuahuan Desert Research Institute and Marfa Public Radio. Visit us online at naturenotesradio.org.