Pack rat middens are widely known among scientists because these strange objects are helping to shed light on our remote history, from the dawn of modern humans 40,000 years ago right up to relatively recent events, such as the fall of the Byzantine empire.
It all sounds very peculiar, so let us take a look at the creature responsible: the pack rat, or the wood rat, or Neotoma, of which there are 22 different species. Pack rats can be pale buff color, gray or reddish brown, with white undersides and feet, and typically measure from between 9 to 19 inches in length.They're found throughout North and Central America in a wide range of habitats, from deserts to forests and mountains.
They sound fairly ordinary and scarcely suggest an animal that's special in any way. However, the pack rat has one particular habit that makes it quite distinctive, and that is its nest-building habits.
Scientists have discovered that pack rats protect themselves by building dens out of any debris that comes their way - twigs, fragments of food, pieces of plant and waste products of other animals.
Pack rats live in these dens, which fill up with their excrement and which also become saturated with their urine. This latter fairly disgusting addition crystallizes and cements the den into a bricklike consistency. It then becomes a midden.
Their scientific importance was only recently appreciated, when researchers discovered in Nevada an old pack rat midden filled with fragments of juniper, a plant that hasn't grown in that region for thousands of years.
So they dated the midden - and found that it was 9,300 years old. Yet inside it were the perfectly preserved remains of the plants and animals that had thrived in that area many millennia ago.
For thousands of years, the pack rat has been creating perfect time capsules of past life on Earth.
Essentially, the pack rat collects detritus for several months from land that ranges about 165 feet around its den. This material is then preserved in crystallized urine, creating tiny archaeological treasure troves.
Studies of these have already produced one learned tome - "Pack Rat Middens," edited by Julio Betancourt, Thomas Van Devender and Paul Martin (University of Arizona Press) - which covers an enormous range of midden investigations: from the reasons why mainland America's big mammals became extinct 11,000 years ago to alterations in the intensity of cosmic ray bombardments over the past 20,000 years. Reports on middens made by other small mammals also are included in the book.
But most intriguing are those chapters that concentrate on the very specific uses of middens, such as the study of the fate of the ancient city of Petra, in Jordan.
Anyone who has seen the Steven Spielberg and George Lucas film, "Indiana Jones and the Last Crusade' will recall the vast, magnificent facades of Petra among which Harrison Ford and Sean Connery search for the Holy Grail.
Petra was once a thriving Neolithic village before it became one of the richest cities in the Roman and, later, the Byzantine empires. Then it was abandoned and was not rediscovered until 1812. The question that archeologists have puzzled over is this: what caused the collapse of the city of Petra?
The solution, decided three researchers from Arizona - Patricia Fall, Steven Falconer and Cynthia Lind-quist - might lie with middens. However, as there are no pack rats in the Middle East, they had to search for another, similar denmaking animal, and selected the rock hyrax, of the Procavia family.
The rock hyrax - a rabbit-size herbivore - fortuitously creates middens like those of pack rats, and the contents of these fossilized dens were studied by the Arizona scientists for three historical periods: 300, 500 and 900 A.D. These dates roughly correspond with three crucial moments in Petra's history: its Roman heyday, the flowering of its Byzantine phase and its post-imperial collapse.
Each midden from these epochs yielded more than 100 plant species, and studies of these painted an all-too-clear picture of the fate of Petra. In the early samples, pollen found in middens came largely from the oak and pistachio trees that once formed vast forests throughout the Mediterranean. But by 500 A.D. these had died out, and most pollen came from shrubs, herbs and grasses. By 900 A.D., even these plants had virtually disappeared.
And the culprit? Mankind, of course.
Two thousand years ago, primitive farmers began clearing land and cutting down trees for firewood and house construction. With the arrival of the Romans, deforestation increased dramatically. Then the Romans left and the Byzantine empire took over. It too collapsed, and the inhabitants of Petra were left to struggle alone in a devastated landscape.
They turned to intensive grazing, letting their goats loose to eat their way through every shrub, herb and piece of grass in the region. In the end, only the sand dunes were left.
In short, Petra flourished because the city was built in prime, verdant real estate. It collapsed because its inhabitants utterly destroyed the countryside around it and eventually brought themselves to the point of starvation. The shining desert around Petra is man-made.
It's a depressing but important story, given our pres-ent fears about the environment. The people of Petra were fairly unsophisticated by today's standards but still managed to wreck their local ecology in spectacular fashion.